Changes in sublingual microcirculatory flow index and vessel density on ascent to altitude

We hypothesized that ascent to altitude would result in reduced sublingual microcirculatory flow index (MFI) and increased vessel density. Twenty-four subjects were studied using sidestream dark-field imaging, as they ascended to 5300 m; one cohort remained at this altitude (n = 10), while another ascended higher (maximum 8848 m; n = 14). The MFI, vessel density and grid crossings (GX; an alternative density measure) were calculated. Total study length was 71 days; images were recorded at sea level (SL), Namche Bazaar (3500 m), Everest base camp (5300 m), the Western Cwm (6400 m), South Col (7950 m) and departure from Everest base camp (5300 m; 5300 m-b). Peripheral oxygen saturation (SpO2), heart rate and blood pressure were also recorded. Compared with SL, altitude resulted in reduced sublingual MFI in small (<25 microm; P < 0.0001) and medium vessels (26-50 microm; P = 0.006). The greatest reduction in MFI from SL was seen at 5300 m-b; from 2.8 to 2.5 in small vessels and from 2.9 to 2.4 in medium-sized vessels. The density of vessels <25 microm did not change during ascent, but those >25 microm rose from 1.68 (+/- 0.43) mm mm(-2) at SL to 2.27 (+/- 0.57) mm mm(-2) at 5300 m-b (P = 0.005); GX increased at all altitudes (P < 0.001). The reduction in MFI was greater in climbers than in those who remained at 5300 m in small and medium-sized vessels (P = 0.017 and P = 0.002, respectively). At 7950 m, administration of supplemental oxygen resulted in a further reduction of MFI and increase in vessel density. Thus, MFI was reduced whilst GX increased in the sublingual mucosa with prolonged exposure to hypoxia and was exaggerated in those exposed to extreme altitude.