Physiology: Why target the MAP?
Theory behind the traditional fluid-first approach to sepsis
Reasons not to delay norepinephrine in a patient with inadequate MAP
(1) Septic shock is not only due to hypovolemia
(2) Norepinephrine increases preload
(3) It's OK to administer peripheral norepinephrine for a short period of time
(4) Duration of time with inadequate MAP increases risk of renal failure
(5) Prolonged hypotension may itself contribute to refractory shock
(6) Not all patients with septic shock are fluid-responsive
Many reasonable approaches
- Vital signs are vital. Despite enthusiasm about gadgets, MAP remains useful. A patient with a severely low MAP is probably malperfused, regardless of how well they may appear.
- Patients with septic shock have hypotension due to multiple factors (i.e., venodilation, arterial dilation, reduced preload). Initiating treatment with fluids alone addresses only one factor (reduced preload), explaining why this often fails.
- Norepinephrine increases preload, MAP, and contractility. This is a more rational approach to the multiple hemodynamic derangements in septic shock.
- It makes little sense to delay norepinephrine while awaiting fluid resuscitation in a patient with septic shock and severely low MAP. Delaying hemodynamic stabilization may increase the risk of renal failure and could contribute to worsening shock.
- There is no solid evidence regarding when to start norepinephrine. The decision to initiate norepinephrine should be made on an individualized basis depending on hemodynamic assessment and the judgment of the treating clinician.
- Mythbusting: Empty IVC and hyperkinetic heart don't prove volume depletion. This post discusses the physiology of early sepsis, and why norepinephrine may make more sense physiologically compared to volume resuscitation.
- Renoresuscitation: Strategy to avoid chronic sepsis. This post discusses an overall strategy to resuscitation of septic shock focused on avoiding renal injury.
- Renal microvascular hemodynamics in sepsis: a new paradigm. Some thoughts about the physiology of acute kidney injury in septic shock and a possible role for early vasopressin.
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