Measurement of circulation time (Ct) from lung to periphery has been investigated as a surrogate for cardiac output. The subject takes a breath of nitrogen and the circulation time is the time elapsing until a desaturation pulse is seen peripherally. Pulse oximetry, using the ear, shows shortening with exercise (12.1 ± 0.37 s, mean ± S.E.M., at rest; 9.1 ± 0.25 s at 100 W, n = 20), lengthening after β-blockade (n = 6) and lengthening in patients with echocardiographic left ventricular systolic dysfunction and clinical left heart failure (8 patients, 16.2 ± 1.1 s; 6 controls, 12.0 ± 0.5 s). The use of pulse oximetry to measure lung-ear delay discriminates heart failure from normal in many, but not all, subjects. In patients referred to a department of nuclear medicine for diagnosis of chest pain, ear and finger oximetry showed unacceptable variability when compared with delays between first pass nuclide arrival at lung and periphery. The nuclide delays for lung to carotid artery correlated significantly with the reciprocals of gated SPECT estimates of cardiac output (Q) (not significant for lung to finger).

In normal subjects a fast response oximeter (non-pulse, Waters oximeter) gave much shorter and more reproducible circulation time estimates than those obtained with pulse oximetry (Ct at rest, mean 5.5 ± 0.21 s (± S.E.M.) and in exercise at 50 W mean 4.1 ± 0.19 s; mean difference 1.4 ± 0.13 s; paired t test, P = 0.002). These include no significant instrument delay. There was a highly significant relationship between Ct using the fast (Waters) oximeter and the reciprocal of cardiac output (indirect Fick derived; Ct = 0.28 X 60/Q + 2.8 s; Q in l min^-1; P < 0.001). Further investigation of fast response oximetry as an indirect indicator of cardiac output is proposed both for physiological measurement and as a potential clinical diagnostic tool.

We thank the Dunhill Trust for financial support, and Mr Ted Carter (Queen Mary and Westfield campus) for technical help.

All procedures accorded with current local guidelines and the Declaration of Helsinki.