Mechanical Ventilation Lecture Review

Mechanical Ventilation Terminology

Mechanical ventilation involves four primary variables during positive pressure delivery: the trigger variable, the control or target variable, the limit variable, and the cycling variable.

The trigger variable is the parameter that initiates the inspiratory phase of a positive pressure breath. There are five main triggers: pressure-triggered, flow-triggered, volume-triggered, time-triggered, and manual-triggered. In pressure-triggering, inspiration starts when the patient causes airway pressure to drop below the established baseline. In flow-triggering, inspiration begins when the patient causes the inspiratory flow to drop below the baseline; this method is generally preferred over pressure-triggering as it may be more responsive to the patient's breathing pattern. Volume-triggering occurs when the ventilator detects a small change in volume in the patient circuit during exhalation. Time-triggering is initiated solely by the ventilator and is commonly used in neonatal or "baby" ventilators. Manual-triggering occurs when a therapist initiates a breath by pressing a button or icon on the user interface.

The control or target variable is the primary variable adjusted by the ventilator to achieve inspiration. There are two primary targets: volume control ($VC$), where the therapist sets a specific volume to be delivered, and pressure control ($PC$), where the therapist sets a specific pressure level.

The limit variable establishes a maximum value that a variable, such as pressure, volume, time, or flow, can reach during inspiration. Reaching this limit does not end inspiration but results in a plateau; the inspiratory phase continues until it is terminated by the target variable or a backup cycling variable.

The cycling variable is the parameter that ends or terminates the inspiratory phase. Volume cycling is the most common method, used during Volume Controlled ($VC$) ventilation. Pressure is applied until a preset volume is delivered, ensuring stable minute volume and blood gases, though airway pressure will fluctuate based on lung compliance and resistance. A significant disadvantage of volume cycling is that worsening resistance or compliance can increase peak inspiratory pressure ($PIP$) and plateau pressure ($P_{plateau}$), potentially leading to barotrauma or volutrauma. Pressure cycling applies positive pressure until a preset pressure value is reached. This is used during Intermittent Positive Pressure Breathing ($IPPB$) and as a safety alarm in $VC$ ventilation. While pressure remains constant, tidal volume ($V_T$) will vary. Time cycling ends inspiration after a preset time is reached, often used in infant ventilation and Pressure Control ventilation where $V_T$ is adjusted via $PIP$, inspiratory time, or flow. Flow cycling applies pressure until a predetermined flow is achieved and is the standard cycling mechanism for Pressure Support Ventilation ($PSV$).

Types of Breaths and Equipment

There are three distinct types of breaths based on control variables. A spontaneous breath is one where the patient controls all variables. A mandatory breath is one where the ventilator controls all variables. An assisted breath is initiated by the patient, after which the ventilator controls the remaining variables.

A typical ventilator circuit includes an inspiratory limb, an expiratory limb, a wye adaptor, a humidifier, and an optional nebulizer. Circuits should not be changed on a regular basis unless they are grossly contaminated or malfunctioning. During a circuit change, the patient must be off the ventilator for the shortest possible time, and manual ventilation with a resuscitation bag must be provided. Pre-oxygenation and post-oxygenation with $100\%\,O_2$ is necessary to prevent iatrogenic hypoxemia. When delivering medication inline with a metered-dose inhaler ($MDI$), the device should be located on the inspiratory side of the wye adapter, as close to the patient as possible.

Ventilator Alarms and Troubleshooting

Common ventilator alarms include:

• High pressure: Set $10\,cmH_2O$ above peak airway pressure.
• Low pressure: Set $10\,cmH_2O$ below peak airway pressure (indicates disconnection).
• Minimum exhaled volume: Set $100\,mL$ below exhaled tidal volume.
• Oxygen: Set $5\%$ above and below the set $F_IO_2$.
• $PEEP/CPAP$: High and low level alarms.
• Failure to cycle or loss of power: Check power supply.
• Oxygen failure: Check the oxygen source.
• Temperature: High and low limits.

In troubleshooting, manual ventilation must always be provided first. A low-pressure alarm may suggest patient disconnection, a leak in the circuit, insufficient flow, or a cuff leak in the endotracheal/tracheostomy tube. High-pressure alarms may be caused by patient obstruction (secretions, bronchospasm, pneumothorax) or equipment obstruction. A low exhaled volume alarm commonly follows an $MDI$ treatment if the ventilator tubing is not properly reconnected. A high temperature alarm can occur after an aerosol treatment because the aerosol flow cools the temperature probe, causing the humidifier to overcompensate; once the treatment ends, the air temperature rises sharply.

Quality Control and Accuracy Verification

The accuracy of ventilator variables should be verified periodically using specialized tools:

• Volume is verified by a spirometer.
• Pressure is verified by a mercury or water manometer.
• Flow is verified by a rotameter.

Clinical Phases and Indications for Mechanical Ventilation

Mechanical ventilation involves three phases. Phase 1 involves recommending or initiating ventilation and selecting initial settings. Phase 2 involves monitoring the patient and adjusting settings. Phase 3 involves assessing weaning readiness and implementing weaning procedures.

Absolute indications for continuous mechanical ventilation include apnea. Other indications include acute ventilatory failure (respiratory failure where spontaneous ventilation cannot maintain normal $P_aCO_2$ and $P_aO_2$), impending ventilatory failure (often seen in neuromuscular patients with rising $P_aCO_2$ and decreasing $V_T$, $VC$, or $MIP$), and the need to reduce the work of breathing ($WOB$). If $P_aCO_2$ is acceptable, oxygenation can be supported with oxygen therapy or $CPAP$; mechanical ventilation is initiated if these fail and $WOB$ increases.

Bedside Ventilatory Parameters and ABG Assessment

Arterial Blood Gas ($ABG$) thresholds for ventilation include a $pH$ level below $7.35$, a $P_aCO_2$ level greater than $45\,torr$, and a $P_aO_2$ level below $80\,torr$. Bedside parameters include:

• Vital Capacity ($VC$): Normal is $65-75\,mL/kg$ ($10 \times V_T$). Acceptable is $\ge 10\,mL/kg$. Unacceptable is $< 10\,mL/kg$.
• Maximum Inspiratory Pressure ($MIP$): Normal is $80\,cmH_2O$. Acceptable is $20\,cmH_2O$. Unacceptable is $< 20\,cmH_2O$.
• Respiratory Rate ($f$): Normal is $12-20/min$. Unacceptable is $> 20/min$ or $< 8/min$.
• Spontaneous Tidal Volume ($V_T$): Normal is $5-8\,mL/kg$. Unacceptable is $< 5\,mL/kg$.
• Minute Ventilation ($\dot{V}_E$): Normal is $5-6\,L/min$. Unacceptable is $> 10\,L/min$.
• Maximum Expiratory Pressure ($MEP$): Normal is $160\,cmH_2O$. Acceptable is $40\,cmH_2O$. Unacceptable is $< 40\,cmH_2O$ (indicates inability to cough).

Physiologic calculations include:

• Deadspace ($V_D/V_T$): Normal is $20-40\%$. Unacceptable is $> 60\%$.
• Static Compliance ($C_{st}$): Normal is $60-100\,mL/cmH_2O$. Unacceptable is $< 25\,mL/cmH_2O$.
• $A-aDO_2$ ($100\%\,O_2$): Normal is $25-65\,torr$. Unacceptable is $> 300\,torr$.
• Shunting ($Q_S/Q_T$): Normal is $\le 5\%$. Unacceptable is $> 20\%$.

Initial Settings for Adults and Infants

Initial adult settings:

• Tidal Volume ($V_T$): $5-10\,mL/kg$ of ideal body weight ($IBW$). For severe asthma, start at $4\,mL/kg$.
• Pressure ($PC$): Set to achieve target $V_T$ or $\le 35\,cmH_2O$.
• Respiratory Rate ($f$): $10-20\,breaths/min$.
• $F_IO_2$: $40-60\%$ (if no prior info) or set at the same level as prior oxygen therapy.
• $PEEP$: $2-6\,cmH_2O$ (if no prior info) or set at prior $CPAP$ level.

Ideal Body Weight ($IBW$) Formula: $IBW = 50\,kg + (2 \times \text{inches over 5 ft})$ For a man $5\,'10\,"$ tall: $50 + (2 \times 10) = 70\,kg$. For a woman $5\,'4\,"$ tall: $50 + (2 \times 4) = 58\,kg$.

Initial infant settings:

• Tidal Volume: $4-6\,mL/kg$.
• $PIP$: $20-30\,cmH_2O$.
• Respiratory Rate ($f$): $20-30\,breaths/min$.
• $F_IO_2$: $40-60\%$ (if no prior info) or prior levels. If $Apgar$ is $0-3$, bag with $100\%\,O_2$.
• $PEEP$: $2-4\,cmH_2O$ (maximum $8\,cmH_2O$).

Monitoring and Alveolar Ventilation

Monitoring includes vital signs, blood pressure, sensorium, breath sounds, and hemodynamic measurements. Exhaled tidal volume is often lower than set volume due to compressible gas volume in the circuit.

Alveolar minute ventilation ($V_A$) is calculated as: $V_A = (V_T - V_D) \times f$ Deadspace ($V_D$) is estimated at $1\,mL$ per pound of $IBW$. Mechanical deadspace is approximately $10\,mL$ per inch of flex tubing. Alveolar ventilation is most effectively increased by increasing tidal volume.

Deadspace types include anatomic (inspired gas not entering alveoli), alveolar (ventilation without perfusion, often in pulmonary embolism), and physiologic (the sum of anatomic and alveolar deadspace).

Lung Compliance and Airway Resistance

Dynamic compliance reflects the movement of gas, while static compliance reflects lung stiffness with no flow. Formulas:

• Dynamic Compliance = $\frac{\text{Exhaled Volume}}{PIP - PEEP}$
• Static Compliance ($C_{st}$) = $\frac{\text{Exhaled Volume}}{P_{plateau} - PEEP}$

Airway Resistance ($R_{aw}$) is the frictional force during breathing. Normal $R_{aw}$ is $0.6-2.4\,cmH_2O/L/sec$. It can be estimated using $(PIP - P_{plateau})$. If $PIP$ increases while $P_{plateau}$ remains constant, the cause is increasing $R_{aw}$ (e.g., secretions or bronchospasm), treated with suctioning or bronchodilators. If both $PIP$ and $P_{plateau}$ increase, the cause is decreasing lung compliance (e.g., atelectasis, ARDS, pneumonia), treated with $PEEP$ and treating the underlying cause.

Mean Airway Pressure ($P_{aw}$) is the average pressure transmitted to the airway from the start of one breath to the next. Factors affecting $P_{aw}$ include $PIP$, rate, inspiratory time, $PEEP$ (which has the most influence), peak flow, and tidal volume. Typical $P_{aw}$ values are $5-10\,cmH_2O$ for normal lungs, $10-20\,cmH_2O$ for obstructive disease, and $15-30\,cmH_2O$ for ARDS.

Ventilation Modes

Primary modes include Assist/Control ($A/C$) and Synchronous Intermittent Mandatory Ventilation ($SIMV$). In $A/C$, the ventilator delivers a minimum number of mandatory breaths, but the patient can trigger assisted breaths. In $SIMV$, patients can breathe spontaneously between mandatory breaths; it is used to avoid hyperventilation and reduce barotrauma.

Secondary modes include:

• Inverse Ratio Ventilation ($IRV$): Reverses the I:E ratio (starting at $2:1$ or greater) to improve oxygenation in ARDS or high-pressure situations ($PIP > 50\,cmH_2O$, $PEEP > 15\,cmH_2O$). Patients must be paralyzed and sedated.
• Airway Pressure Release Ventilation ($APRV$): Spontaneous breathing at positive pressure with periodic releases to a lower pressure.
• Pressure Regulated Volume Control ($PRVC$): A form of ventilation that keeps pressure at the lowest possible level while adjusting breath-to-breath to provide a preset volume.
• Proportional Assist Ventilation ($PAV$): Pressure, volume, and flow are proportional to patient effort.

High Frequency Oscillatory Ventilation (HFOV)

$HFOV$ improves oxygenation in severe lung injury (ARDS) and air leak syndromes like bronchopleural fistula or pneumothorax. It oscillates gas at frequencies of $3-15\,Hertz$ ($Hz$). For example, $5\,Hz$ equals $300\,breaths/min$ ($1\,Hz = 60\,bpm$). Amplitude (Power) is the primary control for $P_aCO_2$. Frequency in $Hz$ is the secondary control for $P_aCO_2$ (lower frequency yields higher tidal volume). Mean airway pressure ($P_{aw}$) controls $P_aO_2$.

Adjusting Settings for Blood Gas Correction

To normalize high $P_aCO_2$: remove mechanical deadspace, increase tidal volume/$PIP$, or increase respiratory rate. To normalize low $P_aCO_2$: evaluate causes like pain or fever, then decrease respiratory rate or tidal volume/$PIP$. To increase low $P_{a}O_2$: first increase $F_IO_2$ by $5-10\%$ (up to $60\%$), then increase $PEEP$ levels by $2-5\,cmH_2O$. To decrease high $P_{a}O_2$: first decrease $F_IO_2$ to below $0.60$, then decrease $PEEP$.

Specialized Protocols: ARDS, Recruitment, and Proning

In ARDSnet protocols, start tidal volume at $8\,mL/kg\,IBW$ and reduce to $6\,mL/kg$. Maintain plateau pressure below $30\,cmH_2O$. Switch from $VC$ to $PC$ if ventilating pressures are too high.

Recruitment maneuvers ($RM$) involve a sustained increase in pressure to open collapsed alveoli. Common methods include increasing $PEEP$ to $40\,cmH_2O$ for $40\,seconds$ or $CPAP$ to $20\,cmH_2O$ for $20\,seconds$. If oxygen saturation falls after an $RM$, it should be repeated.

Prone positioning (face down) can increase $P_aO_2$ by $10-50\,torr$ and decrease shunt by $12-25\%$. It is considered when $F_IO_2 > 60\%$ and $PEEP > 12\,cmH_2O$ or when $RM$ fails. Success occurs in about $75\%$ of ARDS patients, usually within $30\,minutes$.

Ventilator Graphics and Waveforms

Graphics help identify asynchrony and appropriate settings. Scalars plot flow, pressure, or volume against time. Loops plot two parameters against each other (e.g., pressure-volume). Normal flow patterns include sine waves (spontaneous), square waves ($VC$), and decelerating waves ($PC/PSV$).

Troubleshooting with graphics:

• Lower inflection point: Identifies best $PEEP$ level.
• Upper inflection point ("Beak"): Indicates overdistention of the lung; corrected by reducing $V_T$ or $PIP$.
• Broken loops: Indicates a leak in the system (expiratory volume less than inspiratory).
• Scalloped pattern: Indicates water/condensation in the circuit tubing; corrected by draining via water traps.
• Air Trapping (Auto-$PEEP$): Identified when expiratory flow does not return to baseline. Also called intrinsic or occult $PEEP$. Corrected by decreasing inspiratory time, increasing flowrate, or increasing expiratory time.

Patient-Ventilator Asynchrony and Pharmacology

Asynchrony occurs when the ventilator fails to meet the patient's flow or volume requirements. Missed triggers can be caused by Auto-$PEEP$ or weak effort. Auto-triggering (autocycling) may be caused by leaks, inappropriate sensitivity settings, or circuit condensation. Flow asynchrony (flow starvation) is visible as a dip in the pressure graph and is corrected by increasing flow or decreasing inspiratory time.

Pharmacological agents for synchronization include:

• Sedatives: Alprazolam ($Xanax$), Diazepam ($Valium$), Midazolam ($Versed$), Lorazepam ($Ativan$).
• Anesthetics: Propofol ($Diprivan$), Ketamine ($Ketalar$), Etomidate ($Amidate$).
• Analgesics (reverse with Narcan): Morphine, Codeine, Fentanyl, Hydromorphone ($Dilaudid$), Oxycodone ($OxyContin$).
• Neuromuscular blocking agents (paralytics): Pancuronium ($Pavulon$), Vecuronium ($Norcuron$), Rocuronium ($Zemuron$), Cisatracurium ($Nimbex$), Atracurium ($Tracrium$).