Masterclasses in medicine |
The patient with a severe degree of metabolic acidosis: a deductive analysis
From the 1Division of Nephrology, St. Michael's Hospital, University of Toronto, Toronto, Canada and 2Nephrology Unit and Department of Internal Medicine, Stellenbosch University, Cape Town, South Africa
Address correspondence to Professor M.L. Halperin, University of Toronto, St Michael's Hospital Annex, Lab #1, Research Wing, 38 Shuter Street, Toronto, Ontario, M5B 1A6, Canada. email: mitchell.halperin{at}utoronto.ca
This teaching exercise demonstrates how principles of physiology might help in identifying the cause of a particularly severe case of metabolic acidosis and making appropriate decisions about therapy. The patient's plasma pH was 7.00 and their plasma bicarbonate concentration was 2 mmol/l. Because the time course of the patient's illness was believed to be <24 h, this suggested that a large quantity of acid had been added to the body in this short time period, but the medical team managing the case could not identify any acid that could have been produced rapidly by endogenous processes, or was ingested by the patient. Moreover, there was a question about how such a very low arterial PCO2 (8 mmHg) could be sustained. Even once the diagnosis was made, there were issues to resolve concerning therapy. These included questions about how much sodium bicarbonate to administer, and what dangers might arise during this therapy. The missing links in this interesting story emerge during a discussion between the medical team and their imaginary mentor, Professor McCance.