Our teaching and guidelines emphasise using tachycardia as a marker of hypovolemic shock. A paeds study in Anesthesia & Analgesia reminds us how that is far from always the case. The study makes me even more suspicious of the classic staging of hypovolemic shock.
The absence of tachycardia in bleeding adult trauma patients is well known. The incidence is reported to be as high as 44% and is associated with increased mortality. The phenomenon had been confirmed in animal models too. The mechanism for this relative bradycardia remains unclear with a combination of parasympathetic reflexes and ischemia being the chief suspects.
In anesthetized patients the baroreceptor response to hypovolemia is partially or completely stunned, making tachycardia even less useful as a marker for critical hypovolemia. The study in Anesthesia & Analgesia takes a closer look at tachycardia in anaesthetised bleeding paediatric patients.
The study enrolled 53 kids younger than 24 months due for craniofascial reconstructive surgery. This is a type of surgery notorious for causing heavy blood loss. The anesthesia used was a combination of volatile anesthetics and opioids. Using retrospective analysis of recorded data the researchers identified hypotensive episodes.
The researchers identified 29 episodes of hypotension. These episodes were associated with an average blood loss of 1,6 blood volumes and had an average MAP of 34 mmHg. The average blood loss in normotensive patients was 1 blood volume.
Increased heart rate (HR) was not observed in any of the hypotensive episodes. HR was no different at the onset of hypotension when compared to preoperative HR or HR 5 minutes before nor after the hypotensive episode.
Take home message
As an anaesthetist I’m frequently assessing hypovolemia when anaesthetising bleeding or critically ill patients. This, and several other, studies has convinced me the absence of tachycardia is of little value when assessing hypotensive patients. In anaesthetised patients it might be useless.
Absence of tachycardia during hypotension in children undergoing craniofacial reconstruction surgery. Stricker PA, Lin EE, Fiadjoe JE, Sussman EM, Jobes DR. Anesth Analg. 2012 Jul;115(1):139-46. Epub 2012 Apr 13.