Pressure Support Ventilation (PSV) Fundamentals

Fundamentals of Pressure Support Ventilation (PSV)

• Pressure Support Ventilation (PSV) is classified as a spontaneous breathing mode.
• A defining requirement for this mode is that the patient must initiate every breath; there are no mandatory breaths delivered by the ventilator.
• The mechanical characteristics of PSV breaths are explicitly defined as:
  • Patient-triggered.
  • Pressure-limited.
  • Flow-cycled.
• Inspiratory flow dynamics utilize a decelerating flow pattern. This specific waveform is often visually identified as a "shark fin" shape on ventilator monitors.

Ventilation Settings and Operator Controls

• When configuring PSV mode, the primary parameters set by the operator include:
  • $PS$ (Pressure Support): This is the pressure level set above $PEEP$.
  • $PEEP$ (Positive End-Expiratory Pressure).
  • $FiO_2$ (Fraction of Inspired Oxygen).
• Crucially, in PSV mode, the Respiratory Rate ($RR$) and the Inspiratory Time ($I-time$) are NOT set by the operator. These variables are determined by the patient's spontaneous breathing efforts and the flow-cycling criteria.

Management of Arterial Carbon Dioxide ($PaCO_2$) in PSV

• The adjustment of Pressure Support ($PS$) is the primary method for managing $PaCO_2$ levels. The logic for clinical adjustments is based on the correlation between pressure, volume, and ventilation.

• Addressing High $PaCO_2$ (Underventilation):

  • A high $PaCO_2$ level indicates that the patient is underventilating.
  • To correct this, the operator should increase the $PS$ dose.
  • The physiological progression is: $\uparrow PS \rightarrow \uparrow V_T \rightarrow \uparrow V_E \rightarrow \downarrow PaCO_2$.
  • Increasing Pressure Support leads to a higher Tidal Volume ($V_T$), which increases Minute Ventilation ($V_E$), thereby lowering the $PaCO_2$.

• Addressing Low $PaCO_2$ (Overventilation):

  • A low $PaCO_2$ level indicates that the patient is overventilating.
  • To correct this, the operator should decrease the $PS$ dose.
  • The physiological progression is: $\downarrow PS \rightarrow \downarrow V_T \rightarrow \downarrow V_E \rightarrow \uparrow PaCO_2$.
  • Decreasing Pressure Support leads to a lower Tidal Volume ($V_T$), which decreases Minute Ventilation ($V_E$), thereby raising the $PaCO_2$.

Clinical Indications and Patient Workload

• PSV is utilized for several key clinical purposes:

  • Supporting spontaneous breathing for patients who can initiate their own respiratory efforts.
  • Overcoming resistance: PSV helps overcome the work of breathing associated with artificial airways and ventilator circuit resistance.
  • Weaning and Liberation: It is used as an assessment tool to determine if a patient can be liberated from mechanical ventilation support.

• Patient Work of Breathing (WOB):

  • There is an inverse relationship between the dose of PS and the patient's effort.
  • Increasing the $PS$ dose usually results in an increased $V_T$ with less effort required from the patient.
  • Decreasing the $PS$ dose requires the patient to perform more work and take on a greater share of the respiratory load.

Comparative Analysis: Pressure Control (PC) vs. Pressure Support Ventilation (PSV)

• While both modes are pressure-targeted, they differ significantly in delivery and control:

• Pressure Targeted:

  • Pressure Control (PC): Yes.
  • Pressure Support (PSV): Yes.

• Tidal Volume ($V_T$) Guaranteed:

  • Pressure Control (PC): No.
  • Pressure Support (PSV): No.

• Mandatory Breaths:

  • Pressure Control (PC): Yes.
  • Pressure Support (PSV): No.

• Patient Triggering Capabilities:

  • Pressure Control (PC): Yes.
  • Pressure Support (PSV): Yes.

• Cycling Mechanism (Ending Inspiration):

  • Pressure Control (PC): Time-cycled (Inspiration ends when the set $I-time$ is reached).
  • Pressure Support (PSV): Flow-cycled (Inspiration ends based on a decay in inspiratory flow).

• Inspiratory Flow Pattern:

  • Pressure Control (PC): Decelerating (Shark fin).
  • Pressure Support (PSV): Decelerating (Shark fin).

• Set Respiratory Rate (RR):

  • Pressure Control (PC): Yes.
  • Pressure Support (PSV): No.

• Set Inspiratory Time ($I-time$):

  • Pressure Control (PC): Yes.
  • Pressure Support (PSV): No.

• Common Clinical Use:

  • Pressure Control (PC): Full or partial ventilatory support.
  • Pressure Support (PSV): Spontaneous support and weaning/liberation processes.

• Time Triggered:

  • Pressure Control (PC): Yes.
  • Pressure Support (PSV): No.