NIBP monitoring Flashcards
Methods of manual blood pressure measurement
4
- Manual sphygmomanometer
- Mercury sphygmomanometer
- Aneuroid sphygmomanometer
- Oscillotonometer (von Recklinghausen oscillotonometer)
Manual sphygmomanometers: apparatus, how used with palpation
- Cuff enclosing an inflatable bladder
- Inflating bulb with bleed valve (red arrow)
- Gauge to read the pressure
Cuff (appropriately sized) placed circumfurentially around the arm (at level of heart), inflated to pressure above expeced systolic pressure, deflated slowly. BP can be determined by palpation or auscultation
Palpation:
* Cuff inflated while palpating radial pulse, to 30mmHg above point of dissapearance of pulse
* Cuff slowly deflated. Point of reappearance of pulse = systolic pressure
* Disadvantages: Underestimates systolic pressure. Diastolic and mean pressures cannot be detected
Manual sphygmomanometer: BP readings via auscultation
Korotkoff’s phases (5)
Stethoscope over brachial artery. Cuff inflated to above dissapearance of sounds (~systolic). Cuff slowly released at 2-3mmHg/s, listen for sounds.
Korotkoff phases:
1) First appearance of clear tapping sounds -> SBP
2) Sounds soften, ‘swishing’
3) Return of sharper sounds of maximum intensity
4) Abrupt muffling of sounds
5) Sounds dissapear completely -> DBP
Change in quality of sounds is because as deflation of cuff continues, there is greater opening of the artery until it remains open throughout the cardiac cycle
Disadvantage: subject to observer bias, measurement error
Mercury sphygmonanometer
Describe, disadvantages
- Inflatable cuff, inflating bulb, mercury column to read the pressure
- For accuracy: column should be vertical. When not in use the top of the mercury meniscus should read 0. The air vent at the top of the manometer should be patent
- Note rarely used in clinical practice. - concerns about mercury toxicity
Aneroid sphygmomanometer
Describe, disadvantage
- Similar to mercury sphygmomanometer but aneroid gauge replaces the mercury column
- Aneuroid gauge (see image): bellows connected to a dial pointer. Increase in pressure -> expands the bellows -> translated to the dial pointer -> dial pointer moves across the scale to indicate the pressure
- Require stethoscope to read diastolic pressure
- ** Disadvantage: become inaccurate over time.** Accuracy declines with use due to physical defects in the casing and tubing or the indicator needle drifting from 0
Von Recklinghausen oscillotonometer: apparatus
- Double cuff (black overlapping red), connected by black and red rubber pressure tubing, respectively, to a metal block at the base of the dial
- Larger (black) occluding cuff: like sphygmomanometer cuff. Smaller, sensing cuff amplifies the pulsations, seen as oscillations
- At the base of the case, metal block incorporating an adjustable leak valve and control lever (blue, which switches the dial between the two cuffs)
- Pressure tubings are connected to the bellows via the control lever. There is additionally an inflating bulb with adjustable leak valve
Oscillotonometry: method of measuring BP
As in a manual sphygmomanometer, cuff initially inflated above estimated SBP, then slowly deflated by releasing the bleed valve. Pulsation in the artery (once pressure in cuff <SBP) -> transmitted to the cuff -> pressure change within the cuff -> pressure changes displayed on the aneroid gauge as oscillations
- Cuff slowly deflated. Control level pullsed forward so needle jumps slightly in time with pulse
- Needle starts to jump more vigorously -> control lever released -> SBP displayed (1)
- Lever pullsed again and released at point of maximum oscillation -> MAP (2)
- At point where oscillations reduce -> diastolic pressure (3)
Von Recklinghausen oscillotonometer: advantages over manual sphygmomanometer, and disadvantages
Advantages:
* Does not require stethoscope
* Can read SBP, DBP and MAP (while manual sphygmomanometer can only read SBP and DBP)
Disadvantages:
* Manual device, subject to observer bias
* Diastolic pressure is less accurate
* Cumbersome/ complicated - not used clinically since introduction of automated devices
Automated BP measurement
Mechanisms (3), components, example
BP can be determined using one of three detection principles:
* Korotkoff’s sounds using a microphone
* Arterial blood flow using ultrasound
* Arterial blood flow using oscillometry (most common)
Components:
* Pneumatic pump: inflates the arm cuff.
* Solenoid valve: controls deflation of arm cuff
* May have 2 tubes (one for inflation, one for deflation), or a single tube for both.
* Pressure transducer: records oscillations within the cuff caused by the pulse
* Microprocessor: stores data, controls function of pneumatic pump and solenoid valve
* Display unit: SBP, MAP, DBP, HR (median rate during determination period) displayed
Output signal from transducer -> filter -> amplifier that amplifies the oscillations -> analogue digital converter (ADC) -> microprocessor
Example: DINAMAP
Describe function
DINAMAP
* Automated BP measurement
* Single cuff wrapped around patients arm
* Pneumatic pump inflates cuff to level assumed to be >SBP (160 initially, then 25 above pervious reading), then cuff is deflated gradually through solenoid valve
* Sensor measures tiny oscillations in pressure of cuff caused by pulse. Oscillations start -> SBP. Oscillations dissapear -> DBP
* Some devices only measure SBP and mean pressure and calculate the diastolic presure
Describe function
- Finapres (Finger Arterial Pressure) device
- Continuous NIBP measurement using finger clamp method
- Infrared photo-plethysmograph detects changes in volume of blood within the finger. Controller system adjusts pressure in small cuff placed around the finger to keep the volume of blood in the finger constant
- Applied pressure waveform correlates with the arterial blood volume, and therefore, with the arterial blood pressure
- Applied pressure is displayed continuously, in real time, as the arterial blood pressure waveform
Selection of correct cuff size
Method, sizes in infant/child/small/medium/large adult
Estimate the circumference of the upper am at midpoint between shoulder and elbow.
* Bladder inside the cuff should encircle at least 80% of the arm circumference.
* Width of the cuff should be 20% more than the diameter of the arm
* Cuff should cover 2/3 of upper arm
* Cuff placed so midline of bladder is over arterial pulsation
Correct width of cuff:
* Infant: 3cm
* Child: 6cm
* Small adult: 9cm
* Medium adult: 12cm
* Large adult: 15cm
Impact on BP measurement of incorrect cuff size
Too narrow cuff -> over-reading (inaccurately high reading)
Too large cuff -> under-reading (pressure easily transmitted to the artery, which can be compressed with relatively less pressure)
Cuff placed too loosely-> over-reading (because pressure is not adequately transmitted to artery)
Note how a large cuff
Causes of inaccuracy in NIBP reading
- Wrong size cuff: narrow cuff wrapped around fat arm -> abnormally high reading, and vice versa. Bladder of cuff should be >=80% circumference of arm
- Extremes of BP: non-invasive devices tend to over-read at low pressures, under-read at very high pressures
- Arrhythmias e.g. AF (as rely on regular pulse)
- Movement of the arm, shivering or external pressure
- Wrong position: arm should be at level of heart while taking reading
Specific to certain devices:
* von Recklinghausen oscillotonometer is accurate for MAP but inaccurate for diastolic BP
* Accuracy of aneroid sphygmomanometer declines with use
