apparatus and methods for non - invasively determining cardiac output using partial re - breathing techniques are disclosed in which the apparatus is constructed with an instantaneously adjustable deadspace for accommodating differences in breathing capacities of various patients . the apparatus is constructed of inexpensive elements , including a single two - way valve which renders the apparatus very simple to use and inexpensive so that the unit may be readily disposable . the method of the invention provides a novel means of estimating cardiac output based on alveolar co 2 values rather than end - tidal co 2 values as previously practiced . a program for calculating cardiac output is also disclosed .