EFFECTS OF CHRONIC HYPOXEMIA ON CHEMOSENSITIVITY IN PATIENTS WITH UNIVENTRICULAR HEART

Objectives. We sought to compare the arterial blood gas chemosensitivi ty in relation to exercise ventilatory response in patients with unive ntricular heart and cyanosis and in patients with univentricular heart and Fontan-type circulation without cyanosis. Background. Patients wi th univentricular heart demonstrate excessive ventilation during exerc ise, Chronic hypoxemia may alter chemoreceptor function, affecting ven tilation. Methods. Cardiopulmonary exercise testing was performed in 1 0 patients with rest or stress-induced cyanosis (cyanotic group: mean age +/- SE 30.5 +/- 2.3 years; 5 men), 8 patients without cyanosis wit h Fontan-type circulation (Fontan group: mean age 29.4 +/- 1.5 years; 4 men) and 10 healthy control subjects (normal group: mean age 30.7 +/ - 1.9 years; 5 men). Hypoxic and hypercapnic chemosensitivity were ass essed by using transient inhalations of pure nitrogen and the rebreath ing of 7% CO2 in 93% O-2, respectively. Results. Peak O-2 consumption was comparable in both patient groups (21.7 +/- 2.5 [cyanotic group] v s. 21.0 +/- 1.9 ml/kg per min [Fontan group]) but was lower than that in the normal group (34.7 +/- 1.9 ml/kg per min). The ventilatory resp onse to exercise, characterized by the regression slope relating minut e ventilation to CO2 output, was higher in the cyanotic group (43.4 +/ - 4.0) than in the Fontan group (31.4 +/- 3.0, p = 0.02) and the norma l group (23.1 +/- 1.1). Hypoxic chemosensitivity was blunted in the cy anotic group compared with that in the Fontan and normal groups (0.148 vs. 0.448 [p = 0.02] vs. 0.311 liter/min per percent arterial O-2 sat uration, respectively) and did not correlate with the ventilatory resp onse to exercise (r = -0.36, p = 0.29). In contrast, hypercapnic chemo sensitivity represented by the slope of the hypercapnic ventilatory re sponse line,vas similar in the cyanotic, Fontan and normal groups (1.7 1 vs. 1.76 vs. 1.70 liter/min per mm Hg, respectively), but the respon se line had shifted to the left in the cyanotic group (x intercept = 3 1.9 vs. 39.9 mm Hg [p = 0.026]), compared with 45.2 mm Hg in normal su bjects, These findings suggest that in the cyanotic group, ventilation is greater for a given level of arterial CO2 tension and thus may par tly explain the increased exercise ventilatory response in this group. Conclusions. Hypoxic chemosensitivity is blunted in patients with uni ventricular heart and cyanosis and does not determine the exercise ven tilatory response. CO2 elimination appears more important. The bluntin g of hypoxic chemosensitivity is reversible once chronic hypoxemia is relieved, as evident in the Fontan group. (C) 1997 by the American Col lege of Cardiology.