The mass spectrometer (MS) traditionally has been the instrument of choice for 
measuring cardiac output (Q (T)) non-invasively using the foreign gas uptake 
method. However, the size and cost of the MS has hampered widespread adoption of 
this technique outside of the laboratory. Here, we present results, from six 
normal human subjects at rest and during exercise, of simultaneous Q (T) 
measurements by an MS and a new, portable infrared (IR) device developed in our 
laboratories. These measurements are made using on the open-circuit acetylene 
uptake method. The IR device measures inspired and end-tidal concentrations of 
acetylene, sulfur hexafluoride, and carbon dioxide by IR absorption spectroscopy 
with a 10-90% response time of 43 ms; accurate measurements were made down to 
sample flow rates of 50 mL min(-1). Excellent correlation [Q (T)(IR)=0.98 Q 
(T)(MS), R(2)=0.94] was observed between instruments across the range from rest 
to heavy exercise. These results suggest that the IR device, which is small, 
light-weight, and rugged may enable the foreign gas uptake method to be used in 
clinical, field, and point-of-care settings for Q (T) measurement.