Pulmonary mechanics
OVERVIEW
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.
COMPLIANCE
- 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
RESISTANCE
- 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
VIDEOS
Dr Eric Strong on Normal Lung Mechanics
Dr Eric Strong on Monitoring Lung Mechanics
References and Links
Journal articles
- 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]
Critical Care
Compendium
Chris is an Intensivist and ECMO specialist at the Alfred ICU in Melbourne. He is also a Clinical Adjunct Associate Professor at Monash University. He is a co-founder of the Australia and New Zealand Clinician Educator Network (ANZCEN) and is the Lead for the ANZCEN Clinician Educator Incubator programme. He is on the Board of Directors for the Intensive Care Foundation and is a First Part Examiner for the College of Intensive Care Medicine. He is an internationally recognised Clinician Educator with a passion for helping clinicians learn and for improving the clinical performance of individuals and collectives.
After finishing his medical degree at the University of Auckland, he continued post-graduate training in New Zealand as well as Australia’s Northern Territory, Perth and Melbourne. He has completed fellowship training in both intensive care medicine and emergency medicine, as well as post-graduate training in biochemistry, clinical toxicology, clinical epidemiology, and health professional education.
He is actively involved in in using translational simulation to improve patient care and the design of processes and systems at Alfred Health. He coordinates the Alfred ICU’s education and simulation programmes and runs the unit’s education website, INTENSIVE. He created the ‘Critically Ill Airway’ course and teaches on numerous courses around the world. He is one of the founders of the FOAM movement (Free Open-Access Medical education) and is co-creator of litfl.com, the RAGE podcast, the Resuscitology course, and the SMACC conference.
His one great achievement is being the father of three amazing children.
On Twitter, he is @precordialthump.
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