the bicarbonate "rules of thumb", aka Winters' rules
The bicarbonate "rules of thumb", aka Winters' rules, in Australia also known as "Worthley's rules", after L.I.G. Worthley, are a set of
rules relating PCO2 and bicarbonate levels to each other. they are popular as part of the "bicarbonate approach" to acid-base
analysis popular in North America, as opposed to the "base excess approach" that dominates in Europe.
their origin can be traced to a study published in 1967:
Quantitative Displacement of Acid-Base Equilibrium in Metabolic Acidosis, by
Morris S. Albert, M.D., Ralph B. Dell, M.D., and Robert W. Winters, M.D.
Annals of Internal Medicine, Volume 66, No. 2, February 1967
from the Department of Pediatrics, Columbia Univeraity, College of Physicians and Surgeons, and the Babies Hospital.
Colurabia-Presbyterian Medical Center, New York, N. Y.
Theoretically speaking, there is no reason why these rules ought to belong to one or another of the different schools of acid-base
teaching!
Their purpose, after all, is to determine "the interrelationships among the degree of primary reduction of the metabolic
component, the compensatory reduction of the respiratory component, and the resultant reduction of whole blood pH.
The aim of the present study was to ascertain the pattern of such interrelationships in uncomplicated metabolic acidosis and to develop
a series of mathematical expressions that would describe them." (page 312)
In other words, the rules describe the normal physiological reactions of the human body to one isolated acid-base perturbance - the
classical example being the hyperventilatory compensation in an otherwise healthy patient with diabetic ketoacidosis.
The clinical question the rules are designed to answer in this situation is, whether the patient's respiratory compensation is within
the range to be expected or whether there is an additional component of respiratory disturbance, too, e.g. impending respiratory failure.
There is one more caveat:
"Patients with values for blood pH less than 7.10 were also excluded, based upon the observations of Kety, Polis, Nadler, and Schmidt
which demonstrated that the respiratory minute volume of patients with values for blood pH below 7.10 was less than that of patients
with blood pH values above this level." (page 313)
The quoted study lead to the formula:
PCO2 (in mmHg) = 1.54*[HCO3-] (in mmol/l) + 8.36, with a standard error: S.E.= ±1.11 mmHg
This has been simplified to the now widely known formula:
PCO2 (in mmHg) = 1.5*[HCO3-] (in mmol/l) + 8
These rules, thus, only make sense when applied to relatively straightforward situations - typically one derangement and its
corresponding compensation. The original rule, for instance, relating bicarbonate and PCO2 values in cases of primary metabolic acidosis
can only be applied to spontaneously breathing patients!
The formulae used in our scripts are based on the version propagated by Horacio J. Adrogué.
(Assessing acid-base disorders, Kidney International advance online publication, 7 October 2009; doi:10.1038/ki.2009.359)
(the prefix "delta" indicates that the variable represents the amount the value in question deviates from its normal value.)
1- metabolic acidosis as the primary disturbance, PCO2 (in mmHg) in relation to [HC03-]
delta_PaCO2/delta_[HCO3-]= 1.2mmHg per mEq/l
2- metabolic alkalosis as the primary disturbance, PCO2 (in mmHg) in relation to [HC03-]
delta_PaCO2/delta_[HCO3-]= 0.7mmHg per mEq/l
3- respiratory acidosis as the primary disturbance, [HC03-] in relation to PCO2 (in mmHg)
3a- acute compensation:
delta_[HCO3-]/delta_PaCO2= 0.1mEq/l per mmHg
3b- chronic compensation:
delta_[HCO3-]/delta_PaCO2= 0.3mEq/l per mmHg
4- respiratory alkalosis as the primary disturbance, [HC03-] in relation to PCO2 (in mmHg)
4a- acute compensation:
delta_[HCO3-]/delta_PaCO2= 0.2mEq/l per mmHg
4b- chronic compensation:
delta_[HCO3-]/delta_PaCO2= 0.4mEq/l per mmHg
