Hypotension following acute hypovolaemia depends on the caudal midline medulla

Acute hypovolaemia evokes abrupt, life-threatening hypotension and bradycardia. Hypotension can be evoked also by excitation of the caudal midline medulla (CMM). This study investigated the possible contribution of the CMM depressor area to hypotension evoked by acute hypovolaemia. Inactivation of the CMM, with either lignocaine or cobalt chloride did not alter resting arterial pressure. However lignocaine injections blocked the fall in arterial pressure, and cobalt chloride injections delayed the onset and significantly attenuated the size of hypovolaemic-evoked hypotension. These findings suggest that the CMM is a key region triggering hypotension after blood loss, and that the brain areas mediating cardiovascular response to challenges such as acute hypovolaemia are not the same areas that regulate resting arterial pressure.