34 fig1 which may include suitable electronic hardware software and a microcomputer or a combination of the two a software implementation is preferred to carry out the steps shown in fig3 and in that connection it is only necessary to make certain modifications to a program known as com 1 used by equipment available from american edwards laboratories of santa ana calif for the purpose of computing cardiac output with the catheter 11 in the heart 13 as shown in fig1 the thermistor 21 provides continuous temperature information concerning the temperature of the fluid i e blood or blood indicator mixture in the pulmonary artery 21 to the instrument 34 in a digital system this temperature information is sampled periodically such as every 71 milliseconds by a sampler in the instrument 34 with reference to fig3 and excluding the usual preliminaries of the type used in the com 1 program such as noisy baseline identification and thermodrift detection the first step is prebolus baseline determination i e determining the prebolus baseline temperature t b1 to accomplish this the temperature samples are averaged in any suitable manner such as by calculating a running average of the samples although tb1 is actually measured in the pulmonary artery it can be safely assumed that prior to injection of the cold indicator the temperature of the blood entering the right ventricle is the same as in the pulmonary artery with t b1 determined the operator can initiate.