DIURNAL RHYTHM OF HEME TURNOVER ASSESSED BY BREATH CARBON-MONOXIDE CONCENTRATION MEASUREMENTS

Measurements of pulmonary carbon monoxide (CO) excretion can be used a s a quantitative indicator of heme turnover. We determined whether a d iurnal variation of heme turnover occurs by using a recently described technique that estimates breath CO excretion from measurements of alv eolar CO concentration corrected for environmental CO to yield an endo genous PCO. This simple technique, unlike the previously employed rebr eathing method, makes it possible to repeatedly measure CO excretion t hroughout a 24-hour period. Nine studies in seven healthy adult subjec ts demonstrated a diurnal rhythm of CO excretion with a peak excretion rate at about noon that was 26% greater than the nadir, which occurre d at about midnight. The rhythm of CO production underlying the observ ed breath CO excretion was calculated to have an amplitude of about tw ice that of CO excretion and a phase shift relative to excretion of ab out 4 hours. We conclude that a diurnal variation in the rate of heme turnover occurs, and when CO determinations are used to assess minor a lterations in heme turnover, consideration must be given to the time o f day at which the measurements are obtained.