On arrival at high altitude there is a tendency towards hypercoaguiation associated
with an increase in platelet count, factors X and XII, and thrombo-test activity (TA)
which is reflected by decrease in prothrombin time (PT), bleeding time (BT), clotting
time in glass (CT-gl) and in silicone (CT-sl), and stypven time (ST). Clot retraction
is impaired. This hypercoaguiation state is countered by a compensatory rise in fibrinolytic
activity reflected by reduction of clot lysis time (CLT), plasma fibrinogen, and factor
VIII.
This hypercoaguiation state persists throughout the early fortnight after arrival
at high altitude and then starts regressing. On day 3, factor V decreases, BT, and
ST are further reduced, and factor VIII shows a rise. On day 7, a progressive rise
occurs in factors V, VIII, X and XII, TA, platelet counts and platelet factor 3 (PF-3).CT-gl,
CT-sl, PT and ST are further shortened. On day 14, haematocrit rises and of all the
parameters, factors V and X and clot retraction return to normal. Throughout the fortnight,
factor XII remains high, CLT is short, and platelet adhesiveness remains within normal
range.
After 2 years’ stay at high altitude, a regression in hypercoagulability occurs and
is indicated by persistent short CLT, prolonged BT, CT-sl, PT, ST and reduced TA.
Platelet adhesiveness, PF-3, factors V, VIII and XII and clot retraction are restored
to normal.
