ACVC vent mode

What AC/VC Mode Is

  • AC/VC: A mode of mechanical ventilation that delivers a set tidal volume (VT) with every breath, regardless of whether the breath is initiated by the ventilator (machine-triggered) or the patient (patient-triggered).
    • AC = Assist-Control:
    • The ventilator provides full support for every breath taken by the patient.
    • VC = Volume Control:
    • The ventilator guarantees the delivery of a specific tidal volume with every breath, ensuring that each breath has the same volume, regardless of how it is triggered.
    • If a patient tries to breathe on their own, the ventilator will still deliver an additional full breath at the preset tidal volume.

How AC/VC Works

  • Triggering mechanism:

    • Time-triggered: The ventilator delivers breaths at a defined respiratory rate (RR) if the patient fails to take a breath within a certain timeframe.
    • Patient-triggered: When a patient initiates a breath, the ventilator responds by delivering the full set tidal volume for that breath.

  • Limit:

    • Flow-limited: The ventilator regulates the flow pattern during inhalation, commonly maintained as a square wave or constant flow pattern.

  • Cycle:

    • Volume-cycled: The inspiratory phase concludes once the set tidal volume has been delivered into the patient's lungs.

What You Set in AC/VC

  • Important parameters to configure when using the AC/VC mode include:
    • Tidal Volume (VT):
    • Common practice suggests a tidal volume of 6-8 mL/kg of ideal body weight (IBW), with a focus on a lung-protective strategy recommending 6 mL/kg.
    • Respiratory Rate (RR): The frequency of breaths to be delivered by the ventilator.
    • FiO2: The fraction of inspired oxygen, which defines the oxygen concentration delivered to the patient.
    • PEEP: Positive end-expiratory pressure, used to maintain alveolar recruitment and reduce atelectasis.
    • Flow Rate / Inspiratory Time (I-time): Additional parameters that manage how quickly and when the tidal volume is delivered.

Why AC/VC Is Used

  • The AC/VC mode is particularly beneficial in clinical scenarios where patients:

    • Are weak, sedated, or paralyzed; requiring maximal ventilatory support.
    • Are experiencing respiratory failure, where assistance is critical to maintaining adequate ventilation.

  • This mode is effective in ensuring stable minute ventilation, as it guarantees that the tidal volume is fixed and consistent across all breaths.

  • Common clinical situations where AC/VC is utilized:

    • ARDS (Acute Respiratory Distress Syndrome): Utilizes lung-protective tidal volumes to prevent further lung injury.
    • Severe COPD Exacerbations: Assists in managing significant airflow limitation.
    • Neuromuscular Weakness: Support for patients with reduced respiratory muscle function.
    • Post-Intubation Stabilization: Helps stabilize the patient after intubation.

RT Interpretation

  • Clinical Takeaway: AC/VC ensures a guaranteed volume (tidal volume) with every breath delivered to the patient regardless of the trigger.

  • Key Monitoring Considerations:

    • Watch for issues such as auto-triggering, whereby the ventilator delivers breaths even when the patient is not attempting to breathe.
    • Monitor for breath stacking, where multiple breaths may accumulate if the patient triggers more than the ventilator can handle.
    • Be cautious of respiratory alkalosis if the patient becomes tachypneic, as each breath delivered is a full tidal volume that could lead to excessive ventilation.