SIMV VC+ on the Puritan Bennett 980 Ventilator

SIMV VC+ (Synchronized Intermittent Mandatory Ventilation with Volume Control Plus) combines the principles of SIMV and VC+ to deliver effective and adaptive ventilation.

  • SIMV (Synchronized Intermittent Mandatory Ventilation):
    • Mandatory Breaths: The ventilator delivers a set number of breaths per minute with a predetermined tidal volume (VT).
    • Spontaneous Breaths: The patient can breathe spontaneously between mandatory breaths.
  • VC+ (Volume Control Plus): Adaptive Pressure:
    • The ventilator adjusts the inspiratory pressure for each breath to achieve the set tidal volume, adapting to changes in the patient’s lung mechanics (Compliance & Resistance)
  • PS (Pressure Support):
    • This is an adjunct mode used during spontaneous breath, where the ventilator provides a set level of pressure support to help the patient overcome breathing resistance throughout the ventilator circuit.
    • This reduces the work of breathing for the patient during spontaneous breaths.

How SIMV VC+ Works

  1. Preset Mandatory Breaths: The ventilator delivers a predetermined number of mandatory breaths each minute, each with a set tidal volume.
  2. Adaptive Inspiratory Pressure: VC+ adjusts the pressure of each mandatory breath to ensure the set tidal volume is achieved, accommodating changes in lung mechanics. (Compliance & Resistance)
  3. Spontaneous Breathing: Between mandatory breaths, the patient can breathe spontaneously. The ventilator assists these breaths based on the patient’s effort (pressure trigger-P-Trig ) or (flow trigger V-Trig).

Key Benefits

  • Guaranteed Ventilation: Ensures a consistent level of ventilation with mandatory breaths.
  • Spontaneous Breathing: Allows patients to breathe spontaneously between mandatory breaths, promoting patient comfort and reducing ventilator dependence.
  • Adaptive Pressure: VC+ adjusts inspiratory pressure to achieve the desired tidal volume, beneficial for patients with varying lung mechanics. (Compliance & Resistance)

Key Settings for SIMV VC+

  • Tidal Volume (VT): 6-8 ml/kg of ideal body weight.
  • Respiratory Rate (RR): 12-20 breaths per minute.
  • Pressure Support (PS): 5-10 cmH2O above PEEP for spontaneous breaths.
  • Positive End-Expiratory Pressure (PEEP): 5-10 cmH2O.
  • Fraction of Inspired Oxygen (FiO2): 0.21 to 1.0 (Adjust to maintain SpO2 > 94-98%.) (88-92% in COPD patients)
  • Inspiratory to Expiratory Ratio (I:E Ratio): Typically set at 1:2, but adjustable based on patient needs.
  • Rise Time: Adjust to control the speed of inspiratory pressure delivery, typically (50%-70%). (the speed at which inspiratory gas reaches a pressure target)
  • Trigger Sensitivity: Can be flow (Volume) or pressure-triggered; set to ensure the ventilator appropriately senses patient efforts (e.g., 2-3 L/min for flow trigger or -1 to -2 cmH2O for pressure trigger).

P-TRIG (Pressure Trigger)

  • Definition: P-Trig stands for Pressure Trigger.
  • How it works: The ventilator senses a drop in airway pressure caused by the patient’s inspiratory effort. When this pressure drop reaches a preset threshold, the ventilator delivers a breath.
  • Settings: The sensitivity of the pressure trigger can be adjusted, typically set between -1 to -2 cmH2O. The more sensitive the setting, the less effort the patient needs to trigger a breath.
  • Use: Suitable for patients who may have weak inspiratory efforts, such as those in the early stages of weaning from mechanical ventilation.

V-TRIG (Flow Trigger)

  • Definition: V-Trig stands for Volume (or Flow) Trigger.
  • How it works: The ventilator senses the initiation of a patient’s breath by detecting a change in the flow of air in the circuit. When the patient initiates an inspiratory effort, there is a small but detectable flow of air towards the patient. Once this flow reaches a preset threshold, the ventilator delivers a breath.
  • Settings: The flow trigger sensitivity can be adjusted, typically set between 2-3 L/min. A more sensitive setting means the ventilator will trigger a breath with a smaller patient effort.
  • Use: Often preferred in patients with more consistent spontaneous breathing efforts, as it can provide a more natural breathing experience.

Lung Mechanics

Lung mechanics refer to the physical properties and behaviors of the lungs and chest wall that influence the process of breathing.

1. Compliance

  • Definition: Compliance is a measure of the lung’s ability to stretch and expand. It is defined as the change in lung volume per unit change in pressure (ΔV/ΔP).
  • Clinical Importance:
    • High Compliance: Indicates the lungs can easily expand, as seen in conditions like emphysema.
    • Low Compliance: Indicates stiff lungs, requiring more pressure to achieve the same volume, as seen in conditions like acute respiratory distress syndrome (ARDS) or pulmonary fibrosis.

2. Resistance

  • Definition: Resistance refers to the opposition to airflow within the respiratory tract. It is influenced by the diameter of the airways and the flow rate of air.
  • Clinical Importance:
    • High Resistance: Occurs in conditions like asthma or chronic obstructive pulmonary disease (COPD), making it harder for air to flow through the airways.
    • Low Resistance: Easier airflow, generally found in healthy lungs with clear airways.

3. Elastic Recoil

  • Definition: Elastic recoil is the ability of the lungs and chest wall to return to their original shape after being stretched or distended.
  • Clinical Importance:
    • High Elastic Recoil: Ensures efficient expiration, as the lungs naturally deflate. Seen in conditions like pulmonary fibrosis.
    • Low Elastic Recoil: Makes it harder to expel air, as seen in conditions like emphysema.

4. Work of Breathing

  • Definition: The effort required to inhale and exhale.
  • Clinical Importance: Increased work of breathing is seen in many respiratory conditions and can lead to fatigue. Mechanical ventilation aims to reduce the work of breathing for patients.

References