Noninvasive Blood Pressure Measurement © D. J. McMahon 2014 rev

Four types of clinical equipment: Monitoring: Detects and displays physiologic data for the purposes of clinical observation. Diagnostic: Detects, displays, and records physiologic data in order to confirm or rule out a disorder or disease. Therapeutic: Applies treatment in the form of energy or medication in order to treat a disorder or disease. Assistive: Used as an accessory to facilitate a medical or surgical procedure.

Bourdon Tube Manometers:

Diaphragm Type Manometers:

Available on the FacWeb site

‘PSIA’ vs ‘PSIG’ : > PSIA means pounds per square inch, including the ambient pressure. i.e. the absolute total pressure. > PSIG means pounds per square inch, less the ambient atmospheric pressure. i.e. “gauge pressure” Unless marked otherwise, a gauge is assumed to indicate PSIG.

Plethysmography Detection of volume change, not pressure, not flow. ( “plethysmos” = Greek for “to fill or inflate”)

Why was the Mad Hatter ‘mad’ ??

Pressure measured as a column of mercury:

Blood pressure is almost universally expressed in mm of Hg, although the SI unit is the Pascal. For blood pressure, the Pascal is too small, and kiloPascals are too large, resulting in poor resolution for recording blood pressure. Using “mm of Hg” gives better resolution. An alternative name for mm of Hg is the Torr. Note: NOT a B.P. gauge

Electronic analog of the vascular system:

Auto-regulation of blood pressure in the vascular system: Baroreceptors in the vessels, the sympathetic nervous system, the vagus nerve, and the heart act together as a servo-mechanism to regulate blood pressure.

Diastole: The period of ventricular refill; baseline pressure. Systole: The point of maximum ventricular ejection; maximum pulse pressure.

Wiggers diagram

Pressure in the system downstream from the left heart:

Manual determination of blood pressure :

Blood pressure determination by auscultation i.e. the manual (Riva-Rocci) method:

Most common source of error in manual BP: deflating the cuff too quickly AHA recommends deflation of 2mm Hg per second

Cuff size makes a difference:

Importance of cuff size in noninvasive BP methods: A cuff that is too small produces high errors. A cuff that is too large produces low errors.

Blood pressure determination by oscillometry i.e. the automated electronic method:

Pressure pulsations during oscillometry:

What is the difference between BP by auscultation (the ‘manual’ or Riva-Rocci method), and BP by oscillometry (the electronic method) ??

In the auscultatory (manual) method, the systolic and diastolic pressure values are determined by the onset and disappearance of the Korotkoff sounds. In the oscillometric (electronic) method, the systolic and diastolic pressure values are determined by the onset and disappearance of pulsations of an artery. The manual method depends on good hearing and good technique when deflating the cuff. The electronic method depends on a good algorithm in the firmware, and reliable components.

Pressure steps in the cuff as a determination is taken during an oscillometric BP cycle:

Generic automated noninvasive blood pressure monitor:

Example of the architecture of a generic “DinaMAP”

Typical algorithm for a NIBP monitor: There are many variations on the software and hardware, and all are proprietary. Almost all NIBP systems are programmed to pump up to 170 mm Hg on the initial cycle. After that, the unit will pump to 20- or 30-mmHg above the last recorded systolic value. If it detects pulses at that point, it re-tries at a higher point until it captures the systolic.

Two-hose cuffs vs Single-hose cuffs: Primary proponent of the double-hose cuff is Critikon’s (now GE Medical) DinaMAP. With two hoses, one is used for pumping the cuff up, while the other is only used for sensing the pressure waveform during deflation.

Blood Pressure Determination by Ultrasound: Alternative NIBP methods:

The FinaPres © by Ohmeda: Continuous NIBP Late 80’s Difficult to keep stable Never sold well Alternative NIBP methods:

Tensys “TL 300”: continuous NIBP Current, but uncertain success Alternative NIBP methods:

Indirect Blood Pressure Determination Review – > Method of Riva-Rocci: > Cuff compresses a large artery, and the systolic pressure is determined by the first audible ‘snap’ of the artery as turbulent blood escapes past the compressed area. > Diastolic pressure is determined as that point where turbulence is no longer heard.

Review – Indirect Blood Pressure Determination > Oscillometric Method (e.g. the DinaMAP © ): > Cuff compresses a large artery, and the systolic pressure is determined by the first pulsations sensed in the cuff by the electronics. First cycle pumps to ~160mmHg. > Mean arterial pressure taken as that point where the pulsations in the cuff are at maximum amplitude. > Diastolic is determined as that point where pressure pulsations are no longer sensed.

Alternative Indirect BP Measurements - >> Ultrasonic (Acoustic) method --- > Uses crystals that emit a signal toward the artery. > Artery is compressed by a cuff. > Arterial pulsations are detected by small signals returned to the crystal at a shifted frequency. > This Doppler shift is an audible frequency that can be heard, or sensed by electronics. Review – >> Continuous NIBP --- > The FinaPres © or the Tensys system

Troubleshooting NIBP - Most problems with NIBP units are pneumatic: leaks in the cuff, hoses, and hose connectors. Other problems: pump failures or stuck valves If an NIBP monitor has a significant leak in the pneumatics, it will attempt to pump up the cuff to the initial target of 170 mmHg, detect the leak, then re-attempt several more times. Typically after the three attempts, it will stop cycling and alarm.

Philips’ MP-series monitors: M3001A module:

Philips BP pump assembly for their M3000 series monitors:

There are many types of connectors used for BP:

Evolution of the original “DinaMAP”:

Others:

Modules:

Major players in the NIBP market - Free-standing monitors – - GE Medical (Critikon) - Datascope - Welch-Allyn NIBP integrated with other parameters – - Spacelabs - Philips - GE Medical (Marquette)

Consumer grade NIBP devices -