A constricting boa applies pressure with successive body loops, tightening as the prey exhales. For most of the twentieth century the assumed mechanism was suffocation: the ribs cannot expand, so the animal cannot breathe.
Experimental work published in 2015 by Boback and colleagues, using anaesthetised rats fitted with catheters, measured what actually happens under the coils. Arterial pressure fell by roughly half within seconds while venous pressure rose sharply, blood potassium climbed, and cardiac function failed. Prey lost circulation within about a minute, well before oxygen deprivation alone would be fatal. Circulatory arrest, not asphyxia, is the dominant cause of death.
The distinction matters for how the behaviour evolved. A kill that takes seconds rather than minutes reduces the time a constrictor is locked to struggling prey capable of biting or clawing back. Boas also modulate their squeeze in response to the prey's heartbeat, easing off as it fades, which suggests the snake is monitoring circulation directly.