Climbing Everest and K2 without oxygen: what helps?

We tested whether higher minute ventilation (Vm) and higher hypoxic ventilatory response (HVR) were necessary prerequisites to climb Everest or K2 without oxygen, in 11 climbers at sea level (SL), on arrival at the Everest base camp (5200m, HA-1), after 15-day acclimatization (HA-2) (2004 Italian Everest-K2 Expedition). We measured resting oxygen saturation (SaO2), Vm, breathing rate (BR), HVR, vital capacity, maximal voluntary ventilation (MVV), ventilatory reserve (RES) at SaO2=70% (defined as 100 x (MVV- Vm )/MVV), ventilatory efficiency (physiological dead space/tidal volume ratio (Vd/Vt) and SaO2/Vm ). Within 61 days from HA-2, 5 climbers (G1) summited Everest or K2 without oxygen support, 6 climbers (G2) did not (4) or used oxygen. G1 and G2 did not differ at SL and HA-1. At HA-2, G1 showed smaller increases in Vm (to 13.3±0.8 vs 19.2±0.6 L/min, p<0.001), BR (to 10.7±1.9 vs 20.2±0.8 br/min, p<0.001), HVR (to -2.14±0.51 vs 5.09±1.03 L/min/% SaO2, p<0.05), and greater RES (66.6±6.3 vs 26.7±8.8%, p<0.01) vs G2. Ventilatory efficiency was higher in G1 (Vd/Vt: 0.09±0.01 vs 0.16±0.02, p<0.05, SaO2/Vm: 6.66±0.39 vs 4.64±0.13 %/L/min, p