Does visual sphygmomanometer needle oscillation improve accuracy of Doppler ultrasound BP measurement?
New research conducted by Marc Armour, Resident in Anaesthesia and Analgesia at Lumbry Park Veterinary Specialists
New research has identified that visually observing the oscillation of a sphygmomanometer (blood pressure monitor) needle, when compared to the audible pulse via Doppler ultrasound, tends to agree better with a dog’s invasive blood pressure measurements.
The most accurate way to measure blood pressure is via invasive blood pressure measurement. However, this involves inserting a catheter into the artery of an animal which can be tricky and requires specialist equipment.
Led by Marc Armour, Resident in Anaesthesia and Analgesia at Lumbry Park Veterinary Specialists, new research has assessed whether measurement of blood pressure with a less accurate, but non-invasive measure (Doppler), could be improved.
The prospective clinical study investigated three different blood pressure techniques in a clinical population of 35 anaesthetised dogs, who required arterial cannulation at Lumbry Park.
The first method, invasive arterial blood pressure (iABP), is the reference technique providing the most accurate measure. However, this method requires specific equipment and technical skill, so it is infrequently used in general practice and selectively used in referral practice.
The second, the non-invasive Doppler ultrasound, is more typically used in practice. To obtain the blood pressure estimate from Doppler ultrasound, a clinician will listen for an audible return of pulsatile flow (ARPF) via the Doppler device as they deflate a BP cuff. However, this method is not accurate in comparison to iABP or other non-invasive BP monitors (oscillometric) and has never been validated in dogs.
The researchers hypothesised that a third method - visual sphygmomanometer needle oscillation (SNO) would agree more closely with iABP than ARPF.
To gather evidence, the paired measurements of iABP and SNO, and iABP and ARPF were collected by the research team. The non-invasive blood pressure (NIBP) and iABP measurements were analysed with concordance correlation coefficients (CCCs) and Bland–Altman plots. The proportions of SNO and ARPF measurements between 10 and 20 mmHg of iABP were then compared.
The results found that both SNO and ARPF demonstrated greater agreement with invasive systolic (iSAP) than invasive mean (iMAP) pressures, agreeing with current clinical use of Doppler ultrasound to estimate the systolic BP. SNO demonstrated greater agreement with iSAP than ARPF measurements; a significantly greater proportion of visual (SNO) measurements were within 20 mmHg of the invasive technique (iSAP) compared to the audible (ARPF) method. The mean differences (95% limits of agreement) for SNO and APRF were −9.7 mmHg (−51.3–31.9) and −13.1 mmHg (−62.2–35.9), respectively, meaning both techniques tended to underestimate the true iABP value.. The CCC (95% CI) for SNO was 0.5 (0.36–0.64) and ARPF was 0.4 (0.26–0.54), meaning agreement was slightly better for the visual technique.
Overall, the collated evidence shows that when compared to the reference technique, both visual and audible methods performed poorly and underestimated the systolic blood pressure with poor precision. Therefore, they cannot be considered an accurate or reliable measure; when referring to the American College of Veterinary Internal Medicine criteria for a validation of NIBP devices neither technique came close. The invasive method, or a validated NIBP method, should therefore still be used over either SNO or ARPF method if maintenance of blood pressure within narrow margins is of critical importance.
Commenting on the research, Marc Armour said:
“Doppler ultrasound is still commonly used to monitor blood pressure under anaesthesia despite its known shortcomings.
We had noticed clinically that visual sphygmomanometer needle oscillation could occur before an audible pulse when measuring blood pressure via Doppler, so hypothesised this represented a more accurate estimate of iABP. Our findings identified that the visual, rather than the audible, Doppler signal did agree more closely with the reference invasive method.
This adds to the existing literature and helps to inform clinical recommendations that Doppler ultrasound should be utilised mainly for trends of blood pressure. In our study, both NIBP techniques performed poorly, especially compared to validated oscillometric devices. So caution should be used clinically when interpreting values obtained by Doppler ultrasound in anaesthetised dogs, particularly for diagnosing hypotension and hypertension. If there is a strong oscillation preceding audible pulse return whilst using Doppler, this may represent a more accurate iABP value.”
The Agreement of Doppler Ultrasound and Visual Sphygmomanometer Needle Oscillation with Invasive Blood Pressure in Anaesthetised Dogs research was supported by a CVS residency research grant and was undertaken as part of residency training. The research has been published in the journal, Animals.
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