FIGURE 1.

Absolute heart rate response (delta change) during maximal end-inspiratory apnea within Lowlanders (gray circle; N.13), Andean controls (CMS-: black circle; N.9), and Andean CMS patients (CMS +; white circle; N.8). The baseline HR for each group at 4330 m (represented as “0 beats/min”) and nadir response (lowest beats/min obtained during the last 10 cardiac cycles) have also been identified. Lowlanders developed significant bradycardia response prior to volitional breakpoint while both Andean controls and CMS patients had a preserved heart rate response. * Significant difference between cardiac cycle and baseline, P < 0.05. † Significant difference from Lowlanders, P < 0.05.