Ventilator Basics

There are a variety of methods to ventilate patients in the OR once an Airway Devices|Airway Private or Broken Links
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is placed. As the name suggests, controlled modes of ventilation allow the anesthesiologist to set ventilator parameters which will be delivered to the patient. These modes are best if the patient is not interacting with the ventilator (paralyzed, deep anesthetic, strong analgesia). Supportive modes can augment patient's ventilatory efforts which can be useful in unparalyzed patients or those weaning from the ventilator. Spontaneous modes of ventilation require the patient to provide all of the ventilatory work (with the exception of some CPAP).

Terminology

• FiO2
  • The concentration of oxygen within the circuit
• Respiratory Rate (RR)
  • Number of breaths per minute
  • Set by patient or doctor
• Tidal Volume (Vt)
  • Volume of air per breath
  • Based on patient’s ideal body weight, generally targeted at 6-8 cc/kg
• Mean Airway Pressure (MAP)
  • Average pressure in airways during respiratory cycle
• Driving Pressure
  • Difference between plateau pressure and PEEP
  • Reflects pressure to overcome lung and chest wall resistance
• PEEP (Positive End-Expiratory Pressure)
  • Pressure maintained in lungs at the end of expiration
  • Prevents alveolar collapse
• CPAP (Continuous Positive Airway Pressure)
  • Constant positive pressure in airways
  • Keeps airways open
  • Improves oxygenation
• Compliance
  • The amount of tidal volume delivered for a given driving pressure
  • $Compliance = \frac{Tidal~Volume}{Driving~Pressure}$
• Plateau Pressure
  • Pressure applied by the ventilator at the end of inspiration
  • Reflects the pressure in the small airways and alveoli

Ventilator Modes

graph LR
A[Spontaneous Ventilation];
B[PSV + CPAP];
AM[Assist Control];
P[PCV-VG];
N[SIMV];
AN[PCVG & VC-AF];
M[PCV];
RM[VCV];

subgraph Unsupported
A
end
subgraph Supportive Modes of Ventilation
B
end
subgraph Combined Modes of Ventilation
AM & AN
N
end
subgraph Controlled Modes of Ventilation
AN
M
RM
P
end

Controlled Modes of Ventilation

controlled modes do not do well when a patient tries to draw a breath

Volume Control

• tidal volume, respiratory rate, PEEP, FiO2 are set
• ventilator calculates the inspiratory/expiratory time based on respiratory rate
• constant flow is delivered during a breath
• airway pressure will vary based on the patient's compliance

Volume Control - Autoflow ¹

• Appears in Draager machines. Autoflow is available in all ventilator modes but commonly appears in volume control on local machines.
• delivers a decelerating flow similar to pressure control ventilation
• tidal volume, respiratory rate, PEEP, FiO2 are set
• ventilator calculates the inspiratory/expiratory time based on respiratory rate
• airway pressure will still vary based on the patient's compliance

Pressure Control

• driving pressure, respiratory rate, PEEP, FiO2 are set
• ventilator delivers a breath until flow slows down (~20% peak inspiratory flow)
• constant pressure is delivered during a breath
• airway pressure will vary based on the patient's compliance

Pressure Control Volume Guarantee ²

tries to maintain the benefits of pressure control ventilation while allowing the doctor to choose the tidal volume and respiratory rate

• tidal volume, respiratory rate, PEEP, FiO2 are set
• breaths are given in a pressure controlled manner
  • the pressure of a breath is determined by the patient's compliance, and the ventilator will try to pressure that achieves the set tidal volume

• Assisted Modes of Ventilation

  Used when the patient can interact with the ventilator

Requires:

• ventilatory drive
• incomplete paralysis

Assist Control Volume Control

Each triggered breath receives a full breath

• tidal volume, respiratory rate, PEEP, FiO2 are set
• if the patient triggers a breath above the set respiratory rate, a full breath will be delivered
• ventilator calculates the inspiratory/expiratory time based on respiratory rate
• constant flow is delivered during a breath
• airway pressure will vary based on the patient's compliance

Assist Control Pressure Control

Each triggered breath receives a full breath

• driving pressure, respiratory rate, PEEP, FiO2 are set
• if the patient triggers a breath above the set respiratory rate, a full breath will be delivered
• ventilator delivers a breath until flow slows down (~20% peak inspiratory flow)
• constant flow is delivered during a breath
• airway pressure will vary based on the patient's compliance

SIMV modes

SIMV is a mode for weaning patients from the ventilator. Each time the patient triggers the ventilator, they receive a supported breath (vs a full breath in assist control). This can be useful because triggered breaths can be quite different than mandatory breaths.

Requires:

• ventilatory drive
• incomplete paralysis

Manual/Spontaneous Modes

• No inspiratory pressure is delivered
• CPAP can be delivered to prevent atelectasis