Measuring Blood Pressure

Since hypotension is probably the most common “serious” ward call you will be asked to see, it is somewhat dispiriting that zero time is spent in medical education going over how the tools we use to measure this important parameter actually work.

If you’ve sat in on any tutorial on managing hypotension you will likely have been exposed to the old chestnut that if a patient’s radial pulse is palpable their systolic is at least 80, and if their femoral is palpable then it is 70-80, and if the carotid is palpable it is 60-70, or some other similar variation on these parameters.

How accurate is this seemingly simple bedside guide? Unfortunately not very! This (1) elegant study compared arterial line blood pressure measurement with assessment of pulses (done by blinded assessors). The graph below (reproduced without any permission) shows the expected blood pressures in green shading based on which pulses are palpable (groups 1-4), while the scatter points shows the actual systolic blood pressure recorded. As you can see, the degree of hypotension was severely underestimated by the above rules.

However, while the presence of a radial pulse is not reassuring, it would still appear that the absence of one is probably correlated with severe hypotension. Therefore if your BP cuff is not able to record the blood pressure, and you cannot feel radial pulses, it is not a machine fault! For some reason people stop trusting machines when they give extreme readings.

Speaking of which, it is worth understanding how automatic BP cuffs actually work. The machine is actually an oscillometer, which measures oscillations in the brachial artery that are transmitted through the air filled tubing. The mean arterial pressure (MAP) is determined when the amplitude of the oscillations is maximal. Above and below the MAP, the amplitude of oscillations will decline, and the systolic and diastolic are determined when the amplitude reaches a certain percentage of the maximal amplitude (see the figure below for an illustration). The reason this is important is that the cutoff for systolic/diastolic is determined based on a mathematical formula, so it is estimated in a way (2).

Where this becomes relevant is thinking about how reliable your NIBP is in hypotension. This study (3) looked at correlation between NIBP and invasive arterial line measurement in 150 ICU patients, about half of whom had circulatory failure with hypotension or needing vasopressor drugs. They found that non-invasive measurement of MAP was pretty accurate, especially when the patient was hypotensive. However, non-invasive systolic and diastolic measurements were not accurate, with the systolic pressure often being overestimated.

What about manual blood pressure? The literature provides very divergent evidence, but it seems that in the population that we are interested in (the hypotensive) that automated cuffs overestimate the systolic blood pressure, sometimes to quite alarming degrees, at least if this study of trauma patients is to be believed (4). Manual blood pressure is better correlated to injury severity and markers of shock in this group.

The interesting corollary to this is that the often employed delay tactic by house officers when paged about hypotension, of asking the blood pressure to be repeated manually in the hope that it will give a more encouraging reading, is a waste of time. If the automatic cuff is sized properly, any hypotension should likely be believed.

• Either learn to work with MAP rather than SBP, or…
• If you can’t be stuffed doing the above or find it too difficult, then use a manual to assess the SBP; automatic cuffs are unreliable for this parameter and will overestimate it in the setting of hypotension.

Till next time…

1. Charles D Deakin and J Lorraine Low. Accuracy of the advanced trauma life support guidelines for predicting systolic blood pressure using carotid, femoral, and radial pulses: observational study. BMJ. 2000 Sep 16; 321(7262): 673–674.
2. https://lifeinthefastlane.com/ccc/non-invasive-blood-pressure
3. Lakhal K, Macq C, Ehrmann S, Boulain T, Capdevila X. Noninvasive monitoring of blood pressure in the critically ill: reliability according to the cuff site (arm, thigh, or ankle). Crit Care Med. 2012 Apr;40(4):1207-13. doi: 10.1097/CCM.0b013e31823dae42.
4. Are automated blood pressure measurements accurate in trauma patients? J Trauma. 2003 Nov;55(5):860-3. Davis JW1, Davis IC, Bennink LD, Bilello JF, Kaups KL, Parks SN.