When do the sympathoexcitatory and sympathoinhibitory phases occur in acute blood loss?

From the Guidelines

In acute blood loss, the sympathoexcitatory phase occurs initially when blood volume decreases, characterized by increased sympathetic nervous system activity, while the sympathoinhibitory phase follows with more severe blood loss, marked by a decrease in sympathetic activity leading to vasodilation and hypotension. The pathophysiology of acute blood loss involves two distinct phases, with the initial phase being sympathoexcitatory, where the body attempts to compensate for the loss of blood volume through vasoconstriction and increased heart rate, as noted in the context of hemorrhagic shock management 1.

• The sympathoexcitatory phase is mediated by baroreceptors detecting reduced arterial pressure, triggering compensatory mechanisms to maintain blood pressure.
• The sympathoinhibitory phase, which occurs with more severe blood loss, is characterized by a paradoxical decrease in sympathetic activity, leading to vasodilation, bradycardia, and hypotension, as discussed in the management of major bleeding and coagulopathy following trauma 1. Key to managing these phases is understanding that the transition from a compensated to a decompensated state requires immediate intervention, potentially including fluid resuscitation and the judicious use of vasopressors to prevent cardiovascular collapse and end-organ damage, as suggested by guidelines on managing major bleeding and coagulopathy 1.
• The use of vasopressors, such as noradrenaline, may be recommended in cases of severe hemorrhage-induced hypotension, where achieving a target systolic blood pressure of 80–90 mmHg is not possible through volume replacement alone 1.
• Furthermore, the potential benefits of counteracting vasodilation in the presence of hemorrhage, possibly through the supplementation of hormones like arginine vasopressin, highlight the complexity of managing acute blood loss and the need for a balanced approach to maintain intravascular volume and vascular tone 1.

From the Research

Phases of Acute Blood Loss

The sympathoexcitatory and sympathoinhibitory phases in acute blood loss are critical components of the body's response to hemorrhage.

• The sympathoexcitatory phase is characterized by an increase in sympathetic nervous system activity, which helps to maintain blood pressure and perfusion of vital organs through vasoconstriction and increased heart rate 2.
• The sympathoinhibitory phase, on the other hand, is marked by a decrease in sympathetic activity, leading to vasodilation and a potential decrease in blood pressure 3.

Timing of Phases

The timing of these phases can vary depending on the severity and rate of blood loss.

• In general, the sympathoexcitatory phase occurs early in the course of acute blood loss, as the body attempts to compensate for the loss of blood volume 2.
• The sympathoinhibitory phase may occur later, as the body's compensatory mechanisms begin to fail and blood pressure decreases 3.
• A study on anesthetized dogs found that the generalization of vasoconstriction, maintenance of venous return, tachypnea, active spontaneous hemodilution, and increased heart rate are the leading mechanisms of perfusion pressure maintenance in early acute hemorrhage 3.

Mechanisms and Responses

The mechanisms underlying these phases involve complex interactions between the sympathetic nervous system, the renin-angiotensin-aldosterone system, and other hormonal and neural pathways.

• The body's response to acute blood loss is aimed at maintaining adequate tissue oxygenation and perfusion of vital organs 2.
• The use of fluid resuscitation, including crystalloids and colloids, can help to support blood pressure and perfusion, but the optimal approach remains a topic of debate 4, 5, 6.