Why shock is not all about the blood pressure

When calling the surgical or gastro registrar on the phone, one of the surefire questions you will be asked is ‘is the patient hemodynamically stable?’ This is no doubt an important question but unfortunately hemodynamic stability means different things to different people. There is generally a lack of willingness to look beyond a ‘normal’ vs ‘low’ blood pressure.

It should be made clear that hypotension is a sign of ‘decompensated’ shock. The idea that hypotension is a late sign of shock has been long recognized in the trauma literature where 18% of penetrating abdominal trauma can have over 750ml of blood in the abdomen despite normal vital signs (including 7% who had over 1500ml). In the majority of patients evidence of tissue hypoperfusion precedes the development of hypotension (1).

Why is this? Well, it is all about the concept that pressure does not equal flow. The body is quite adept at trying to maintain a normal blood pressure through various compensatory mechanism, chief amongst which is the sympathetic response. But this does not tell us whether perfusion at the tissue level is adequate- inadequacy of this is after all the definition of shock. This is especially true in young patients where the compensatory mechanisms are quite strong and blood pressure may be preserved till late in the piece, as illustrated by the graph at the top, of my creation (credit to MS paint).

This phenomenon is true in all situations- not just trauma- so it is relevant to the patients you will be seeing on ward calls. For example ‘normotensive shock’ is recognized in sepsis (2), cardiogenic shock (3), and just generally (4). What then are the signs of ‘normotensive shock’ you should look out for? Well, these are simply the signs of inadequate tissue perfusion- cool and clammy skin, oliguria, mental state changes and elevated lactate. Other signs which may accompany this that are not necessarily indicative of hypoperfusion but that do indicate a compensatory response are tachycardia and tachypnea.

It is also important to note two slightly related things. Firstly, a systolic blood pressure of 110 may be normal for a 20 year old but grossly hypotensive for a 70 year old with chronic hypertension. Secondly, it is sometimes difficult to tell whether a ‘soft’ blood pressure in a young person is just normal for them or whether they are actually hypotensive- a normal heart rate cannot be used to reassure you in this instance because not uncommonly shocked patients may have paradoxically increased parasympathetic tone (1) the exact mechanism for which is unclear. Older patients may also be on Beta Blockers.

The conclusion in all of this- next time you call the gastro reg with a patient who has vomited blood and is clammy with a lactate of 4 but has a normal blood pressure, the answer to the question ‘is the patient hemodynamically stable?’ is a ‘HELL NAH”.

Today’s post may sound basic but it is all about fundamentals- a fundamental which is often simply not done. Rather than spending 10 minutes documenting dual heart sounds, look and feel for the signs of shock!

Till next time.

 

 

1. Identification and Resuscitation of the Trauma Patient in Shock Michael N. Cocchi, MDa , Edward Kimlin, MDa , Mark Walsh, MDb , Michael W. Donnino, MD. Emerg Med Clin N Am 25 (2007) 623–642
2. Septic Shock. Advances in Diagnosis and Treatment. Christopher W. Seymour, MD, MSc and Matthew R. Rosengart, MD, MPH. JAMA. 2015 Aug 18; 314(7): 708–717.
3. Menon V et al. Acute myocardial infarction complicated by systemic hypoperfusion without hypotension: Report of the SHOCK trial registry. Am J Med 2000 Apr 1 108 374380
4. Approach to Hemodynamic Shock and Vasopressors. Stefan Herget-Rosenthal, Fuat Saner and Lakhmir S. Chawla. CJASN March 2008, 3 (2) 546-553; DOI:https://doi.org/10.2215/CJN.01820407

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.

The conclusion of the second part of this article is basically

• 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.

 

Supine masking of hypotension

Today’s post regards a common pitfall regarding the assessment of the hypotensive patient. Unfortunately, it will be based on anecdote rather than literature, but hopefully you won’t find the underlying assumptions too controversial.

Often you are called to the bedside of a patient with a hypotensive episode. You arrive to find them flattened on the bed, sometimes in the Trendelenburg position. You take stock of the situation and find the airway to be intact, the patient fully responsive, with warm peripheries and a good strong peripheral pulse. They are breathing comfortably. A blood pressure is taken and this is 110/70. The other vitals are normal as is the rest of the examination.

It is easy to conclude that the blood pressure has normalised and that this was therefore a self- resolving period of hypotension, most likely postural or vasovagal. What is important to remember is that, if you leave at this point, you have left the patient in a highly unnatural position. People don’t live their lives supine, nor indeed in Trendelenburg, and often their previous blood pressure recording taken in hospital will be with them sitting up. It is easy to gloss over this fact as you only arrive after the fact and therefore tend to only ever see the patient supine.

Before being reassured that the blood pressure is normal you must make sure it is normal in the sitting position. If they are sat up and now the blood pressure is 85/40, you clearly have an ongoing problem. Even the most posturally fragile old patients should maintain a normal blood pressure while sitting.

I have certainly ignored my own rule multiple times and it is extremely easy to do so because the human mind tends to only see what is there in front of you. Also, given the multitude of calls for abnormal vital signs, we latch onto any indication that the patient is actually OK and doesn’t need intervention. A few times when I ignored this rule I was later called for sustained hypotension (of course after they had been sat up) which turned out to be proper pathology.

The first of many lessons that you always need to take into account what we have done to the patient when making your assessment!