Accuracy of Non-invasive blood pressure in critically ill patients

Haber E.N et al. Accuracy of Noninvasive Blood Pressure Monitoring in Critically ill Adults. J Int Care Med. 2024, Jan 12 (Ahead of Print.) PMID: 38215002

I always have anxiety when the sick, shocked patient comes into the emergency department. I feel that I can’t rely on non-invasive blood pressure (NIBP) readings, especially when the pressures are low. This study was of interest.

This paper wanted to answer the question of how accurate NIBP readings are in comparison to an arterial line blood pressures(BP).

They specifically looked at the ICU setting in critically ill patients.

What they did

This was a retrospective, observational study, conducted at one centre.

There had to be at least one BP reading taken simultaneously from NIBP and arterial line.

 N=1852

 Location of arterial lines were:

• Radial artery (81%)
• Femoral artery (11%)

 Primary Outcome

Less than or equal to 10% difference in BP between non-invasive and invasive methods.

What they found:

There was a median difference in Mean arterial pressure (MAP) readings of 6mmHg.

67% of these readings were in agreement.

The patients had a median systolic blood pressure (SBP) of 110-120mm Hg.

They found 5 predictors of measurement discrepancy:

1. Increasing doses of Noradrenaline
2. Lower MAP values
3. Higher BMI
4. Increased age
5. Radial arterial line

The authors conclusion was:

“There was broad agreement between IABP and NIBP in critically ill patients over a range of BPs and severity of illness. Several variables are associated with measurement discrepancy; however, their predictive capacity is modest.”

My take on this

 A study like this got me excited…. Initially anyway.  If I felt that NIBP results were reliable, then it would save a great deal of anxiety until an arterial line was placed. We could carry out all the initial stabilisation without that feeling of not really knowing what the blood pressure is. Although we often don’t. How many times do we have to try and take a manual BP, because the NIBP won’t read?

 This was a retrospective study in only one centre. It doesn’t try to be what its not. It doesn’t look at mortality or morbidity. It tries to answer the simple question of is there a major difference in blood pressure readings between invasive versus non-invasive means.

 The main diagnoses studied were:

• respiratory failure,
• sepsis and
• central nervous system disease such as stroke. 

Cardiogenic shock was not studied. This is a potentially significant group with causes including cardiac ischaemia, pulmonary embolism and drug related shock.

 We are not sure of how long the patients were in ICU when these measurements were made. Will the blood pressure respond in the same way when patients present acutely to the Emergency department?

 The fact that the median SBP was between 110 mmHg and 120 mmHg leads me to ask two questions:

• How sick were these patients? They may have been stable patients in the ICU (we are not told how long they were in ICU for).
• Would the discrepancies be larger with a lower SBP. The answer is probably yes given that the greater discrepancies occurred in those who had a lower MAP.

 The predictors of discrepancy are my greatest concerns here.

 Most of the arterial lines we place are radial, although there is a trend towards femoral. Many of my patients have a high BMI, they tend to be older and Noradrenaline is my pressor of choice in most cases.

 …. So if the patient is sick, old, large, on increasing doses of Noradrenaline, has lower MAP values and a radial arterial line and has cardiogenic cause of shock, I can’t use NIBP reliably.

Great idea for a study, but it won’t change my practice. I’ll still not trust the NIBP is the sick acute shocked patients and stay a little anxious until I get the arterial line in.

Reference

Haber E.N et al. Accuracy of Noninvasive Blood Pressure Monitoring in Critically ill Adults. J Int Care Med. 2024, Jan 12 (Ahead of Print.) PMID: 38215002