Determining compliance and resistance within a respiratory system is important during the management of mechanical ventilation. Measurements that help us determine these include:
- -> tidal volume
- -> peak inspiratory flow rate
- -> peak airway pressure
- -> end-inspiratory plateau pressure
- -> end-expiratory pressure in circuit
- -> occult end-expiratory pressure measured during an end-expiratory pause maneuver.
- change in volume/change in pressure
- can be static (when there is no air flow) or dynamic (during breathing – where airflow resistance becomes a factor)
- normal dynamic compliance during mechanical ventilation – 50-100mL/cmH2O
- when paralysed and mechanically ventilated, peak airway pressure = the force required to overcome resistive and elastic recoil of the lung and chest wall
- to distinguish resistive from elastic recoil-related pressures requires an introduction of an end-inspiratory circuit occlusion after VT delivery.
- peak pressure will decrease down to a stable plateau pressure (3 second hold) -> this corresponds to the elastic recoil pressure
“Quasi-static” Compliance = VT/Pplat – PEEPtotal
- when patient spontaneously breathing -> compliance becomes uncertain
- can decrease the pause time to 1 second but is difficult to measure
- Flow = change in pressure/resistance
- Resistance = change in pressure/flow
Respiratory system resistance
= Paw – Pplat/Peak inspiratory flow rate
- peak inspiratory flow rate = preocclusion flow rate and is expressed in terms of cmH20/L/second
- resistance is expressed as cmH20/L/second
- respiratory resistance can only be accurately determined with a constant inspiratory flow (square wave) pattern -> usually set at 1L/second when measuring resistance -> this happens to be a standard setting for patient comfort.
NORMAL AND PATHOLOGICAL STATES
- in mechanically ventilated, normal patients: compliance = 50-100mL/cmH20, resistance = 1-8cmH20/L/s
- ARDS or cardiogenic pulmonary oedema: low compliance (around 40mL/cmH2O), and elevated resistance (15cmmH2O/L/s).
- COPD or asthma: high compliance (66mL/cmH2O) and higher resistance (25cmH2O/L/s).
- restrictive lung disease = decreased compliance -> small, rapid breaths -> decrease WOB
- obstructive lung disease = increased compliance -> large, slow breaths -> decreased WOB
Dr Eric Strong on Normal Lung Mechanics
Dr Eric Strong on Monitoring Lung Mechanics
References and Links
- Bersten AD. A simple bedside approach to measurement of respiratory mechanics in critically ill patients. Crit Care Resusc. 1999 Mar;1(1):74-84. PubMed PMID: 16599866. [Free Full Text]