What causes the drop in pressure in the airways (Paw) as flow continues during mechanical ventilation?

Airway Pressure Drop During Continuous Flow in Mechanical Ventilation

Airway pressure (Paw) drops as flow continues during mechanical ventilation primarily due to the resistive pressure component dissipating once flow ceases or decreases, leaving only the elastic recoil pressure of the respiratory system. 1

Fundamental Mechanism: Pressure Components During Ventilation

During mechanical ventilation with constant inspiratory flow, the total airway pressure consists of three components:

• Resistive pressure: Generated by airflow through airways and the endotracheal tube, directly proportional to flow rate 1, 2
• Elastic pressure: Related to lung and chest wall compliance, proportional to volume 1
• PEEP: Baseline pressure maintained at end-expiration 3

The pressure drop occurs because the resistive component disappears when flow stops or decreases, while the elastic component remains constant at a given lung volume. 1

Physical Basis of Pressure Drop

Resistance-Related Pressure Loss

• The endotracheal tube creates significant flow-dependent pressure drop (ΔPETT), which is nonlinearly related to flow and inversely related to tube diameter 2
• In pediatric patients, the difference between Paw (measured at the ventilator) and actual tracheal pressure averages 2.3 cm H₂O due to ETT resistance alone 2
• Peak airway pressure (Ppk) is predominantly determined by inspiratory flow rate rather than lung volume, explaining why high Ppk does not necessarily indicate dangerous hyperinflation 4

Time Constant Effects

• When inspiratory flow becomes constant after initial transients decay, pressure-volume relationships become linear 1
• The immediate drop in airway pressure after occluding the airway during steady-state inspiration, divided by inspiratory flow, yields the total respiratory system resistance 1
• This pressure drop represents the resistive component that was maintaining flow against airway resistance 1

Clinical Implications for Mechanical Ventilation

Distinguishing Resistive from Elastic Pressure

• Total respiratory system compliance can be estimated from the slope of the pressure-volume relationship during constant flow, independent of breathing frequency 1
• The intercept of this relationship, divided by constant flow, provides total resistance 1
• Monitoring only peak airway pressure is misleading because it reflects flow resistance more than lung distension or hyperinflation risk 4

Hyperinflation and Auto-PEEP Context

• In patients with severe airflow obstruction, progressive hyperinflation occurs when expiratory time is insufficient, reaching lung volumes up to 3.6 L above FRC 4
• Alveolar pressure, central venous pressure, and esophageal pressure rise in parallel with lung volumes, while peak airway pressure primarily reflects inspiratory flow 4
• Auto-PEEP develops when expiratory flow limitation prevents complete exhalation, trapping gas and elevating end-expiratory pressure 5

Practical Monitoring Considerations

• End-inspiratory lung volume measured during apnea provides more accurate assessment of hyperinflation than peak airway pressure 4
• Tracheal pressure (Ptrach) should be calculated or estimated rather than relying solely on ventilator-measured Paw, especially in pediatric patients with small ETTs 2
• The pressure drop across the ETT is influenced by tube inner diameter, length, and manufacturer, with 25% shortening reducing pressure drop by approximately 14% 2

Hemodynamic Consequences

While not the primary mechanism of pressure drop, the cardiovascular effects are clinically significant:

• Mean airway pressure (mPaw) increases pleural pressure, reducing venous return gradient and potentially decreasing cardiac output 6, 3
• In ARDS, pleural pressure rises relatively little as mPaw increases, but pulmonary vascular resistance escalates progressively 6, 7
• Hypotension commonly occurs at highest lung volumes due to impaired venous return, independent of peak airway pressure 4