Noninvasive Blood Pressure Measurement D. J. McMahon 141018 rev cewood 2017-10-23
Key Points Noninvasive Blood Pressure Measurement: - ways to measure pressure, direct mechanical measurement: Bourdon Tube Manometers Diaphragm aneroid Manometers Pressure measured as a column of Mercury - common unit of measure for blood pressure, why it is used (and not kpa, psi, etc) - the other name for mmhg - understand the relationship of diastolic pressure, systolic pressure, and mean arterial pressure - understand auscultatory blood pressure method (Riva-Rocci) - be familiar with the Karotkoff sounds video at: http://natok24.com/video/korotkoff-blood-pressure-sights-and-sounds-sd/ragxm32ftbu.html - understand oscillometric blood pressure method (eg DinaMAP, etc) - know the effects of cuff leaks or tubing kinks and wrong-sized cuff - know how frequently NIBP measurements may be taken - know how ultrasonic measurement of BP works - is ultrasonic measurement auscultatory or oscillometric?
Plethysmography ple-thys-mog-raph-y Detection of volume change, not pressure, not flow. ( plethysmos = Greek for to fill or inflate )
Bourdon Tube Manometers:
Diaphragm Type aneroid Manometers: Aneroid: barometer that measures air pressure by the action of the air in deforming the elastic lid of an evacuated box or chamber
Available on the FacWeb site
Pressure as a column of mercury barometer manometer
Manometers, two styles; Pressure measured as a column of mercury: barometer manometer absolute pressure closed end (contains vacuum) measures atmospheric pressure 760 mmhg normal (at sea level) higher atmospheric pressure = higher number (pressure in outer space vacuum would be 0 mmhg) gauge pressure open end (contains ambient air pressure) measures applied pressure 0 mmhg is unpressurized state higher applied pressure = higher number
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 Displaying ambient room air pressure, a gauge showing PSIA would display 14.7 psi a gauge showing PSIG would display 0 psi Unless marked otherwise, a gauge is assumed to indicate PSIG.
Pressure Conversions (most useful conversions shown in bold) PSI KiloPascal cm of H 2 O mm of Hg atmosphere millibar 1 PSI = 1 6.89 70.3 51.7 0.068 68.9 1 KiloPascal = 0.145 1 10.19 7.5 0.0098 10 1 cm of H 2 O = 0.014 0.1 1 0.735 0.00097 1 1 mm of Hg = 0.019 0.133 1.36 1 0.0013 1.33 1 atmosphere = 14.7 101 1033 760 1 1013 1 millibar = 0.0147 0.1 1.02 0.75 0.0009 1 remember: 1 atmosphere = 14.7 psi = 760 mmhg = 1033 cmh 2 O = 101 kpa Hg: 760 mm = 29.92 inches H 2 O: 1033 cm = 33.9 feet
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. poor resolution 120 mmhg = 16,000 Pascal = 16 kpa = 2.32 psi
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 ( di-as-toe-lee ): The period of ventricular refill; baseline pressure. Systole ( SIS-toe-lee ): The point of maximum ventricular ejection; maximum pulse pressure.
the blood pathway, showing decreasing pressure
Pressure in the system downstream from the left heart:
Wiggers diagram
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.
Alternative NIBP methods: 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 http://tensysmedical.com/us/
Indirect (non-invasive) Blood Pressure Determination > 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.
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.
Proper method for measuring blood pressure When measuring blood pressure in the home, an accurate reading requires that one not drink coffee, smoke cigarettes, or engage in strenuous exercise for 30 minutes before taking the reading. A full bladder may have a small effect on blood pressure readings; if the urge to urinate arises, one should do so before the reading. For 5 minutes before the reading, one should sit upright in a chair with one's feet flat on the floor and with limbs uncrossed. The blood pressure cuff should always be against bare skin, as readings taken over a shirt sleeve are less accurate. The same arm should be used for all measurements. During the reading, the arm that is used should be relaxed and kept at heart level, for example by resting it on a table. Hazard of frequent NIBP measurements Do not repeat NIBP measurements (particularly STAT measurements) at intervals less than 3-5 minutes over an extended period of time. Rapidly repeating measurement can impair circulation in the monitored limb.
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. >> 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 -