Relationship between dyspnea increase and rate variability during an incremental ventilatory gas exchange thresholds during exercise in children with surgically corrected heart impairment

To study the relationship between the onset of an increase in dyspnea and ventilatory threshold (VT) in children with congenital heart impairment, sixteen young subjects underwent a cardiopulmonary exercise test with dyspnea perception and ventilatory gas exchange assessments. Dyspnea score was measured from a visual analogical scale at rest and during each step of an incremental exercise test. Dyspnea score was plotted against oxygen uptake and the onset of an increase in dyspnea (DT) was determined when a brutal disruption occurs on the dyspnea score-oxygen uptake curve. VT was defined from gas exchange according to Beaver's method [7]. The first breakdown point in the oxygen uptake-carbon dioxide production relationship locates VT. Oxygen uptake (VO2), pulmonary ventilation (VE), heart rate (HR), oxygen pulse (O-2 pulse = VO2/HR), carbon dioxide production (VCO2) and power output (P) were measured both at VT and DT effort level. Results pointed out that there was no significant difference between the cardiorespiratory variables measured respectively at VT and DT: VO2 (VTVO2 = 16.71 +/- 2.65 vs. DTVO2 = 18.34 +/- 5.74 ml . kg(-1) . min(-1)), VE (VTVE = 24.33 +/- 6.86 vs. DTVE=26.82 +/- 9.59 l.min(-1)), (VTVCO2=789.31 +/- 165.17 vs. DTVCO2=924.02 +/- 342.28ml . min(-1)), HR (VTHR=116 +/- 10 vs. DTHR = 123 +/- 20 beat . min(-1)), O-2 pulse (VT O-2 pulse = 7.83 +/- 2.00 vs. DT 02 pulse = 8.01 +/- 2.13 ml - kg-1 - beat-), and P (VTP = 43 +/- 16 vs. DTP = 52 +/- 27 W). Moreover, the cardiorespiratory variables measured at DT and VT were closely related: VO2 (r = 0.64, p < 0.01), VE (r = 0.51, p < 0.01), HR (r = 0.75, p < 0.02), O-2 pulse (r = 0.90, p < 0.001), and P (r = 0.80, p < 0.01). In addition, according to Bland and Altman's procedure [8], the onset of dyspnea increase and ventilatory threshold were shown in close agreement for the cardiorespiratory variables measured at these effort levels. The standard errors of the estimates were low. It was concluded that dyspnea and ventilatory gas exchange thresholds occur concomitantly and were strongly correlated in children with congenital heart impairment. The use of the onset of dyspnea increase for aerobic capacity assessment may be a good alternative to ventilatory gas exchange threshold measurement.