Andrew Stephen, MD
Background
The two most common forms of shock in the TICU and SICU are hemorrhagic and septic shock. It cannot be overstated that for hemorrhagic shock the initial key tenets are bleeding control(OR, IR, direct pressure, scalp laceration repair) and blood product resuscitation. In cases of hemorrhagic shock vasopressors are usually only used when the MAP is dangerously low and bleeding is still occurring and the intravascular volume deficit has not been corrected. For surgical cases of septic shock the initial tenets are similar-source control of the septic focus and IV fluid resuscitation. It remains unclear at what juncture vasopressors should be started in cases of septic shock. Surviving sepsis guidelines in 2018 continue to suggest resuscitating patients with hypotension(MAP<65 mm) with 30 mL/kg of IV fluid in the first 3 hours after presentation. The authors acknowledge a lack of evidence to support this strategy.
We suggest starting a vasopressor for septic shock in surgical critical care patients when MAP remains < 65 mm after 1-2 liters of bolused IV fluid. This does not mean further concurrent fluid resuscitation is not needed however.
MAP is the key number to understand in terms of organ and tissue perfusion. MAP = CO x SVR. It does not make sense though in cases of hemorrhagic or septic shock to only try to drive up the SVR through use of an exclusive alpha agonist, phenylephrine. It likely does not improve forward flow and perfusion of the organ or tissue bed. It makes more sense to use a vasopressor that augments both CO and SVR to some degree. This is behind the theory of why norepinephrine is an effective vasopressor for a number of shock states.
Vasopressor mechanisms
It is valuable to have knowledge of the receptors and how the vasopressors act on them.
For a review of receptor mechanisms one can refer to this blog entry.
Is one vasopressor superior to the others? VASST, VANCS and VANISH
It remains unclear whether norepinephrine, vasopressin or epinephrine is superior as the first line agent for treating septic shock. Based on trials dopamine now has a minimal role as it causes higher rates of tachyarrythmias. For septic shock generally we start with norepinephrine titrated to goal MAP 65 mm. When doses start to escalate consider adding vasopressin. Vasopressin can be used first line and may be more effective in states of significant acidemia, pH < 7.20.
The important recent trials:
-VASST-NEJM 2008, RCT. 778 patients with septic shock. Norepi and then vasopressin added vs. norepi at increasing doses. No difference in mortality.
-VANCS-Anesthesiology 2017, RCT. 330 patients. Vasopressin can be used as a first-line vasopressor agent in postcardiac surgery vasoplegic shock and improves clinical outcomes.
-VANISH-JAMA 2016, RCT. 409 patients with septic shock. Vasopressin compared to norepinephrine. 2×2 design with hydrocortisone included. No difference in AKI but less renal replacement therapy required in vasopressin group compared to norepinephrine.
This is important data and suggests vasopressin is useful as a first line agent and that certain patients may benefit from it more than others. Pulmcrit has a nice review and presents a table of whom may benefit more from vasopressin or catecholamine pressors:
Should we be using individualized MAP goals?
This has been discussed quite a bit this decade with our growing elderly population. An RCT of patients with septic shock compared high (80–85 mm) to low (65–70 mm) MAP target and saw no difference in 28 day mortality between groups. A subset of 167 patients with chronic hypertension had lower incidence of sCR doubling and required less RRT when randomized to the high MAP target. The higher MAP target group had a higher incidence of atrial fibrillation.
Is MAP push for spinal cord injury effective?
It remains unclear. Most of the studies are retrospective and do not adjust for surgical decompression. A careful study would need to be done that relates MAP push and surgical timing to outcomes. It remains in many published SCI algorithms despite limited evidence so expect to have it requested by the spine services. Consult with the ICU attending before starting it. Starting pressors or giving additional fluid to augment MAP can lead to a number of complications: atrial fibrillation, line placement issues etc.