In spine surgery , patients are susceptible to significant blood loss .
Patients are positioned face down for long periods of time , which increases venous pressure , decreases arterial perfusion pressure , and often causes facial swelling ( increased tissue pressure ) .
In spine surgery , patients are susceptible to significant blood loss , and they are positioned face down for long periods of time , which increases venous pressure , decreases arterial perfusion pressure , and often causes facial swelling ( increased tissue pressure ) .