Introduction. The relation between minute ventilation (VE) and carbon dioxide production (VCO2) can be characterised by the instantaneous ratio of ventilation to carbon dioxide production, the ventilatory equivalent for CO2 (VEqCO2). We hypothesised that the time taken to achieve the lowest VEqCO2 (time to VEqCO2 nadir) may be a prognostic marker in patients with chronic heart failure (CHF). Methods. Patients and healthy controls underwent a symptom-limited, cardiopulmonary exercise test (CPET) on a treadmill to volitional exhaustion. Results. 423 patients with CHF (mean age years; 80% males) and 78 healthy controls (62% males; age years) were recruited. Time to VEqCO2 nadir was shorter in patients than controls ( ？s versus ？s; ). Univariable predictors of all-cause mortality included peak oxygen uptake ( ), VEqCO2 nadir ( ), and time to VEqCO2 nadir ( ). In an adjusted Cox multivariable proportional hazards model, peak oxygen uptake ( ) and VEqCO2 nadir ( ) were the most significant independent predictors of all-cause mortality. Conclusion. The time to VEqCO2 nadir was shorter in patients with CHF than in normal subjects and was a predictor of subsequent mortality. 1. Introduction Cardiopulmonary exercise testing (CPET) is used to stratify risk in patients with cardiorespiratory disease [1]. In patients with chronic heart failure (CHF), the normal linear relation between ventilation (VE) and carbon dioxide production (VCO2) is maintained, but the slope of the relation is greater than normal, so that, for a given volume of carbon dioxide production, the ventilatory response is greater [2–6]. Another way of characterising the relation between minute ventilation and carbon dioxide production is the instantaneous ratio of ventilation to carbon dioxide production, the ventilatory equivalent for CO2 (VEqCO2). Recently, we have shown that the lowest VEqCO2 (VEqCO2 nadir) provides greater prognostic value than other CPET-derived variables in patients with suspected CHF [7]. Other studies have reported that the lowest VEqCO2 has similar prognostic power to the VE/VCO2 slope derived from the whole of exercise [8]. During an incremental CPET, as exercise intensity increases, both VCO2 and VE increase linearly. However, VEqCO2 falls at the onset of exercise, possibly due to a reduction in dead space ventilation. Beyond the ventilatory compensation point (VCP), lactic acid production causes an increase in ventilation relative to carbon dioxide production, and thus the VEqCO2 rises. Although patients with CHF have the same pattern of VEqCO2 during exercise as normal
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