The extent of respiratory compensation for a metabolic disorder is determined by:
- the balance between the abnormality in the pH (hence the drive to change)
- how hard it is to get there (eg. work of respiratory muscles in hyperventilation)
A knowledge of the expected degree of compensation for a given acid base status is necessary to determine the presence of an additional respiratory disorder.
Two traditional methods have been used:
- use of formulae
- use of a standardized diagram
The direction of change in the CO2 should be to normalise the pH for the underlying disorder. A normal pH indicates an additional process is present. The commonest relevant formulae to estimate the PaCO2 in use are:
- the expected PaCO2 = the last two digits of the pH (+/- 2-5 mmHg; from pH 7.1 to 7.6)
- the expected PaCO2 = (1.5 X measured bicarbonate) + 8 (+/- 2)
- other approaches: expected change in PaCO2 = Standard Base Excess or 1.2 mmHg fall in PaCO2 for each 1 mmol/L reduction in HCO3.
- the same equation is used, though the reliability may be less than with a metabolic acidosis. Expected PaCO2 = the last two digits of the pH (+/- 2-5; from pH 7.1 to 7.6).
- other approaches: change in PaCO2 = 0.6 X Standard Base Excess, 0.7 mmHg rise in PaCO2 for each 1 mmol/L increase in HCO3.
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.