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Central venous pressure is measured in critically ill patients. The venous pressure is transmitted to a strain gauge transducer through a catheter inserted into the right atrium or superior vena cava. Through a signal conditioning technique using a low-pass filter with a cutoff at 0.8 c/s, the pulsatile variations related to heart action that are superimposed on the low-frequency respiratory changes are removed. The output of the respirometer is sampled every five minutes by art IBM 1710 control system computer and printed on an output typewriter, together with a number of variables, in a comprehensive patient monitoring system.