Time to read: 2 minutes 14 seconds
In 1944 the Lancet published a study wherein a group of Irish medical students were bled, until they fainted. (I’ll just leave that there and move on for the sake of time).
Their cardiac output, heart rate, blood pressure, and vascular resistance were monitored during the time they were well, deliberately being exsanguinated. Their bleeding is where we pick up the discussion from last week about the challenge with monitoring blood pressure as a primary indicator of perfusion status.
What this study demonstrated (other than what it was like to go to medical school in 1944) was unsurprising on the face of it for those of us in 2024. Our physiology has not changed, and this study has since been replicated with lower body negative pressure suits which are not as interesting but certainly safer.
Cardiac output began to fall immediately and continued to fall without rebounding until reperfusion efforts were started, heart rate began to climb immediately, as did the systemic vascular resistance which we would expect due to our built-in compensatory mechanisms that make our bodies innately resilient.
Now onto the primary point: blood pressure remained steady, until the volunteers fainted.
Stroke volume was not monitored but I submit to you that if the formula for cardiac output is heart rate multiplied by stroke volume, then a rising heart rate automatically mitigates the fall of cardiac output. Therefore, stroke volume (being less affected by heart rate), would have fallen at an even steeper rate than cardiac output.
Stroke volume then, can help you see around the corner on blood pressure as it changes first, ahead of our other vitals.
However, we do not currently monitor stroke volume on every patient. We can then look to heart rate and respiratory rate, knowing that tachycardia and tachypnea occur well before systolic blood pressure begins to fall in the presence of a perfusion deficit. There are a multitude of studies on the value of monitoring respiratory rate early and often, but the biggest challenge with this – and I can say this as a nurse – is getting nurses to count respirations ;). Then again, we all know that sometimes the simplest things, are the hardest to implement.
May our hearts stay responsive,
Stephanie Griffeth, MSN RN CCRN
P.S. – The bleeding study can be found here: https://www.sciencedirect.com/science/article/abs/pii/S0140673600741730
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