Does Continuous Positive Airway Pressure (CPAP) deliver a set tidal volume?

CPAP Does Not Deliver a Set Tidal Volume

CPAP does not deliver a set tidal volume but rather maintains a constant positive airway pressure throughout the respiratory cycle while the patient determines their own tidal volume through spontaneous breathing. 1

How CPAP Works

CPAP functions fundamentally differently from ventilation modes that deliver set tidal volumes:

• Pressure Maintenance: CPAP maintains a constant positive airway pressure throughout both inspiration and expiration 1
• Patient-Determined Breathing: With CPAP, the patient's respiratory effort determines:
  • Respiratory frequency
  • Timing of each breath
  • Tidal volume achieved 1
• No Ventilatory Support: Unlike other modes of non-invasive ventilation, CPAP does not actively deliver breaths or augment tidal volume 1

Physiological Effects of CPAP

CPAP works through several mechanisms:

• Permits higher inspired oxygen content than other oxygen supplementation methods
• Increases mean airway pressure
• Improves ventilation to collapsed areas of lung
• Recruits underventilated lung tissue (similar to PEEP in intubated patients)
• Unloads inspiratory muscles, reducing work of breathing
• Can offset intrinsic PEEP in COPD patients 1

Comparison to Ventilation Modes That Do Deliver Set Tidal Volumes

Unlike CPAP, these modes deliver specific tidal volumes:

• Controlled Mechanical Ventilation (CMV):

  • Full ventilatory support with no patient effort required
  • Either pressure or volume is set along with frequency and timing
  • In volume control, tidal volume is explicitly set 1

• Assist/Control Ventilation (ACV):

  • Delivers preset number of mandatory breaths
  • Patient triggering permitted but machine delivers identical breaths to mandatory ones 1

• Volume-Targeted BPAP (VT-BPAP):

  • Automatically adjusts IPAP-EPAP difference to deliver a target tidal volume 1

Technical Considerations with CPAP

• CPAP systems can be categorized as either constant pressure or constant flow 2
• Breath-dependent pressure fluctuations can occur in both constant-flow and variable-flow CPAP devices 3
• The work of breathing is minimized when there is minimal change in airway pressure at the mouth during respiration 4
• Interface selection impacts effectiveness - nasal masks may be more effective than full face masks for maintaining airway patency and achieving effective tidal volumes in certain scenarios 5

Common Pitfalls

• Confusing CPAP with bilevel positive airway pressure (BPAP) modes that provide ventilatory support
• Assuming CPAP will increase tidal volume directly (it may indirectly improve it by reducing work of breathing)
• Setting inappropriate pressure levels that may either be insufficient to maintain airway patency or excessive, causing discomfort and potentially increasing work of breathing
• Failing to recognize that in hyperinflated patients with airflow obstruction, CPAP may adversely affect inspiratory muscle function 1

In clinical practice, when a specific tidal volume delivery is required for a patient, a ventilation mode such as pressure support, assist-control, or volume-targeted BPAP would be more appropriate than CPAP alone.