Since arm exercise affects the respiratory muscles the CO2 rebreathing
method for determining cardiac output (Q) has to be evaluated during
arm exercise. The purpose of this study was (1) to compare three diffe
rent methods of determining arterial CO2 tension (PaCO2) during arm ex
ercise, (2) to verify the linearity of the relationship between Q and
oxygen uptake (VO2) during arm exercise, and (3) to investigate whethe
r the CO2 rebreathing method according to Collier can determine accura
tely Q during arm exercise. Sixty male subjects performed arm-cranking
exercise at 20%, 40% and 60% of their individual maximal load. Carbon
dioxide output (VCO2) was measured by gas exchange measurement, and m
ixed venous CO2 tension (P $($) over bar$$ vCO(2)) was determined from
the CO2 rebreathing plateau at each exercise level. PaCO2 was estimat
ed in three different ways: (A) by the modified Bohr formula for dead
space, (B) by an arterialized blood sample from the hyperaemic ear-lob
e, and (C) by the end-expiratory CO2 tension. A, B, and C were used to
calculate Qa, Qb and Qc, respectively. The Pearson's correlation coef
ficient was high (P<0.01) among the three different ways of estimating
PaCO2. The Q-VO2 relationship proved to be linear (P<0.01). The Q-val
ues showed a good agreement with the direct Fick measurements, and wer
e in the same range compared to other results obtained by dye dilution
, electrical impedance cardiography and the exponential CO2 rebreathin
g method during arm exercise. In conclusion, the CO2 rebreathing metho
d appeared to be accurate to determine Q during submaximal arm exercis
e.