Introduction: In cardiopulmonary exercise testing (CPET) respiratory exchange ratio (RER) at maximum load or oxygen uptake is used as indicator of ?good effort?; i.e. significant lactic acidosis. The usual cutoff point being an RER of 1.10. The correlation of RER with change in base excess (BE) however is poor and the association between RER and good effort therefore weak.

Aim: To assess the CO2-production per unit body weight at peak exercise as a parameter for good effort in exercise testing.

Methods: On 220 patients (132 male) aged 58 (SD 15) CPET was performed using incremental stepwise load on a bike-ergometer. Data on gas-exchange was collected and arterial blood gases were taken at rest, maximum load and 2 minutes in recovery. A decline in BE of 9 mEq.L-1 or more between rest and recovery was considered a ?good effort?. Analysis of receiver-operating-characteristics was applied to determine or verify optimal cutoff-points for V'CO2 per unit body weight and RER.

Results: Correlation between RER and change in BE was poor (r2= 0.1718). Between V?CO2 per unit body weight and change in BE correlation was better (r2= 0.5169). Association of V?CO2 with ?good effort? (AUC: 0.929, p= 0.000) outperformed association of RER (AUC: 0.737, p= 0.000) with "good effort". Assessment of effort was accurate in 86.8% of the cases using a cutoff-point of 26 ml.min-1.kg-1 for V?CO2 per kilogram, versus 71.8% with RER using a cutoff-point of 1.10.

Conclusion: A cutoff-point of 26 ml.min-1.kg-1 in V?CO2, rather than 1.10 in RER, yields a more accurate non-invasive assessment of effort in CPET.