Comprehensive Study Guide on Mechanical Ventilation: Breath Delivery, Modes, and Clinical Selection

Fundamentals of Breath Delivery and the Equation of Motion

• The Equation of Motion: Mechanical ventilation is fundamentally understood through the equation of motion. This equation describes the exact relationships among pressure, volume, and flow during either a spontaneous or a mechanical breath.

• Transrespiratory Pressure ($P_{TR}$): This represents the total energy (recorded as pressure) required to establish a pressure gradient to move gas into the lungs. It is the amount of pressure necessary to overcome the impedance of the respiratory system.

• Elastic Recoil Pressure ($P_E$): This is the pressure required to overcome the elastic load offered by the lungs and the chest wall. Factors influencing this include lung compliance and chest wall stiffness.

• Flow-Resistance Pressure ($P_R$): This is the airway resistance load produced as gas flows through the conducting airways. It represents the energy needed to push air through the tubes of the respiratory tract.

Factors Controlled and Measured During Inspiration

• Pressure-Controlled Breathing: In this mode, the ventilator maintains a specific pressure waveform pattern. The pressure waveform remains constant and is unaffected by changes in lung characteristics, such as compliance or resistance.

• Volume-Controlled Breathing: The ventilator maintains specific volume and flow waveforms. These waveforms remain unchanged regardless of alterations in the patient's lung characteristics.

• Flow-Controlled Breathing: In this approach, flow and volume waveforms remain constant, while the pressure waveform fluctuates based on changes in the patient's lung condition.

• Time-Controlled Breathing: Both the pressure, volume, and flow waveforms are affected by variations in lung characteristics. This type of control is typically seen in many high-frequency jet ventilators and oscillators.

Phases of a Breath and Phase Variables

• Trigger Variable: This variable is responsible for beginning the inspiratory phase.

• Limit Variable: This variable limits the maximum value of pressure, volume, flow, or time during the inspiratory phase but does not necessarily end the breath.

• Cycle Variable: This variable is responsible for the termination of the inspiratory phase and the beginning of expiration.

• Baseline Variable: This establishes the baseline pressure (typically during expiration) before the next inspiration is triggered.

Beginning of Inspiration: Triggering Mechanisms

• Time Triggering: The ventilator delivers a mandatory breath after a specific designated time has elapsed.

  • The timing is based on the Total Cycle Time (TCT).

  • $TCT = T_I + T_E$, where $T_I$ is Inspiratory Time and $T_E$ is Expiratory Time.

  • Time triggering is established using the rate (frequency) control settings.

• Patient Triggering: Inspiration is initiated by the patient's own effort. Setting this involves the clinician adjusting "sensitivity."

  • Pressure Triggering: The ventilator detects a drop in pressure below the baseline caused by the patient's inspiratory effort.

  • Flow Triggering: Occurs when the ventilator senses a drop in flow through the circuit during the exhalation phase.

  • Volume Triggering: Occurs when the ventilator interprets a specific drop in volume as a sign of patient effort.

Limiting Variables During Inspiration

• General Definition: The ventilator can determine the waveform for variables (pressure, volume, flow, or time) and can prevent them from exceeding a specific limit.

• Pressure Limiting: Pressure is allowed to rise to a set value but cannot exceed it.

  • If the limit is reached, excess pressure is usually vented via a spring-loaded pressure release or "pop-off" valve.

  • Infant ventilators often utilize pressure limiting for the inspiratory phase while using time to cycle the breath.

• Volume Limiting: A specific volume is set as a limit; reaching this limit does not always terminate the inspiration immediately.

• Flow Limiting: Flow is delivered up to a maximum value but does not exceed that value before the end of the inspiratory phase.

• Maximum Safety Pressure:

  • This is used to prevent excess pressure from damaging the lungs.

  • It is typically set to a value of $10\,cmH_2O$ above the average Peak Inspiratory Pressure (PIP).

  • In most adult ventilators, the breath ends (cycles) if this safety limit is reached.

Tubing Compressibility and Compliance Factor (TCF)

• The Concept of Compressible Volume: During inspiration, the ventilator circuit expands due to pressurization. This means the actual volume delivered to the patient is lower than the set Tidal Volume ($V_T$) because a portion of the gas (compressible volume) remains in the expanded circuit.

• Tubing Compliance Factor (TCF): Usually found on the circuit label, this factor identifies how much gas is lost to the circuit per unit of pressure.

• Formulas for Corection:

  • $\text{Compressible volume} = (PIP - PEEP) \times \text{tubing compliance factor}$

  • $\text{Corrected } V_T = \text{set } V_T - \text{compressible volume}$

• Significance: Calculation is critical when delivering low tidal volumes, such as the $4\,ml/kg$ to $6\,ml/kg$ range.

• Example Calculation:

  • Patient weight: $80\,kg$

  • Set $V_T = 480\,ml$

  • $TCF = 3.0\,ml/cmH_2O$

  • $PIP = 45\,cmH_2O$

  • $PEEP = 5\,cmH_2O$

  • $\text{Compressible volume} = (45 - 5) \times 3.0 = 120\,ml$

  • $\text{Corrected } V_T = 480 - 120 = 360\,ml$

Termination of Inspiratory Phase: Cycling Mechanisms

• Volume-Cycled Ventilation (VCV): The breath ends once the preset volume is delivered.

• Time-Cycled Ventilation: The ventilator applies positive pressure until a preset time limit is reached. In this case, $V_T = \text{flow (volume/time)} \times T_I$.

• Flow-Cycled Ventilation: The ventilator cycles into expiration once the flow decreases to a predetermined value.

  • This is the most common mechanism in Pressure Support (PSV) mode.

  • Flow termination occurs when flow reaches a specific percentage (ranging from $5\%$ to $80\%$) of the peak inspiratory flow, as selected by the clinician.

• Pressure-Cycled Ventilation: Inspiration ends when a set pressure threshold is reached at the mouth or upper airway.

  • Advantage: Limits peak airway pressures to reduce barotrauma.

  • Disadvantage: Delivers variable and often lower tidal volumes if lung compliance decreases or resistance increases.

Inflation Hold and Baseline Parameters

• Inflation Hold (Inspiratory Pause): This maneuver maintains gas in the lungs at the end of inspiration before the exhalation valve opens.

  • The pressure measured starts at the Peak Inspiratory Pressure (PIP) and levels off to the Plateau Pressure ($P_{plat}$).

• Baseline Pressure: The parameter controlled during the expiratory phase.

  • Zero End-Expiratory Pressure (ZEEP): Baseline pressure is atmospheric ($0\,cmH_2O$).

  • Positive End-Expiratory Pressure (PEEP): Baseline pressure remains above zero throughout exhalation.

Expiratory Phase Maneuvers and Methods

• Time-Limited Expiration: Clinicians control $T_I$ and $T_E$.

  • In Airway Pressure-Release Ventilation (APRV), $T_1$ controls high-pressure time and $T_2$ controls the release (low pressure) time.

• Continuous Gas Flow: Many ICU ventilators provide gas flow through the circuit during the late part of exhalation to minimize expiratory resistance.

• Expiratory Hold (End-Expiratory Pause): A transient maneuver at the end of exhalation used to measure pressure. Note: Accurate readings of end-expiratory pressure are impossible in spontaneously breathing patients.

• Expiratory Retard: Designed to mimic "pursed-lip breathing" by adding resistance to exhalation. This prevents early airway closure in patients with specific diseases. It is not commonly used in contemporary clinical practice.

Framework for Ventilator Selection (Chapter 5)

• Selection Criteria:

  • Indication: Why does the patient need support?

  • Pathology: Does the problem require a specific mode?

  • Treatment Goals: What therapeutic outcome is desired?

  • Patient Interface: Intubation vs. Mask.

  • Location: ICU, home, or extended care.

  • Duration: Brief vs. Long-term.

  • Staff Training: Familiarity with the available equipment.

Noninvasive Positive Pressure Ventilation (NIV)

• Interfaces: Face and nasal masks are standard.

• Methods:

  • Continuous Positive Airway Pressure (CPAP): Baseline pressure above zero for spontaneous breathers.

  • Noninvasive Positive Pressure Ventilation (NIV): Bi-level support.

• Device Types: Microprocessor-controlled critical care ventilators or specific pressure-triggered, pressure-limited, flow-cycled devices.

Full and Partial Ventilatory Support

• Full Ventilatory Support (FVS): The ventilator provides all the energy for effective alveolar ventilation. Typically involves rates greater than $8\,\text{breaths/min}$, adequate $V_T$, and preset volume or pressure.

• Partial Ventilatory Support (PVS): The patient participates in the Work of Breathing (WOB). Typically involves rates less than $6\,\text{breaths/min}$.

Breath Delivery Classifications

• Mandatory Breaths: Ventilator controls the timing, tidal volume, and/or inspiratory pressure.

• Spontaneous Breaths: Patient controls the timing and the tidal volume based on their own demand and lung characteristics.

• Assisted Breaths: Features both mandatory and spontaneous characteristics; the ventilator generates all or part of the breath in response to patient effort.

Comparative Targeting: Volume vs. Pressure

• Volume-Targeted Ventilation:

  • Advantages: Guarantees specific volume and minute ventilation; helps maintain stable $PaCO_2$.

  • Disadvantages: May result in high pressures if lung conditions worsen; risk of patient-ventilator asynchrony if flow/sensitivity are inadequate.

• Pressure-Targeted Ventilation:

  • Advantages: Allows set maximum pressure to reduce overdistention; decelerating flow pattern; often more comfortable for spontaneous breathers.

  • Disadvantages: Tidal volume changes as lung compliance or resistance changes; $V_T$ and $V_E$ decrease when conditions deteriorate.

Ventilation Modes: CMV, IMV, and CSV

• Continuous Mandatory Ventilation (CMV): All breaths are mandatory. Can be time-triggered (controlled) or patient-triggered (assisted). "Locking out" a patient from exercising effort is rarely recommended.

• 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.

Detailed Modes of Ventilation

• Volume-Controlled CMV (VC-CMV): All breaths are volume-targeted. While intended to minimize WOB, patients may still perform $33\%$ to $50\%$ or more of the inspiratory work.

• Pressure-Controlled CMV (PC-CMV): All breaths are pressure-targeted and time-cycled. Uses a decelerating ramp flow curve which can improve gas distribution.

• Pressure Support Ventilation (PSV): The ventilator provides constant pressure once patient effort is sensed. Essential for overcoming gas-flow resistance from the circuit and endotracheal tube (ETT). The clinician calculates the initial level using Airway Resistance (RAW), defined as $PIP - P_{plat}$.

• Mandatory Minute Ventilation (MMV): Primarily used for weaning. The ventilator ensures a minimum minute volume; if the patient's spontaneous effort falls short, the ventilator increases rate or pressure to compensate.

• Adaptive Support Ventilation (ASV): A variation of MMV where the ventilator automatically selects $V_T$ and rate based on patient weight and lung mechanics.

• Airway Pressure-Release Ventilation (APRV): Provides two levels of CPAP. The high-pressure level is intermittently released to allow brief drops in pressure. Auto-PEEP is intentionally present because flow does not return to baseline.

• Proportional Assist Ventilation (PAV): A positive feedback system where pressure, flow, and volume are delivered proportional to the patient's spontaneous effort and the degree of amplification set by the clinician.