Mech Vent test 1

Fundamental Functions of a Mechanical Ventilator

• Lung Inflation (I): The primary purpose of the ventilator is to inflate the patient's lungs by delivering a volume of gas.

• Decision to Stop Inflation (I:E): The device determines the transition from inspiration to expiration, establishing the Inspiratory to Expiratory ratio.

• Passive Exhalation (E): The ventilator allows for the lungs to empty through passive recoil.

• Decision to Re-start Inspiration (E:I): The device determines when to terminate the expiratory phase and begin the next inspiratory cycle.

Types of Mechanical Ventilation

• Negative-Pressure Ventilation (NPV):

  • Objective: This method attempts to mimic the physiological function of the respiratory muscles to facilitate breathing through normal mechanisms.

  • Examples:

    • Iron Lung

    • Chest Cuirass

    • Poncho Wrap

    • Porta-Lung

• Positive-Pressure Ventilation (PPV):

  • A mechanical device is utilized to deliver gas into the lungs through an artificial airway (endotracheal tube) or a positive pressure mask.

• High-Frequency Ventilation:

  • Oscillation

  • Jet Ventilation

Negative Pressure Ventilation (NPV) Mechanics and Clinical Applications

Physiological Pressure Changes during NPV

• Inspiration:

  • Pleural pressure ($P_{pl}$) drops from $-5$ to $-10   cmH_2O$.

  • Alveolar pressure ($P_{alv}$) drops from $0$ to $-5   cmH_2O$.

  • Air flows into the lungs; $P_{alv}$ returns to $0$ as the lungs fill.

  • Gas flow stops when the pressure gradient between the mouth and the lungs is equalized.

• Exhalation:

  • $P_{pl}$ increases from $-10$ back to $-5   cmH_2O$.

  • $P_{alv}$ increases from $0$ to $+5   cmH_2O$.

  • The chest wall and lung tissue recoil to their resting position, causing air to flow out.

  • Flow stops once $P_{alv}$ returns to $0$.

Advantages and Disadvantages of NPV

• Advantages (Pros):

  • Effective for patients with relatively normal airway and lung compliance.

  • Appropriate for nocturnal ventilation (nighttime use).

  • Non-invasive technique.

  • Allows the patient to speak and communicate verbally.

  • Associated with a lower risk of infection compared to invasive methods.

• Disadvantages (Cons):

  • Highly confining for the patient.

  • The equipment is often large and cumbersome.

  • Limits physical access to the patient's body for nursing care.

  • Negative pressure applied to the abdominal cavity may lead to blood pooling and impaired venous return to the heart.

  • Ineffective for patients with significant lung disease.

Positive Pressure Ventilation (PPV) Mechanics and Effects

Physiological Pressure Changes during PPV

• Inspiration:

  • Airway pressure ($P_{aw}$) rises to $+15   cmH_2O$, while the initial $P_{alv}$ is $0$.

  • Air is forced into the lungs until $P_{alv}$ rises to between $+9$ and $+12   cmH_2O$.

  • $P_{pl}$ rises from $-5   cmH_2O$ (pre-inspiration) to $+5   cmH_2O$ at the end of the inspiratory phase.

  • Flow ceases when the ventilator cycles into the exhalation phase.

• Expiration:

  • $P_{aw}$ drops to $0$ as gas flow from the ventilator stops.

  • $P_{alv}$ drops from the range of $+9$ to $+12   cmH_2O$ down to $0$.

  • Passive recoil of the lung tissue and chest wall occurs.

  • Air flows out of the lungs.

  • $P_{pl}$ returns to its resting baseline of $-5   cmH_2O$.

Physiological Impacts of PPV

• Cardiovascular/Cardiac: PPV can significantly reduce venous return. During the positive pressure breath, the increased intrathoracic pressure compresses the great veins, leading to decreased left ventricular output.

• Pulmonary Blood Flow: Changes in intrathoracic pressure alter flow dynamics within the pulmonary vasculature.

• Lung Injury: Risk of Barotrauma (pressure-related injury) and Volutrauma (volume-related injury).

• Ventilation/Perfusion (V/Q) Mismatch: The distribution of gas may not perfectly match the distribution of blood flow.

• Acid-Base Balance: Changes in ventilation levels directly impact $CO_2$ levels and pH.

• Cerebral Effects: May cause cerebral vasoconstriction.

High Frequency Ventilation Categories

• High Frequency PPV (HFPPV): Delivers rates of approximately $60$ to $100   \text{breaths/min}$.

• High-frequency Jet Ventilation (HFJV): Operates at rates between approximately $100$ and $400$ to $600   \text{breaths/min}$.

• High-frequency Oscillatory Ventilation (HFOV): Delivers extremely high rates, reaching up to approximately $4000   \text{breaths/min}$.

Internal Components and Power Systems

Control Systems

• Open-Loop Systems: Most non-microprocessor-controlled ventilators. The operator sets a variable (e.g., tidal volume), and the device delivers it without the ability to respond to changing patient conditions.

• Closed-Loop Systems: Referred to as "intelligent" systems. These compare the set control variable to the measured variable. If a difference is detected (e.g., set tidal volume vs. exhaled tidal volume), the system automatically alters delivery to compensate.

Power and Generation

• Power Source: Ventilators can be electrically powered or pneumatically powered.

• Pressure Generator: Can generate either positive or negative pressure.

• Control Panel Variables: Allows regulation of four main variables: Flow, Volume, Pressure, and Time.

• Fluidic Control: Uses principles like the Flip Flop Valve (Beam Deflection) and the Wall Attachment Phenomenon (Coanda Effect).

Control Mechanisms and Compressors

• Drive Mechanisms: Device used to produce gas flow. Can include:

  • Piston-driven mechanisms

  • Rotating blades

  • Moving diaphragms

  • Bellows

• Hospital-Level Compressors: Facilities typically use large, piston-type, water-cooled compressors to provide the gas supply for wall outlets.

Volume-Displacement Drive Designs

• Spring-Loaded Bellows:

  • An adjustable spring on top of a bellows applies force.

  • Pressure is determined by the formula: $P = \frac{\text{Force}}{\text{Area}}$ ($P = F/A$).

  • Tightening the spring increases force and resultant pressure.

  • Example: Servo 900c.

• Linear Drive Piston:

  • An electric motor uses a gearing mechanism to move a piston rod inside a cylinder at a constant rate.

  • Example: PB760.

• Rotary Drive Piston (Eccentric/Nonlinear):

  • An electric motor rotates a drive wheel.

  • Produces a sinusoidal (sine) flow pattern: slow at the start, peak at mid-inspiration, and tapering off at the end.

  • Example: PB Companion 2801 (used in home care).

Flow-Control Valves

• Most modern ICU ventilators (e.g., PB 980/840, Servo I/U, Draeger V500) use these to regulate gas flow.

• They utilize digital valves or proportional solenoid valves for rapid response and flexibility.

External Ventilator Components (The Patient Circuit)

Primary Circuitry Blocks

• Inspiratory Line: Tubing from the ventilator outlet to the patient connector. Often includes a bacterial filter and an optional heated vaporizer.

• Patient Wye (Connector): The junction point where the inspiratory line, patient interface, and expiratory line meet. It may include extension tubing, closed suction, or a Heat and Moisture Exchanger (HME).

• Expiratory Line: Carries exhaled gas back to the ventilator. May include heated wires, a water trap, and a bacterial filter.

• Expiratory Valve: Located at the end of the expiratory line, it controls gas exit and maintains Positive End-Expiratory Pressure (PEEP).

Ancillary Components

• Bacterial Filters: Placed at the ventilator outlet (patient protection) and at the end of the expiratory limb (equipment/staff protection).

• Oxygen ($O_2$) Analyzer: If external, placed on the inspiratory line before the HME.

• Nebulizer: Positioned in the inspiratory limb before the humidifier container.

• Humidification:

  • Heated Vaporizer: Attached to the inspiratory limb. If set correctly, provides Body Temperature Pressure Saturated (BTPS) gas.

  • HME: Connected to the patient connector. Offers roughly $70\%$ relative humidity but can cause resistance issues.

• Water Traps: Used in the expiratory (and rarely inspiratory) line to collect condensation from temperature drops.

• Monitoring: Pressure manometers and flow/volume sensors are often integrated into the ventilator but can be placed close to the patient.

Monitoring and Measurement Devices

Pressure and Volume

• Pressure Monitoring: Uses manometers located at the mouth or the ventilator.

• Volume Measurement: Older ventilators used bulky but durable spirometers (expandable bellows).

Flow Measurement Mechanisms

• Turbine Flowmeters: Use rotary vanes proportional to gas flow (e.g., Wrights Respirometer). Must be kept dry to prevent gear rust.

• Pneumotachometers: Calculates flow based on Poiseuille's law by measuring the pressure drop across a known resistance. Affected by gas viscosity and turbulence (e.g., Fleisch pneumotachograph).

• Thermal (Heated Wire/Thermistor) Flowmeters: Measures the electrical current required to maintain a wire's temperature as gas flow cools it (e.g., Draeger ventilators).

• Ultrasonic Devices: Measures the number of vortices created by a fixed obstruction that interrupt an ultrasonic beam (e.g., Servo I).

• Strain Gauge: Resistance changes as gas flow deforms (stretches) a filament (e.g., Servo 900c and 300).

Core Definitions of Ventilation and Respiration

Ventilation: Defined as the physical movement of air into and out of the lungs.

Respiration: The physiologic exchange of oxygen ($O_2$) and carbon dioxide ($CO_2$) between an organism and its environment.

External Respiration: This component involve the diffusion of $O_2$ and $CO_2$ between the alveoli and the pulmonary capillaries.

Internal Respiration: This component involves the exchange of $O_2$ and $CO_2$ between the systemic capillaries and the specific cells of the body.

Indications for Mechanical Ventilation

Respiratory Failure (RF): Mechanical ventilation is indicated when a patient is in respiratory failure. Clinical thresholds for RF include:

PaO_2 < 60\,mmHg

PaCO_2 > 50\,mmHg (while breathing room air)

Types of Respiratory Failure:

Type 1: Hypoxic respiratory failure.

Type 2: Hypercapnic respiratory failure, also known as Ventilatory Failure.

Other Specific Indications:

Apnea.

Acute Ventilatory Failure.

Impending Ventilatory Failure.

Refractory Hypoxic Failure accompanied by an increased Work of Breathing (WOB).

Physical Mechanics of Breathing

Primary Muscles:

Diaphragm.

Intercostal muscles.

Accessory Muscles of Inspiration:

Scalene.

Sternocleidomastoids.

Pectoralis.

Trapezius.

Accessory Muscles of Expiration:

Rectus abdominus.

External oblique.

Internal oblique.

Transverse abdominal.

Serratus.

Latissimus dorsi.

Spontaneous Breathing Cycles

Inspiration:

The diaphragm, external intercostals, and accessory muscles contract.

The ribcage swings upward and outward while the sternum elevates, leading to an increase in the size of the thoracic cavity.

According to Boyle's Law ($\Delta V \propto \frac{1}{\Delta P}$), the change in volume causes intrapleural pressure ($P_{pl}$) to decrease and intraalveolar pressure ($P_A$) to drop below atmospheric pressure.

Air flows down the pressure gradient; pressure in the lungs rises until $P_A$ equals atmospheric pressure.

During a forced inspiration, pleural pressure can drop as low as $-50\,cmH_2O$.

Exhalation:

The diaphragm relaxes, and the lungs and thoracic wall recoil.

The volume of the thoracic cavity decreases.

$P_{pl}$ returns to normal, and $P_A$ becomes slightly greater than atmospheric pressure.

Air follows the gradient out of the lungs until pressures are equalized and airflow stops.

Quantifiable Variables in Ventilation

Primary Variables: Inspiration vs. Expiration times, pressures, volumes, and flows.

Graphical Comparisons (Scalars):

Time vs. Volume.

Time vs. Pressure.

Time vs. Flow.

Loops:

Pressure vs. Volume.

Pressure vs. Flow.

Respiratory Timing and I:E Ratios

Inspiratory Time ($T_i$): The duration of inspiration. Normal range is $0.8\,s \text{ to } 1.2\,s$. It can also be expressed as a percentage of the Total Cycle Time (TCT).

Expiratory Time ($T_e$): The time elapsed between inspirations.

Total Cycle Time (TCT): Includes inspiration plus active and passive exhalation.

Frequency ($f$): Also known as Respiratory Rate (RR). Normal range is $12\,bpm \text{ to } 20\,bpm$.

I:E Ratio: The ratio of Inspiratory Time to Expiratory Time.

Calculated based on the RR.

Normal ratio is considered $1:2 \text{ to } 1:3$.

Ideally, Inspiratory time should be less than Expiratory time (I < E).

Inverse Ratio: When I > E. This can lead to gas trapping or Auto/Intrinsic PEEP.

Example Calculation:

Given: $RR = 15$

$TCT = \frac{60\,sec}{15} = 4\,s$

Given: $I\text{-Time} = 0.75\,s$

$T_e = TCT - T_i = 4\,s - 0.75\,s = 3.25\,s$

$Ratio = 1:\frac{T_e}{T_i} = 1:\frac{3.25}{0.75} = 1:4.33$

Pressure Units and Equivalents

$1\,mmHg = 1.36\,cmH_2O$

$1\,kPa = 7.5\,mmHg$

$1\,torr = 1\,mmHg$

$1\,atm = 760\,mmHg = 1034\,cmH_2O$

Pressure Definitions and Lung Gradients

Airway Opening Pressure ($P_{awo}$): Also known as mouth pressure ($P_m$), airway pressure ($P_{aw}$), mask pressure, or proximal airway pressure. It is typically zero or atmospheric unless external pressure is applied.

Body Surface Pressure ($P_{bs}$): Pressure measured at the surface of the body.

Intrapleural Pressure ($P_{pl}$): Pressure in the space between the parietal and visceral pleura.

End of exhalation: $-5\,cmH_2O$.

End of inspiration: $-10\,cmH_2O$.

Estimated using Esophageal Pressure ($P_{ES}$) via a specialized balloon.

Alveolar Pressure ($P_A$ or $P_{alv}$): Also called lung pressure or intrapulmonary pressure.

During spontaneous inhalation: $-1\,cmH_2O$.

During spontaneous exhalation: $+1\,cmH_2O$.

Transairway Pressure ($P_{TA}$): The gradient between the airway opening and the alveolus.

$P_{TA} = P_{aw} - P_{alv}$

Represents the pressure required to overcome resistance to gas flow in the conductive airways.

Transthoracic Pressure ($P_W$ or $P_{TT}$): The gradient between the alveolar space and the body surface.

$P_{TT} = P_{alv} - P_{bs}$

Represents the pressure required to expand or contract the lungs and chest wall.

Transpulmonary Pressure ($P_L$ or $P_{TP}$): The difference between alveolar space and pleural space.

$P_{TP} = P_{alv} - P_{pl}$

Known as the alveolar distending pressure; it maintains alveolar inflation.

Under static conditions (such as an inspiratory hold), $P_{TP} = P_{AO} - P_{pl}$.

Transrespiratory Pressure ($P_{TR}$): The gradient between the airway opening and the body surface.

$P_{TR} = P_{awo} - P_{bs}$

Describes the pressure needed to inflate the lungs and airways during positive-pressure ventilation.

Clinical Pressure Measurements on a Ventilator

Peak Inspiratory Pressure (PIP): The highest pressure reached during inspiration. It is a measurement, not a setting. It reflects the pressure needed to inflate the lungs plus the pressure to overcome airway resistance ($P_{TA}$). Target is often to stay below $30\,cmH_2O$.

Plateau Pressure ($P_{plat}$): Measured using an inspiratory hold/pause ($0.5 \text{ to } 1.5\,s$) to stop airflow. This equilibrates mouth and alveolar pressures to find the true pressure felt at the alveolar level at the end of inhalation. It is inaccurate if the patient is breathing actively during measurement.

Baseline Pressure: The pressure remaining in the lungs at the end of exhalation.

Positive End Expiratory Pressure (PEEP): Baseline pressure above atmosphere applied to the Functional Residual Capacity (FRC). It prevents alveolar collapse and increases oxygen diffusion. Measured in $cmH_2O$.

Constant Positive Airway Pressure (CPAP): Similar to PEEP, but provides one single pressure set to prevent upper airway obstruction.

Mean Airway Pressure ($P_{aw}$): The average area under the pressure/time curve. It mediates venous return to the heart. High $P_{aw}$ values are responsible for harmful cardiovascular side effects of intermittent positive-pressure ventilation (IPPV).

$P_{aw} = 0.5 \times (PIP - PEEP) \times \frac{T_{i}}{TCT} + PEEP$

Driving Pressure

Definition: The difference between plateau pressure and PEEP ($P_{plat} - PEEP$).

Alternate Formula: Ratio of tidal volume to respiratory system compliance ($\frac{V_T}{C_{rs}}$).

Clinical Significance: Represents the stress applied to the respiratory system. It is a strong predictor of mortality in lung-protective ventilation strategies.

Ideal Range: Maintain below $15\,cmH_2O$.

Compliance and Resistance

Compliance ($C$): The ease with which a structure distends; describes elastic forces opposing lung inflation.

$C = \frac{\Delta V}{\Delta P}$

Normal Spontaneous: $0.05 \text{ to } 0.17\,L/cmH_2O$ ($50 \text{ to } 170\,mL/cmH_2O$).

Intubated Male: $40 \text{ to } 50\,mL/cmH_2O$ (up to 100).

Intubated Female: $35 \text{ to } 45\,mL/cmH_2O$ (up to 100).

Static Compliance ($C_s$): Measured during no-flow conditions.

$C_s = \frac{V_T}{P_{plat} - PEEP}$

Elastance ($e$): The tendency of a structure to return to its original form after stretching. It is the mathematical opposite of compliance.

Resistance ($R_{aw}$): The opposition to gas flow through the airways.

$R_{aw} = \frac{P_{TA}}{\text{flow}}$

Normal (Non-intubated): $0.6 \text{ to } 2.4\,cmH_2O/(L/s)$ at a flow of $0.5\,L/s$.

Normal (Intubated): $\approx 6\,cmH_2O/(L/s)$ or higher.

Asthma/Emphysema: Can range from $13 \text{ to } 18\,cmH_2O/(L/s)$.

Lung Volumes and Flow Patterns

Tidal Volume ($V_T$): The volume targeted for each individual breath.

Average: $6-8\,ml/kg$ of predicted body weight (PBW) for healthy lungs.

Lung Protective: $4-6\,ml/kg$ PBW.

Minute Volume ($V_E$): Total volume delivered per minute.

$V_E = V_T \times RR$

Inspiratory Flow: Can be a preset value to achieve a specific $V_T$ or can flow freely to achieve a set pressure. Different flow patterns (waveforms) can be selected.

Expiratory Flow: Usually passive and reflected in waveforms provided by the ventilator.

The equation of motion

The law of motion equation describes the relationships among pressure, volume, and flow during a spontaneous or mechanical breath.

It determines whether the ventilator will deliver breaths with a volume controller or a pressure controller.

The physical model used for the equation consists of a single flow-conducting tube (representing the airways) and a single elastic compartment (representing the lungs and chest wall).

The fundamental equation is expressed as: $P_{mus} + P_{vent} = P_E + P_R$

Components of the equation:

$P_{mus}$: Pressure generated by the respiratory muscles.

$P_{vent}$: Pressure generated by the ventilator.

$P_E$ (Elastic Pressure): Measured as $\text{Elastance} \times \text{Volume}$. Since Elastance is the inverse of Compliance ($E = \frac{1}{C}$), $P_E$ is calculated as $\frac{\text{Volume}}{\text{Compliance}}$ ($\frac{V}{C}$).

$V/C$ also represents Alveolar Pressure ($P_{alv}$).

$P_R$ (Resistive Pressure/Frictional Resistance): Calculated as $\text{Airway Resistance} \times \text{Flow}$ ($R_{AW} \times \dot{V}$).

Expanded Equation: $P_{mus} + P_{vent} = \left(\frac{V}{C}\right) + (R_{AW} \times \dot{V})$

Relevant Variables and Definitions:

Transairway Pressure ($P_{TA}$): Represented by the term $R_{AW} \times \dot{V}$.

Compliance ($C$): $C = \frac{\Delta V}{\Delta P_{TT}}$.

Transthoracic Pressure ($P_{TT}$): Calculated as $P_{alv} - P_{bs}$ (Pressure at body surface).

Elastance ($E$): $E = \frac{P_{TT}}{V}$.

Simplified Equation for Ventilators:

$P_{vent} = P_{TR}$ (Measured on the ventilator during inspiration).

$P_E = P_{alv}$.

$P_{TA} = R_{AW} \times \dot{V}$.

$P_{TA}$ can be measured on the ventilator using the difference between Peak Inspiratory Pressure and Plateau Pressure ($PIP - P_{plat}$).

Final simplified form: $P_{mus} + P_{TR} = P_{plat} + P_{TA}$.

Understanding Compliance

Compliance is considered as an average for the whole lung because it varies between regions due to lung position and disease.

Lung Compliance ($C_L$) is affected by:

Surfactant levels.

Fibrotic disease.

Tissue structure.

Edema.

Pneumonia.

Atelectasis.

Ascites (inflammation and swelling of tissues).

Chest Wall Compliance ($C_{CW}$) is affected by:

Kyphoscoliosis.

Musculature.

Pregnancy.

Obesity.

Gravity and posture.

Bandaging.

Ascites.

Dynamic Compliance ($C_D$): Measured while airflow is occurring; it does not distinguish between compliance and resistance problems.

Formula: $C_D = \frac{V_T}{PIP - PEEP}$

Static Compliance ($C_s$): Measured when there is no airflow; it helps distinguish between compliance and resistance problems.

Formula: $C_s = \frac{V_T}{P_{plat} - PEEP}$

Normal Compliance Thresholds:

Spontaneously breathing: $0.05\,L/cmH_2O$ to $0.17\,L/cmH_2O$ ($50\,mL/cmH_2O$ to $170\,mL/cmH_2O$).

Intubated Males: $40\,mL/cmH_2O$ to $50\,mL/cmH_2O$, reaching up to $100\,mL/cmH_2O$.

Intubated Females: $35\,mL/cmH_2O$ to $45\,mL/cmH_2O$, reaching up to $100\,mL/cmH_2O$.

Differentiating Compliance vs. Resistance:

The normal pressure difference between $PIP$ and $P_{plat}$ is $3\,cmH_2O$ to $5\,cmH_2O$.

If the difference is greater than $5\,cmH_2O$, it indicates a frictional resistance problem.

If both $PIP$ and $P_{plat}$ increase while the difference remains within $3-5\,cmH_2O$, the problem is lung compliance.

Respiratory Resistance

Total Resistance represent the flow-dependent part of the lung-thorax system and includes three subtypes:

Airway Resistance ($R_{AW}$): Frictional resistance to gas flow. Normal is $0.5-1.5\,cmH_2O/L/sec$. For intubated patients, it is approximately $6\,cmH_2O/L/sec$.

Tissue Viscous Resistance ($R_{VT}$): Resistance created when lung tissue, rib cage, diaphragm, and abdominal tissues are displaced.

Frictional (Non-Elastic) Resistance: Required to overcome flow resistance; affected by gas molecule viscosity, friction between gas and tube walls, and resistance through the tube.

Changes in Resistance can be caused by:

Distribution of $R_{AW}$ variation between the oropharynx and small airways.

Airway narrowing (both physiological and mechanical).

Higher set flow rates during mechanical ventilation.

Relationship with Inspiratory Time: Decreasing inspiratory time (implies faster flow) increases resistance; conversely, slower flow (longer inspiratory time) decreases resistance.

Mathematical relationship: $R_{AW} = \frac{\Delta P}{\dot{V}}$.

Clinical Implications of Compliance and Resistance

Distribution of inspired air.

Pressure gradients in Positive Pressure Ventilation (PPV): Monitoring the flow/time waveform is critical for identifying gas trapping. Appropriate I:E times are needed to allow full exhalation.

Work of Breathing (WOB) and Ventilator Pattern: Settings are adjusted to reduce WOB. As patients improve, settings are reduced to preserve muscle function and prevent deterioration, though clinicians must avoid reducing venous return excessively.

Energy cost of breathing.

Breath Delivery: Control and Phase Variables

Control Variables (Primary variable adjusted to achieve inspiration):

Pressure Controller: Ventilator maintains the pressure waveform; waveform is unaffected by lung characteristics.

Volume Controller: Ventilator maintains volume and flow regardless of lung characteristics.

Flow Controller: Flow and volume waveforms remain unchanged despite changes in lung mechanics.

Time Controller: Pressure, volume, and flow waveforms are all affected by lung characteristics; common in high-frequency jet ventilators and oscillators.

Note: Based on Sanborn (2005), a ventilator can control only one variable (Pressure, Volume, Flow, or Time) at a single time.

Phase Variables:

Trigger: The mechanism used to begin inspiration.

Limit: Sets the maximum value for pressure, volume, flow, or time during inspiration (does not end the breath).

Cycle: The variable that ends the inspiratory phase and begins exhalation.

Baseline: Establishes the baseline during expiration (e.g., PEEP).

Output Waveforms

Waveform shapes at the mouth include:

Rectangular (Square or constant): Common for pressure in Pressure Control or flow in Volume Control.

Exponential (Increasing/rising or decreasing/decaying).

Sinusoidal (Sine wave).

Ramp (Ascending or descending/decelerating ramp).

Trigger Variables

Machine (Time) Triggering:

Mandatory breath delivered after a set time elapses based on the respiratory rate.

Total Cycle Time ($TCT$) is calculated as: $TCT = T_I + T_E$.

Example: If $RR = 20$, $TCT = \frac{60}{20} = 3\,seconds$. The ventilator triggers every $3\,seconds$.

Patient Triggering (Adjusted via sensitivity):

Flow Triggering: Occurs when the ventilator detects a drop from the baseline bias flow. If baseline flow is $6\,L/min$ and trigger is set to $2\,L/min$, inspiration begins at a detected flow of $4\,L/min$. This generally requires less WOB.

Pressure Triggering: Occurs when the ventilator detects a drop in airway opening pressure. Normal setting is roughly $-1\,cmH_2O$. A trigger of $-1\,cmH_2O$ requires less effort than $-5\,cmH_2O$.

Volume triggering: Detected as a drop in circuit volume.

Neural triggering.

Improper Trigger Settings can lead to: Increased WOB, respiratory distress, patient-ventilator dyssynchrony, increased risk for Ventilator-Induced Lung Injury (VILI), discomfort, and slower weaning.

Limit and Cycle Variables

Limit Variable:

Maximum value a variable can hit without ending inspiration. If the limit is reached, inspiration continues at that value until the set inspiratory time ($T_I$) ends.

Example: Pressure-limited, time-cycled breath. If $P_{max} = 25\,cmH_2O$ and $T_I = 1\,s$, if the pressure is reached at $0.4\,s$, it stays at $25\,cmH_2O$ for the remaining $0.6\,s$.

Cycling Mechanisms:

Volume-Cycled: Ended when set volume is delivered. Pressures vary with lung changes. Choosing an inspiratory pause makes the breath volume-limited and time-cycled.

Time-Cycled: Ended when a set time has elapsed. Tidal volume is predictable if flow is constant.

Flow-Cycled: Ended when flow decreases to a specific percentage of peak inspiratory flow (e.g., 5% to 80%). Common in Pressure Support modes.

Pressure-Cycled: Ended when a set pressure threshold is reached. Reduces risk of excessive pressure but volume delivery becomes highly variable.

Maximum Safety Pressure: Usually set $10\,cmH_2O$ above the average PIP to prevent damage. High-pressure alarms often cycle the breath to exhalation early.

Expiratory Phase and Technical Considerations

Expiration begins when the expiratory valve opens. It is usually passive, relying on lung recoil.

Baseline Variable: Pressure is the most common baseline variable.

ZEEP: Zero End-Expiratory Pressure ($0\,cmH_2O$).

PEEP: Positive End-Expiratory Pressure.

Inspiratory Pause: Maintains air in lungs before exhalation. Allows calculation of $P_{plat}$ for static compliance and improves peripheral gas distribution.

Expiratory Hold: A maneuver where both valves are closed at the end of exhalation to measure Auto-PEEP (Intrinsic PEEP). Auto-PEEP is visible on flow waveforms if flow does not return to zero before the next breath.

Tubing Compressibility: Gas is compressed in the circuit based on Boyle's Law. Approximately $1$ to $3\,mL$ of gas is lost for every $1\,cmH_2O$ of measured airway pressure.

Exhaled Tidal Volume: The actual volume reaching the patient (Measured volume at exhalation valve minus compressed volume).

Calculation Examples

Resistance Calculation Example 1:

$P_{AW} = 30\,cmH_2O$, $P_{alv} = 20\,cmH_2O$, $\dot{V} = 60\,L/min$

$\dot{V} = \frac{60\,L}{60\,s} = 1\,L/s$

$R_{AW} = \frac{30\,cmH_2O - 20\,cmH_2O}{1\,L/s} = 10\,cmH_2O/L/s$

Resistance Calculation Example 2:

$PIP = 25\,cmH_2O$, $P_{plat} = 15\,cmH_2O$, $\dot{V} = 2\,L/s$

$R_{AW} = \frac{25\,cmH_2O - 15\,cmH_2O}{2\,L/s} = 5\,cmH_2O/L/s$

Static Compliance Example:

$PEEP = 5\,cmH_2O$, $P_{plat} = 20\,cmH_2O$, $V_T = 500\,mL$

$C_s = \frac{0.5\,L}{20\,cmH_2O - 5\,cmH_2O} = 0.033\,L/cmH_2O$

Dynamic Compliance Example:

$PEEP = 5\,cmH_2O$, $PIP = 25\,cmH_2O$, $V_T = 500\,mL$

$C_D = \frac{0.5\,L}{25\,cmH_2O - 5\,cmH_2O} = 0.025\,L/cmH_2O$

Introduction to Mechanical Ventilation and Clinical Decision-Making

• In the acute care setting, clinicians traditionally relied heavily on arterial blood gas ($ABG$) analysis to identify respiratory failure and the necessity of ventilatory support.

• Modern clinical practice has evolved to include ventilatory measurements, such as respiratory muscle strength, to support the decision to initiate mechanical ventilation.

• Interventions must be beneficial and associated with effective clinical outcomes, including:

  • Improved quality of life.

  • Reduced length of hospital stay ($LOS$).

  • Lower mortality rates.

• Respiratory failure is defined as the point when the body can no longer achieve an appropriate level of ventilation to maintain adequate gas exchange and acid–base balance.

• The primary purpose of mechanical ventilation is to maintain homeostasis.

Objectives of Mechanical Ventilation

• Physiological Objectives:

  • Support or manipulate pulmonary gas exchange.

  • Increase lung volume.

  • Reduce the work of breathing ($WOB$).

  • Prevent or reverse atelectasis and maintain functional residual capacity ($FRC$).

  • Reduce systemic or myocardial oxygen consumption.

• Clinical Objectives:

  • Reverse acute respiratory failure ($ARF$).

  • Reverse respiratory distress.

  • Reverse hypoxemia.

  • Reverse respiratory muscle fatigue.

  • Permit sedation or paralysis (or both) for clinical procedures.

  • Minimize associated complications and reduce mortality.

Recognizing Respiratory Distress and Clinical Failure

• Assessment Steps:

  • Determine the patient’s level of consciousness ($LOC$).

  • Assess the patient’s work of breathing ($WOB$).

  • Assess the appearance and texture of the skin.

  • Evaluate vital signs, specifically:

  • Respiratory rate ($RR$).

  • Heart rate ($HR$).

  • Blood pressure ($BP$).

  • Body temperature.

  • Oxygenation status.

• Respiratory Failure Definition:

  • Respiratory activity is absent or insufficient to maintain adequate $O_2$ uptake and $CO_2$ clearance.

  • Acute Respiratory Failure ($ARF$) is the inability to maintain $PaO_2$, $PaCO_2$, and $pH$ at acceptable levels, characterized by:

  • $PaO_2$ below the predicted normal range for the patient’s age under ambient conditions.

  • $PaCO_2$ greater than $50\,mmHg$ and rising.

  • A falling $pH$ of $7.25$ or lower.

Types of Respiratory Failure

• Hypoxemic Respiratory Failure:

  • Characterized by acute life-threatening or vital organ–threatening tissue hypoxia.

  • Causes include:

  • Severe ventilation/perfusion ($V/Q$) mismatch.

  • Diffusion defects.

  • Right-to-left shunting.

  • Alveolar hypoventilation.

  • Aging.

  • Inadequate inspired oxygen levels.

• Hypercapnic Respiratory Failure:

  • Also known as acute ventilatory failure.

  • Occurs when adequate ventilation cannot be achieved to maintain a normal $PaCO_2$.

  • Results from ventilatory pump failure involving the respiratory muscles, thoracic cage, and nerves controlled by the brainstem respiratory centers.

  • Three main categories of hypercapnic $RF$:

  • Central nervous system ($CNS$) disorders.

  • Neuromuscular disorders.

  • Disorders that increase $WOB$.

Symptomatology of Hypoxemia and Hypercapnia

• Recognizing Hypoxemia:

  • Mild to Moderate: Tachypnea, dyspnea, paleness, tachycardia, mild hypertension ($HTN$), restlessness, disorientation, headaches, and lethargy.

  • Severe: Tachypnea, dyspnea, cyanosis, eventual bradycardia and hypotension, somnolence, confusion, blurred vision, loss of consciousness, and coma.

• Recognizing Hypercapnia:

  • Mild to Moderate: Tachypnea, dyspnea, tachycardia, $HTN$, vasodilation, headaches, drowsiness, dizziness, confusion, and sweating/skin redness.

  • Severe: Tachypnea (eventually leading to bradypnea), eventual hypotension, hallucinations, hypomania, convulsions, loss of consciousness, and eventual coma.

Specific Disorders Leading to Respiratory Failure

• CNS - Reduced Drive to Breathe:

  • Depressant drugs: Barbiturates, tranquilizers, narcotics, and general anesthetic agents.

  • Brain or brainstem lesions: Stroke, head/neck trauma, cerebral hemorrhage, tumors, and spinal cord injury.

  • Hypothyroidism.

  • Sleep apnea syndrome caused by idiopathic central alveolar hypoventilation.

• CNS - Increased Drive to Breathe:

  • Increased metabolic rate (causing increased $CO_2$ production).

  • Metabolic acidosis.

  • Anxiety associated with dyspnea.

• Neuromuscular Disorders:

  • Paralytic disorders.

  • Paralytic drugs.

  • Drugs affecting neuromuscular transmission.

  • Impaired muscle function.

• Disorders that Increase Work of Breathing:

  • Pleura-occupying lesions.

  • Chest wall deformities.

  • Increased airway resistance due to secretions, mucosal edema, bronchoconstriction, inflammation, or foreign body aspiration.

  • Lung tissue involvement.

  • Pulmonary vascular problems.

  • Postoperative pulmonary complications.

  • Dynamic hyperinflation.

Physiological Measurements in Acute Respiratory Failure

• Maximum Inspiratory Pressure ($MIP$):

  • Also called Negative Inspiratory Force ($NIF$).

  • The lowest (most negative) pressure generated during forceful inhalation against an occluded airway.

  • Measured at bedside using a pressure manometer via mouthpiece or $ETT$ adapter.

  • Normal Range: $-100\,cmH_2O$ to $-50\,cmH_2O$.

  • Critical Value: $-20\,cmH_2O$ to $0\,cmH_2O$.

• Vital Capacity ($VC$):

  • Volume exhaled maximally after maximum inspiration.

  • Essential for assessing the ability to produce a strong cough for airway clearance.

  • Normal Range: $65$ to $75\,cm^3\,kg^{-1}$ of Ideal Body Weight ($IBW$); can reach $100\,cm^3\,kg^{-1}$.

  • Critical Value: <10 to $15\,cm^3\,kg^{-1}$.

• Peak Expiratory Flow ($PEF$):

  • Indicator of airway resistance and patency.

  • Normal Range: $350$ to $600\,dm^3\,min^{-1}$.

• Forced Expiratory Volume in 1 Second ($FEV_1$):

  • Measure of airway resistance.

  • Normal: Roughly $80\%$ of $VC$, or $50$ to $60\,cm^3\,kg^{-1}\;IBW$.

  • Critical Value: <10\,cm^3\,kg^{-1}\;IBW.

• Respiratory Rate ($f$):

  • Normal: $12$ to $20\,breaths/min$.

  • Critical Value: >35\,breaths/min for extended periods.

• Minute Ventilation ($\dot{V}_E$):

  • Formula: $\dot{V}_E = RR \times V_T$.

  • Normal: $5$ to $6\,dm^3\,min^{-1}$.

  • Critical Value: >10\,dm^3\,min^{-1}.

• Tidal Volume ($V_T$):

  • Normal: $4$ to $8\,cm^3\,kg^{-1}$.

  • Critical Value: <4\,cm^3\,kg^{-1}.

Formulas for Ideal Body Weight ($IBW$)

• Weight in Pounds (Imperial):

  • Women: $IBW\,(lb) = 105 + 5(H - 60)$ (where $H$ is height in inches).

  • Men: $IBW\,(lb) = 106 + 6(H - 60)$ (where $H$ is height in inches).

  • Conversion to $kg$: $\text{lb} \div 2.2$.

• Weight in Pounds (Metric height input):

  • Women: $IBW\,(lb) = 45.5 + (0.91 \times [\text{height in cm} - 152.4])$.

  • Men: $IBW\,(lb) = 50 + (0.91 \times [\text{height in cm} - 152.4])$.

• Examples:

  • Height $70\,in$ (Woman): $105 + 50 = 155\,lbs$ ($70\,kg$).

  • Height $75\,in$ (Man): $106 + 90 = 196\,lbs$ ($89\,kg$).

Clinical Indicators for Initiating Mechanical Ventilation

• Failure of Ventilation:

  • Best indicator is $PaCO_2$.

  • Acute ventilatory failure defined as PaCO_2 > 50 to $55\,mmHg$ with a pH < 7.25.

  • Suggests increased dead space (ventilation without perfusion) relative to $V_T$.

• Failure of Oxygenation:

  • PaO_2 < 70\,mmHg or SpO_2 < 90\% on FiO_2 > 0.6 indicates refractory hypoxemia.

  • If accompanied by high $WOB$, rising $PaCO_2$, and low $pH$, mechanical ventilation is required.

• Standard Criteria:

  • Apnea or absence of breathing.

  • Acute ventilatory failure.

  • Impending ventilatory failure.

  • Refractory hypoxemic respiratory failure with increased $WOB$ or ineffective breathing patterns.

• Special Criteria for Airway Management:

  • Endotracheal tube ($ET$) size $\le7\,mm$ internal diameter ($ID$) with \dot{V}_E > 10\,dm^3\,min^{-1}.

  • $ET \le 8\,mm\;ID$ with \dot{V}_E > 15\,dm^3\,min^{-1}.

Invasive Ventilation in Chronic and Acute Disease

• COPD Exacerbation Indications:

  • Requires dyspnea, tachypnea, and acute respiratory acidosis plus one of the following:

  • Cardiovascular instability.

  • Altered mental status.

  • Inability to protect the airway.

  • Viscous/copious secretions.

  • Facial abnormalities preventing $NIV$.

• Neuromuscular Disease Indications:

  • Acute respiratory acidosis.

  • Progressive decline in $VC$ to <10 to $15\,cm^3\,kg^{-1}$.

  • Progressive decline in $MIP$ to below $-20$ to $-30\,cmH_2O$.

Non-Invasive Ventilation ($NIV$) and Alternatives

• Non-invasive Positive Pressure Ventilation ($NIV$):

  • Treatment of choice for acute-on-chronic respiratory failure (unless cardiovascular instability exists).

  • Benefits: Reduces intubation, complications, hospital stay, and mortality.

  • Indications (at least two present):

  • RR > 25\,breaths/min.

  • pH $7.25$ to $7.30$; $PaCO_2$ $45$ to $60\,mmHg$.

  • Moderate to severe dyspnea using accessory muscles.

• Heated High Flow Oxygen Therapy ($HFOT$):

  • Common in pediatrics; used with or without artificial airways.

• Absolute Contraindications for $NIV$:

  • Respiratory or cardiac arrest; cardiovascular instability ($MI$, hypotension).

  • Nonrespiratory organ failure (encephalopathy, $GI$ bleed).

  • Tracheoesophageal fistula.

  • High aspiration risk or inability to protect the airway.

  • Head/facial surgery or trauma.

• Relative Contraindications for $NIV$:

  • Copious/viscous secretions; fixed nasopharyngeal abnormalities; extreme obesity.

• Indications to Switch from $NIV$ to Invasive Ventilation:

  • RR > 35\,breaths/min.

  • Severe acidosis (pH < 7.25) and and hypercapnia (PaCO_2 > 60\,mmHg).

  • Life-threatening hypoxemia: PaO_2 < 40\,mmHg or PaO_2/FiO_2 < 200.

Framework for Selecting Ventilatory Support

Appropriate selection of a ventilator is guided by several critical questions regarding the patient's status and the environment of care:

Indication: Why does the patient require ventilatory support?

Pathology: Does the specific ventilatory problem necessitate a specialized mechanical ventilation mode?

Treatment Goals: What therapeutic outcomes are intended through the use of a specific ventilator?

Patient Interface: Does the patient require intubation (invasive), or can a mask be utilized (noninvasive)?

Location: Where will the support be provided (e.g., Intensive Care Unit (ICU), the patient’s home, or an extended care facility)?

Duration: Is the support expected to be brief or long-term?

Staff Training: How familiar are the clinicians and staff with the ventilators being considered?

Essential Decisions in Ventilatory Management

Clinicians must navigate a series of decisions when managing a ventilated patient:

Confirming if indications for support are present.

Choosing between noninvasive and invasive ventilation.

Determining the type and method used to establish an airway.

Selecting between pressure-controlled or volume-controlled ventilation.

Deciding on partial versus full ventilatory support.

Determining the specific mode of ventilation.

Selecting appropriate ventilator settings for the chosen mode.

Setting backup values and appropriate alarm limits.

Keys to Lung Protection

Adhering to lung-protective strategies is essential to prevent Ventilator-Induced Lung Injury (VILI). Key parameters include:

Transpulmonary Pressure: Should be less than $27\,cm\,H_2O$. This typically corresponds to a plateau pressure ($P_{plat}$) of less than $28\,cm\,H_2O$, provided that chest wall compliance is normal.

Plateau Pressure ($P_{plat}$): Maintain at less than $28\,cm\,H_2O$.

Driving Pressure ($\Delta P$): Keep at less than $15\,cm\,H_2O$. Driving pressure is the difference between plateau pressure and PEEP ($P_{plat} - PEEP$).

Tidal Volume ($V_T$): Target a range of $4\,ml/kg$ to $8\,ml/kg$ of Ideal Body Weight (IBW).

Positive End-Expiratory Pressure (PEEP): Set at a level sufficient to avoid alveolar derecruitment during the exhalation phase.

Noninvasive Positive Pressure Ventilation (NIV)

NIV is widespread; more than 75% to 80% of all patients receiving ventilatory assistance do so noninvasively.

Patient Interfaces: Nasal masks and face masks are commonly used.

Methods of Administration:

Continuous Positive Airway Pressure (CPAP): A continuous level of pressure is administered throughout the entire respiratory cycle.

Noninvasive Positive Pressure Ventilation (NIV/Bilevel): Utilizes two different pressure levels to facilitate the movement of air.

Device Types:

Pressure-triggered, pressure-limited, flow-cycled devices: An example is the Philips Respironics BiPAP.

Critical Care Ventilators: Most modern ICU ventilators include integrated NIV modes.

Disorders Frequently Managed with NIV

Chronic Conditions: Chronic respiratory failure, chest wall deformities, neuromuscular disorders, central alveolar hyperventilation, Chronic Obstructive Pulmonary Disease (COPD), Cystic Fibrosis (CF), bronchiectasis, and Obstructive Sleep Apnea (OSA).

Acute Conditions: Acute Respiratory Failure (ARF), Acute Respiratory Distress Syndrome (ARDS), pneumonia, asthma, cardiogenic pulmonary edema, and heart failure.

Other: Post-operative complications and post-extubation failure or complications.

Advantages and Disadvantages of NIV

Advantages: Avoids the complications associated with artificial airways; provides flexibility for initiation or removal; reduces the need for heavy sedation; preserves natural airway defense mechanisms, including the ability to speak and swallow.

Disadvantages: Gastric distension, skin irritation or pressure sores, dryness of the nose/eyes, claustrophobia, patient discomfort, and poor sleep quality due to air leaks.

Full vs. Partial Ventilatory Support

Full Ventilatory Support (FVS)

In FVS, the ventilator provides all the energy required to maintain effective alveolar ventilation.

Respiratory Rate: Typically set at rates greater than $8\,breaths/min$.

Settings: Provides an adequate tidal volume ($V_T$) for the patient.

Patient Interaction: The patient may still trigger breaths but receives a preset volume or pressure. Patient-ventilator synchrony can be more challenging than in partial support.

Common Modes: PC-CMV and VC-CMV.

Partial Ventilatory Support (PVS)

In PVS, the ventilator performs only a portion of the work of breathing (WOB).

Respiratory Rate: Machines are set at rates less than $6\,breaths/min$, though the patient participates in the WOB.

Benefits: Minimizes the loss of respiratory muscle function due to atrophy; requires less sedation; assists in the recruitment and stabilization of alveolar units.

Common Modes: Pressure Support (PS), SIMV, Proportional Assist Ventilation (PAV), Neurally Adjusted Ventilatory Assist (NAVA), and Volume Support (VS).

Characteristics of Breath Delivery

Breath delivery is defined by the type of breath, targeted control variables, and timing.

Type of Breath

Mandatory: The ventilator controls the timing, tidal volume, or inspiratory pressure. Inspiration is machine-triggered, limited, and/or machine-cycled.

Assisted: Shares characteristics of both mandatory and spontaneous breaths. All or part of the breath is generated by the ventilator. The patient triggers the breath, allowing breathing above the set rate, but this can increase WOB or dyssynchrony if settings are incorrect.

Spontaneous: The patient controls the timing and the tidal volume based on their demand and lung characteristics.

Timing of Breath Delivery

Continuous Mandatory Ventilation (CMV): Every breath is mandatory and can be either time-triggered (controlled) or patient-triggered (assisted).

Intermittent Mandatory Ventilation (IMV): The patient receives a set number of mandatory breaths but can breathe spontaneously between them.

Continuous Spontaneous Ventilation (CSV): All breaths are spontaneous and patient-triggered.

Choice of Control Variables: Volume vs. Pressure vs. Dual

Volume Control (VC)

In Volume Control, the volume remains constant while pressure fluctuates based on the patient's lung characteristics.

Advantages: Guarantees specific volume delivery and expired gas volume; maintains a stable $PaCO_2$.

Disadvantages: Volume is delivered regardless of changes in lung compliance or resistance, which can lead to high peak pressures and VILI. Risk of asynchrony if flow and sensitivity are not optimized.

Factors Affecting Pressures in VC:

Lung Characteristics: Reductions in lung or chest wall compliance increase peak and plateau pressures. Increased airway resistance ($R_{aw}$) increases peak pressures.

Inspiratory Flow Pattern: Constant flow produces higher peak pressures compared to decelerating flow (though decelerating flow generates higher mean airway pressure). Faster flow leads to higher pressures.

Volume Settings: Larger volumes produce higher peak and plateau pressures.

PEEP/Auto-PEEP: Increased PEEP or the presence of Auto-PEEP raises peak and mean airway pressures.

Pressure Control (PC)

In Pressure Control, maximum pressure is the set independent variable.

Advantages: Lung-protective strategy that reduces overdistension risk; often more comfortable for spontaneously breathing patients as it employs a decelerating flow pattern and reduces WOB.

Disadvantages: Tidal volume ($V_T$) varies as lung characteristics change. Alarms must be set carefully to monitor volume.

Factors Affecting Volume in PC:

Pressure Setting: Higher set pressures produce higher $V_T$.

Pressure Gradient ($\Delta P$): Increasing the difference between set pressure and PEEP increases peak pressures and $V_T$.

Lung Characteristics: Reduced compliance or increased resistance lowers $V_T$.

Inspiratory Time ($T_I$): Longer $T_I$ allows more time for volume delivery, increasing $V_T$.

Patient Effort: Active inspiratory efforts can increase the delivered $V_T$.

Dual Control

Dual control involves mandatory breaths with combined limiting/controlling factors (flow and pressure). The ventilator can switch between control modes during a single inspiration or on a breath-by-breath basis.

Mechanism: For example, a PC breath with a volume target. If the $V_T$ target is not met, the pressure is increased for the following breath. Alternatively, a breath may switch from pressure to flow control within the same inspiration calculations suggest the target $V_T$ will not be met.

Examples: VC+ on the PB980; PRVC on the Servo, PB840, and Evita XL.

Detailed Modes of Ventilation

Continuous Mandatory Ventilation (CMV)

Controlled Ventilation (Time-Triggered): Only appropriate when patients cannot make any effort to breathe (e.g., obtunded due to drugs, cerebral malfunction, spinal cord injury, or paralysis). Usually requires deep sedation or pharmacological paralysis.

Assisted Ventilation (Patient-Triggered): Operator sets minimum respiratory rate (RR), trigger sensitivity, and breath type. Complications (asynchrony, increased WOB) often arise from poorly optimized trigger sensitivity.

VC-CMV: Also called volume-targeted CMV. All breaths are mandatory. Though intended to minimize WOB, patients may perform 33% to 50% of the work if the flow rate does not meet demand ("air hunger"), indicated by an inflection on the peak of the pressure waveform.

PC-CMV: Also called pressure-controlled ventilation (PCV). All breaths are time- or patient-triggered, pressure-targeted, and time-cycled. Decelerating ramp flow improves gas distribution. The maximum pressure limit is typically set at $+10\,cm\,H_2O$ above the target pressure.

Intermittent Mandatory Ventilation (IMV) and Synchronized IMV (SIMV)

IMV: Periodic mandatory breaths (VC or PC) occur at set intervals. Patients can breathe spontaneously between them. Advantage: Used for weaning. Disadvantage: Spontaneous efforts may not stop when a mandatory breath is delivered, increasing WOB.

SIMV: Refined IMV. The ventilator waits for the patient's effort within a predetermined interval to synchronize the mandatory breath. Spontaneous breaths can be supported by Pressure Support (PS).

Breath Stacking: A problem in original IMV where a mandatory breath is delivered during a spontaneous inhalation, resulting in high volumes and potential barotrauma. SIMV was designed to eliminate this.

Continuous Spontaneous Ventilation (CSV)

Spontaneous Breathing: Breathing through a circuit without mandatory breaths (e.g., T-Piece).

CPAP: Continuous pressure to improve oxygenation in refractory hypoxemia and patients with low Functional Residual Capacity (FRC).

Pressure Support Ventilation (PSV/PC-CSV): Constant pressure provided once an inspiratory effort is sensed. Requires a consistent respiratory drive. Patient establishes rate, flow, and $T_I$. $V_T$ is determined by $\Delta P$ ($Set\,Pressure - PEEP$), lung characteristics, and effort. Flow-cycled based on a decrease in flow (descending ramp).

Additional Settings in PSV:

Rise Time: Time required to reach set pressure (also called slope, flow acceleration %, or inspiratory rise time).

Flow Cycle Criterion: Adjusts when to cycle to exhalation, ranging from 5% to 80% of peak flow. COPD patients need a shorter $T_I$ (higher flow cycle %), while ARDS patients need a longer $T_I$ (lower flow cycle %).

Advanced and Specialized Modes

Proportional Assist Ventilation (PAV): A positive feedback system where pressure, flow, and volume are delivered proportional to patient effort. It tracks changes in effort and lung mechanics but cannot compensate for system leaks or Auto-PEEP.

Spontaneous Breathing Trial (SBT): A diagnostic trial (15–30 minutes) where support is reduced to evaluate readiness for extubation/discontinuation.

Bilevel Positive Airway Pressure: Sets IPAP (Inspiratory) and EPAP (Expiratory). Different brands use names like BiPAP (Philips), BiLevel (PB 840), Bi-Vent (Maquet), Duo PAP (Hamilton), and BIPAP (Drger).

Pressure-Regulated Volume Control (PRVC): Patient- or time-triggered, volume-targeted, time-cycled. Ventilator adjusts pressure over several breaths to reach target $V_T$. Also known as VC+.

Pressure Augmentation (PAug): Also called Volume-Assured Pressure Support (VAPS). Provides pressure-limited ventilation with volume targeted for every individual breath.

Volume Support (VS-CSV): Pressure support with a volume target. Patient-triggered and flow-cycled.

Mandatory Minute Ventilation (MMV): Primarily for weaning. Operator sets a minimum minute ventilation (typically 70%-90% of current $V_E$). The ventilator makes up the difference if the patient fails to reach it.

Adaptive Support Ventilation (ASV): A variation of MMV based on Ideal Body Weight (IBW). Automatically selects $V_T$ and RR based on changes in mechanics.

Airway Pressure-Release Ventilation (APRV): Two CPAP levels (high and low). High CPAP is interrupted briefly to allow exhalation. Mode resembles inverse ratio ventilation (PC-IRV). Auto-PEEP is intentionally present because flow does not return to baseline.

Summary Comparison: Advantages and Risks

Mode

Advantages

Risks/Disadvantages

VC-CMV / PC-CMV

Guaranteed volume/pressure; sets minimum $V_E$; supports non-breathing patients.

Respiratory alkalosis; high mean airway pressures; muscle atrophy; asynchrony if flow/sensitivity is poorly set.

VC-SIMV / PC-SIMV

Lower mean airway pressure; maintains muscle strength; used for weaning.

Variable WOB; potentially increased weaning time; asynchrony during spontaneous efforts during mandatory breaths.

PSV (PC-CSV)

Patient-controlled; comfortable; useful for weaning.

Requires reliable respiratory drive; $V_T$ varies with compliance/resistance.

APRV

Recruits alveoli; allows spontaneous breathing at two levels.

Intentional Auto-PEEP; potentially high mean airway pressures.