Non-invasive determination of effective stroke volume. Evaluation of a CO2-rebreathing method in normal subjects and patients.

A CO2-rebreathing method for the determination of stroke volume (SV) was evaluated at rest by comparison with the direct Fick technique in 50 randomly selected patients with valvular heart disease. Patients with intracardiac shunts were excluded. Objective criteria for acceptance of a measurement were set to ensure reliable results. Forty-six of the 50 patients fulfilled these criteria. The rebreathing manoeuvre is, in itself, an effort for the patient, leading to a change in steady state which excludes simultaneous comparison with the direct Fick method. Day-to-day variation of the SV measured with the CO2-method was therefore assessed first, and found to be low. Because of this low day-to-day variation, a comparison of stroke volumes measured one day with the CO2-method and next day with the direct Fick technique was found to be acceptable. In the determination of SV in the supine position, there was no significant difference between the two methods (SVCO2 = 5.2 + 0.90 X SVFick, r = 0.90, SDres = 9.4 ml, n = 46), while cardiac output was significantly higher when measured with the CO2 technique than with the direct Fick method (22%, P less than 0.001). Ten of 12 patients with signs of obstructive lung disease managed to produce registrations which fulfilled the criteria of acceptance. The method is well suited for clinical use.