Ventilator Breath Types and Sequences

Comparison of Breath Delivery Types: Volume Control vs. Pressure Control

• When managing a patient on a ventilator, the first decision is determining the breath type, while the second is determining the sequence of the breaths.
• Volume Control (VC) Ventilation
  • Definition: A mode where the ventilator delivers a consistent tidal volume ($V_T$) with every breath.
  • Advantages:
    • The clinician can precisely set the tidal volume ($V_T$).
    • This makes it very easy to predict and maintain the patient's minute ventilation ($\dot{V}_E$).
  • Disadvantages:
    • Fixed flow rate: The ventilator delivers a set inspiratory flow rate (e.g., $60\,L/min$) with every breath.
    • Non-physiological: Over a normal minute, humans naturally vary their inspiratory flow rates; a fixed flow can be uncomfortable for patients.
    • Pressure variability: Any changes in airway resistance ($R_{aw}$) or lung compliance ($C_L$) will cause the peak inspiratory pressure ($PIP$) to fluctuate, even though volume remains constant.
    • Oxygenation: Due to the shape of the pressure waveform, VC results in a lower mean airway pressure ($\bar{P}_{aw}$), which may make it harder to oxygenate certain patients.
• Pressure Control (PC) Ventilation
  • Definition: A mode where the ventilator maintains a consistent peak inspiratory pressure ($PIP$) with every breath.
  • Advantages:
    • Consistent peak pressure reduces the risk of barotrauma and pressure-related injuries.
    • Variable flow rate: If a patient wants $30\,L/min$ on one breath and $60\,L/min$ on the next, the ventilator allows for this, making the mode more comfortable and physiological.
    • Improved Oxygenation: PC provides a higher mean airway pressure ($\bar{P}_{aw}$), which is particularly beneficial for patients experiencing a shunt.
  • Disadvantages:
    • Volume variability: Changes in airway resistance ($R_{aw}$) or lung compliance ($C_L$) will directly affect the tidal volume ($V_T$) delivered to the patient.

Strategic Approach to Exam and Board Questions on Breath Types

• When faced with a question regarding a patient on Volume Control where resistance increases or compliance decreases, use the following strategy:
  • Rule of Constancy: If the patient is on Volume Control, identify the variable that cannot change. In this case, the tidal volume ($V_T$) is fixed.
  • Eliminate Choices: Remove any answer choices suggesting that $V_T$ will increase or decrease.
  • Analyze the Pressure: Focus on whether the peak inspiratory pressure ($PIP$) will go up or down.
• Conditions Leading to Increased $PIP$ in Volume Control:
  • High airway resistance ($R_{aw}$).
  • Low lung compliance ($C_L$) (the lungs do not want to expand, requiring more pressure).
• Conditions Leading to Decreased $PIP$ in Volume Control:
  • Decreased airway resistance ($R_{aw}$).
  • Increased lung compliance ($C_L$).

Levels of Ventilatory Support: Full vs. Partial

• Definitions:
  • Full Support: Defined by a set respiratory rate ($f$) of $8$ breaths per minute or higher. The goal is for the ventilator to perform most of the work of breathing ($WOB$).
  • Partial Support: Defined by a set rate of $6$ breaths per minute or less. The goal is for the patient to perform some or potentially all of the work of breathing ($WOB$).
• Clinical Application and Risks:
  • Patients are usually started on full support immediately following intubation, especially for hypercapnic respiratory failure, to allow exhausted respiratory muscles to heal.
  • Hazard - Muscle Atrophy: Muscles break down quickly when not used. Ultrasonic changes in the diaphragm can be observed within $12\,hours$ of initiating mechanical ventilation.
  • Transition to Weaning: It is preferable to start "wean trials" within $24$ to $36\,hours$. Allowing a patient to breathe on their own for even $30\,min$ can help rehabilitate respiratory muscles.
  • Prolonged Full Support: Patients who are hemodynamically unstable or difficult to oxygenate may stay on full support longer, but every day increases the risk of muscle weakness and difficult extubation.

Classification of Breath Types (Mandatory, Spontaneous, Assisted)

• Mandatory Breath: A breath delivered and controlled by the machine based on set parameters.
  • In Volume Control, mandatory parameters are tidal volume ($V_T$) and inspiratory flow rate.
  • In Pressure Control, mandatory parameters are inspiratory pressure ($P_I$) and inspiratory time ($T_I$).
• Spontaneous Breath: A breath that is both triggered and cycled by the patient.
  • Trigger: The start of the breath.
  • Cycle: The end of the breath.
• Assisted Breath: A breath that is either triggered or cycled by the patient, but receives some mechanical assistance.
  • Patient-Triggered Mandatory Breaths: In Assist Control, if a patient triggers a breath above the set rate, it is an assisted breath because the machine still controls the limit and the cycle.
  • Pressure Support ($PS$): A form of assisted spontaneous breathing. While the patient triggers and cycles the breath, the machine provides a pressure "boost" to increase tidal volume and decrease $WOB$.
• The Metaphor for Pressure Support ($PS$): Pressure support is like a friend giving you a "boost" through a window when you are locked out of your house. Without the boost, you have to pull yourself up by the windowsill, which causes muscle strain (work of breathing). The boost makes the process easier and allows you to reach further (higher $V_T$).
• CPAP Waveforms: A purely spontaneous breath on CPAP (without pressure support) shows negative pressure during inspiration and positive pressure during exhalation. Over time, breathing through a small endotracheal tube without assistance causes exhaustion.

Continuous Mandatory Ventilation (CMV) / Assist Control (AC)

• Definition: Every breath delivered is a mandatory breath based on the clinician's design. It does not allow for truly spontaneous breathing.
• Functionality:
  • The clinician sets a minimum respiratory rate (e.g., $12\,bpm$). These are controlled breaths.
  • If the patient's respiratory drive returns, they can trigger breaths above the set rate; these are assisted breaths.
• Clinical Example: Patients arriving in the ICU after cardiothoracic surgery. They are often paralyzed and anesthetized initially (breathing only at the set rate). As the paralytic wears off, their drive returns, and they begin triggering assisted breaths.
• Settings and Variables:
  • The patient has no control over the limit (volume or pressure) or the cycling mechanism.
  • Can be time-triggered (the machine) or patient-triggered (via flow or pressure triggers).
• Advantages:
  • Guaranteed volume or pressure for every breath.
  • Lowest work of breathing ($WOB$) for the patient; they only need to provide a minimal "sip" of air to trigger a full breath.
• Disadvantages:
  • Respiratory Alkalosis: If a patient triggers too many breaths, they may blow off too much $CO_2$. For example, if a patient wakes up and their rate jumps from $12$ to $18$, their pH may rise and $CO_2$ may drop significantly.
  • Muscle Atrophy: High risk if used for more than $36\,hours$.
  • Asynchrony: Patients may "fight" the ventilator because they cannot change the way the mandatory breath is delivered. This often requires higher levels of sedation.

Synchronized Intermittent Mandatory Ventilation (SIMV)

• Definition: A mode that delivers a mix of mandatory breaths and spontaneous breaths.
• Mechanism:
  • The clinician sets a mandatory rate (e.g., $12\,bpm$).
  • Any breaths the patient takes above that set rate are spontaneous (often assisted with pressure support) rather than mandatory.
• Breath Stacking and Synchronization: Older IMV modes risked delivering a machine breath on top of a spontaneous breath. Modern SIMV uses a "refractory period" (approx. $1$ to $2\,seconds$) after a spontaneous breath where a mandatory breath cannot be delivered, preventing discomfort.
• Settings in SIMV:
  • In VC-SIMV: Respiratory rate, $V_T$, flow, PEEP, $FiO_2$, and Pressure Support ($PS$).
  • Note: Pressure support is set directly on modern ventilators (no need for $IPAP - EPAP$ math).
• Historical Context - Weaning: Clinicians used to wean by dropping the IMV rate gradually ($12 \rightarrow 6 \rightarrow 4 \rightarrow 0$). This is now considered a waste of time. Current practice favors transitioning directly from full support to pressure support for a brief period before extubation.
• Disadvantages of SIMV:
  • High Asynchrony: Can confuse the patient's respiratory center. The brain's respiratory center reacts to pressure and stretch receptors; receiving inconsistent breath sizes (small spontaneous vs. large mandatory) can decrease respiratory drive.
  • Increased $WOB$: May actually increase work of breathing compared to AC.
  • Weaning Time: Can dramatically increase the time it takes to get a patient off the ventilator.
• Specific Clinical Use Case: SIMV is useful for patients with neurological injuries (e.g., Cheyne-Stokes-like irregular breathing) who trigger very high rates on Assist Control. Moving them to SIMV allows the mandatory rate to be capped, preventing severe respiratory alkalosis.

Summary Comparison: AC vs. SIMV

• Scenario: Paralyzed/Heavily Sedated Patient: There is no functional difference between AC and SIMV at the same set rate (e.g., $12\,bpm$ and $500\,mL$) because the patient is not triggering extra breaths.
• Scenario: Patient with Respiratory Drive:
  • In Assist Control, every breath above the set rate results in a full mandatory breath ($500\,mL$).
  • In SIMV, every breath above the set rate results in a spontaneous/assisted breath (size determined by patient effort and pressure support level).