The Lowly Blood Pressure Cuff: Is It Accurate?

Yesterday, I described how the typical automated oscillometric blood pressure cuff works. We rely on this workhorse piece of equipment for nearly all pressure determinations outside of the intensive care unit. So the obvious question is, “is it accurate?”

Interestingly, there are not very many good papers that have ever looked at this! However, this simple question was addressed by a group at Harvard back in 2013. This study utilized an extensive ICU database from 7 ICUs at the Beth Israel Deaconess Medical Center. Seven years of data were analyzed, including minute by minute blood pressure readings in patients with both automated cuffs and indwelling arterial lines. Arterial line pressures were considered to be the “gold standard.”

Here are the factoids:

  • Over 27,000 pairs of simultaneously recorded cuff and arterial line measurements from 852 patients were analyzed
  • The cuff underestimated art line SBP for pressures at or above 95 torr
  • The cuff overestimated SBO for pressures below 95 torr (!)
  • Patients in profound shock (SBP < 60) had a cuff reading 10 torr higher
  • Mean arterial pressure was reasonably accurate in hypotensive patients

sbp-cuff-v-aline

Bottom line: The good, old-fashioned automated blood pressure cuff is fine for patients with normal pressures or better. In fact, it tends to understimate the SBP the higher it is, which is fine. However, it overestimates the SBP in hypotensive patients. This can be dangerous! 

You may look at that SBP of 90 and say to yourself, “that’s not too bad.” But really it might be 80. Would that change your mind? Don’t get suckered into thinking that this mainstay of medical care is perfect! And consider peeking at the mean arterial pressure from time to time. That may give you a more accurate picture of where the patient really is from a pressure standpoint.

Reference: Methods of blood pressure measurement in the ICU. Crit Care Med Journal, 41(1): 34-40, 2013.

How Does It Work? The Lowly Blood Pressure Cuff

The blood pressure cuff is one of those devices trauma professionals don’t give a second thought to. Old timers like me remember using the cuff with a sphygmomanometer and stethoscope to get manual blood pressures. I’ve had to do this twice in the past few years on airplanes, and I had forgotten how much work this is.

But technology makes things easier for us. Now you just slap a cuff on the arm (or wrist, or finger), push a button, and voila! You’ve got the pressure.

But have you stopped to think about how this actually works? Why don’t we need the stethoscope any more? Here’s the scoop:

When you take a manual blood pressure, the cuff is inflated until a pulse can no longer be auscultated with the stethoscope. The pressure is slowly released using a little thumb wheel while listening for the pulse again. The pressure at which it is first audible is the systolic, and the pressure at which it softens and fades away is the diastolic.

The automatic blood pressure device consists of a cuff, tubing that connects it to the monitor, a pressure transducer in line with the tubing, a mini air pump, and a small computer. The transducer replaces the analog pressure gauge, and the pump and computer replace the human.

The transducer can “see” through the tubing and into the cuff. It is very sensitive to pressure and pressure changes. The computer directs the pump to inflate to about 20 torr above the point where pulsations in the air column cease. It then releases the pressure at about 4 torr per second, “feeling” for air column vibrations to start. Using a manufacturer specific computer algorithm, the mean arterial pressure is measured. A little more proprietary calculation results in an estimated systolic and diastolic pressure.

bpcuff

Piece of cake! But here’s the question: is it accurate? In my next post, I’ll write about how the automated cuff compares to an indwelling arterial line.

And Yet Another Update On How Fast Can You Warm Up A Hypothermic Patient

The cold snap has finally broken in Minnesota, but Texans had a severe problem with it last week. Unfortunately, it resulted in several dozen deaths. Most were due to carbon monoxide poisoning, but there were a number of people who died from hypothermia as well.

I published a revised compilation of my rewarming rate table a month ago. Since then, I’ve been informed that we are using a new device here at Regions Hospital, the Zoll Thermoguard XL. I had to do a little legwork to get the rewarming rate estimate for it. I am republishing the whole table, including the new device, for your reference.

Warming Technique Rate of Rewarming
Bladder lavage no data
probably
< 0.5° C / hr
Passive external (blankets, lights) 0.5 – 1° C / hr
Active external (lights, hot water bottle) 1 – 3° C / hr
Bair Hugger (a 3M product, made in Minnesota of course!) 2.4° C / hr
Hot inspired air in ET tube 1° C / hr
Fluid warmer 2 – 3° C / hr
GI tract irrigation (stomach or colon, 40° C fluid, instill for 10 minutes, then evacuate) 1.5 – 3° C / hr
Peritoneal lavage (instill for 20-30 minutes) 1 – 3° C / hr
Cool Guard system 1° C / hr
Cool Guard system with thoracic lavage 2° C / hr
Cool Guard system with peritoneal lavage 2.7° C / hr
Thoracic lavage (2 chest tubes, continuous flow) 3° C / hr
Continuous veno-venous rewarming 3° C / hr
Zoll Thermoguard XP with 4-balloon rewarming catheter 3 – 4° C / hr
Continuous arterio-venous rewarming 4.5° C / hr
Mediastinal lavage (thoracotomy) 8° C / hr
Cardiopulmonary bypass 9° C / hr
Warm water immersion (Hubbard or therapy tank) 20° C / hr

One of the most important things to consider is the length of time for rewarming. Do the math using the numbers above! For most patients with severe hypothermia, it’s going to take several hours to rewarm. So make sure you are in a suitable location, such as an OR or ICU!

New Tech: Augmented Reality In The OR?

Virtual reality headsets have been all the rage for some time. They immerse the wearer in a complete virtual world, and are typically used for gaming. Augmented reality, on the other hand, overlays virtual items on the real world so both are visible at the same time. Think the notorious Pokemon Go app:

What if this technology could be used for medical purposes? You could overlay diagnostic or anatomic information on your patient to help guide therapy, surgery, etc.

A group at the University of Alberta in Canada have been playing with this technology. ProjectDR takes any kind of image-related information and projects it directly on a patient.The patient is first scanned using surface markers like they do when making movies:

Obviously, no fancy suit or huge number of markers is needed. Once this is accomplished, the diagnostic information can be projected onto the patient. The patient can move, and the projector will compensate and keep the projection anatomically correct.

Here is a short video that demonstrates the system:

So is this useful? Unfortunately, not yet. It may eventually be good for office-based trauma professionals, but it needs further refinement. This version uses an actual digital projector, which means it will be subject to shadows which will interfere with viewing, especially if a surgeon gets his or her big head in the way.

Here’s how it will really have to work: The system could function quite well in surgical procedures. Imagine the surgeon being able to don a VR headset (lightweight, please!) and see the surgical field with key information overlaid on the display. Or even easier, incorporate it in the DaVinci robotic system display. Add vital signs in the upper corner and details of anatomic structures that have not been surgically exposed yet. It could help show anatomic anomalies in great detail, such as vascular variants. And heck, why not throw in some on-demand magnification as well?

As with most new and exciting tech that hits the general media, a version that is actually usable by clinicians is several years away. But it should be fun when it finally gets here!

Reference: University of Alberta ProjectDR system

Routine Duplex Screening For Venous Thromboembolism

Venous thromboembolism (VTE) is a potential problem for all hospitalized patients, and traumatic injury is yet an additional risk factor for its occurrence. Most trauma centers have some kind of risk assessment tool to help the tailor their chemoprophylaxis regimen to patients most at risk. But far fewer have adopted the use of screening ultrasounds to monitor for new onset VTE that would dictate conversion to therapeutic treatment.

Unfortunately, in the US, the Centers for Medicare and Medicaid Services (CMS) has deemed VTE as a “never” event and penalizes hospitals when they report it. One of the unintentional consequences of this (or is it?) is that hospitals may then pressure trauma programs to avoid surveillance in order to “make the numbers look better.” Remember Law X from Samuel Shem’s House of God?

X. If you don’t take a temperature, you can’t find a fever.

Similarly, if you don’t do a duplex screen, you probably won’t detect VTE. Now granted. some patients develop classic symptoms like edema, pain, and tenderness. But not that many.

But is this wise? My contention has been that if the patient doesn’t develop symptoms that catch your attention, yet they develop VTE that you don’t know about, they are at risk for more serious complications like pulmonary embolism (PE). And you are blithely unaware.

The trauma group at Intermountain Medical Center in Salt Lake City performed an elegant study to determine the impact of screening for VTE in their trauma patients. They performed a prospective, randomized trial on trauma patients admitted over a 30-month period. Patients were included if they were judged to be at moderate to high risk based on their risk assessment profile (RAP) score. Patients were excluded if they were children, had VTE or PE within 6 months prior to hospitalization, or had some type of hypercoagulable state.

Patients were sequentially randomized to no duplex screening vs screening on days 1, 3, 7, and then weekly thereafter. The primary outcome measure was PE during the hospital stay. Secondary outcomes consisted of a number of factors relating to development of DVT.

Here are the factoids:

  • Nearly two thousand patients were enrolled, with about 995 patients in each group and no differences in demographics
  • The ultrasound group had significantly more below-knee (124 vs 8) and above knee (19 vs 8) DVT identified (no surprise there)
  • The ultrasound group had significantly fewer pulmonary emboli than the no ultrasound group (1 vs 9) (lots of surprise here!)
  • Mortality was similar during the hospital stay and for 90 days after

Bottom line: If you look for it, you will find it! This is the definition of surveillance bias. But in in this study, looking for clots in the legs may also decrease the number of patients who develop symptomatic pulmonary embolism. How could this be?

There are a few possibilities. The majority of DVT found in the surveillance group were located distally. Although there is some uncertainty as to how likely these are to embolize, it is probably very low. So let’s ignore them for now and assume that only the proximal clots might embolize.

This leaves an extra 11 DVT found in the surveillance group over and above the no-ultrasound group. Despite that, the surveillance group had only one PE vs 9 in the no-ultrasound group!

Another explanation was that the ultrasound guided changes in management, shifting to management to therapeutic drug dosing. The authors did not find a significant difference between the use of therapeutic vs prophylactic dosing between the groups. But there was a difference. Although the overall study was well-powered, there really weren’t enough numbers to show whether there was a true difference in therapeutic dosing. Fourteen patients in the ultrasound group got therapeutic anticoagulation compared to only 4 in the no-surveillance group. I think this is the actual reason.

Overall, this is a well-designed and well-executed study that shows why taking the Ron Popeil approach to DVT prophylaxis (“set it and forget it”) doesn’t work. Patients do occasionally develop proximal DVT on standard chemoprophylaxis (and frequently develop distal DVT), but it doesn’t always result in obvious signs and symptoms. This study shows that if you don’t look for it, you may not know until they suddenly develop chest pain, air hunger, and worse! So consider carefully if your practice guideline doesn’t yet include surveillance.

Reference: Trauma Patients at Risk for Venous Thromboembolism who Undergo Routine Duplex Ultrasound Screening Experience Fewer Pulmonary Emboli: A Prospective Randomized Trial. J Trauma, publish ahead of print, Publish Ahead of Print. DOI: 10.1097/TA.0000000000003104, February 4, 2021.

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