Blood Pressure Measurement

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Blood Pressure Measurement

• Jay Horrow, MD
• Professor and Chair, Anesthesiology
• Drexel University College of Medicine
• Philadelphia, PA

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• No conflicts to disclose

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ABA Content Outline Items

• I.B.1.b Transducers
• I.B.8.a Vascular Pressures
• I.B.9.d Pressure transducers resonance, damping
• I.B.9.e Non-Invasive BP Doppler, oscillometry,
  Korotkoff sounds, palpation
• II.B.1.d Blood Pressure
• Systolic, diastolic, mean, perfusion pressure
• Intracardiac, pulmonary, venous
• Systemic and pulmonary vascular resistance
  viscosity
• Baroreceptor function

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Outline

• What is blood pressure?
• Physiology of blood pressure
• Invasive measurement techniques
• Noninvasive techniques

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What is blood pressure?

• noun pressure exerted by the blood upon the
  walls of the blood vessels and especially
  arteries, usually measured on the radial sic
  artery by means of a sphygmomanometer, and
  expressed in millimeters of mercury either as a
  fraction having as numerator the maximum pressure
  that follows systole of the left ventricle of the
  heart and as denominator the minimum pressure
  that accompanies cardiac diastole or as a whole
  number representing the first value only lta blood
  pressure of frac120/80gt lta blood pressure of
  120gt -- abbreviation BP

www2.merriam-webster.com/cgi-bin/mwmednlm?bookMed
icalvablood20pressure, accessed 20-Aug-06
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Physiology of blood pressure
Bruner JMR Handbook of blood pressure
monitoring. PSG, Littleton MA, p.52
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Physiology of blood pressure

• All BPs are not created equal
• Arterial Site of measurementWhat the BODY does
  to the pressure wave
• Technique of measurementWhat our DEVICES do to
  the pressure wave
• PRESSURE ? FLOW
• Pressure wave travels rapidly in arteries
• Flow lags behind pressure
• Tsunami
• Buoy (or cork) floating in ocean

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What the BODY does
PRESSURE WAVE
Bruner JMR Handbook of blood pressure
monitoring. PSG, Littleton MA, p.57taken from
Hamilton WF, Dow P Am J Physiol 1939 125 48-59
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HOW this occurs

• Resonance in great vessels
• Energy reflection from periphery

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Physiology of blood pressure

• TIME v. FREQUENCY domains
• Continuous, repetitive signal (Fourier)
• Sum of sine waves, varying amplitude

From Bruner JMR Handbook of blood pressure
monitoring. PSG, Littleton MA, p.56 www.cage.curti
n.edu.au/mechanical/info/vibrations/tut1.htm
(20-Aug-06)
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Example Time v Frequency domain
www.answers.com/topic/triangle-td-and-fd-png
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The CV system is a vibrating system

• Periodic energy source
• Resistive impedance to flow (viscosity)
• Capacitive impedance (springiness)less at higher
  frequencies
• Accepts and stores energy spring
• Elasticity of aorta
• Capacitor in electric circuit
• Inductive impedance (inertia)more at higher
  frequencies
• Resists change in flow dashpot
• Mass of blood in aorta
• Inductor in electric circuit

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Vibrating systems Resonance

• Resonant (fundamental) Frequency
• Swing analogy T ?(L/g) ? 3 sec f 1/T
• Bay of Fundy 225 deep X 160 mi ? T6 hrs ? 25
  tide
• Resonant frequency ? pulse rate
• Cymbals or drum v. sound wave

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Resonance in Great Vessels

• fn ? 2- 10 Hz
• Two parallel circuits upper / lower

From Hamilton WF. Handbook of Physiology, 1963,
cited by Bruner JMR, p.59
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Resonance and Reflection

• Resonance in the arterial tree
• fn ? 2- 10 Hz
• Two parallel circuits upper / lower
• Pressure wave has most energy 5 -20 Hz, and CV
  system has fn 2 10 Hz ? amplification and
  attenuation occur
• Abrupt increase in impedance at arterioles ?
  reflection

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Impedance Matching

• Transfer of energy
• REFLECTED if low ? high impedance
• TRANSMITTED if matched impedance
• RETAINED if high ? low impedance
• The car analogy

• Back to the body
• Elastic aorta matches impedance of LV
• Mismatch at arterioles ? reflections
• Sea inlet analogy

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Short Review Elastic Aorta

• Minimizes work of the heart
• Low impedance to ejection
• Convert pulsatile flow to a more continuous flow
• Decreases required power (energy)
• Old fire engines same need
• windkessel air chamber
• Store energy at peak release later
• Aorta accomodates stroke volume via elasticity

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Recall
QUIZ Compare areas under the curves do they
change?
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Outline

• What is blood pressure?
• Physiology of blood pressure
• Invasive measurement techniques
• Noninvasive techniques

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How does a transducer work?
From Cliffe P. Transducers for the measurement of
pressure, inScurr C, Feldman S. Scientific
Foundations of Anaesthesia. Year Book, Chicago,
1982, p 40-52
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Two things to remember

• RESONANCE
• DAMPING

From Cliffe P. Transducers for the measurement of
pressure. In Scurr C, Feldman S. Scientific
Foundations of Anaesthesia. Year Book, Chicago,
1982, p 46
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DAMPING

• Tendency to stop oscillating
• Varies with frequency
• Hydraulic systems underdamped (?ltlt0.7)

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How to measure system fn and ?
From Blitt CD, Hines RL Monitoring in
anesthesia and critical care medicine, Churchill,
New York, 1995.
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Hydraulic system also has fn
RESISTIVE viscous drag of fluid CAPACITIVE elast
icity of tubing INDUCTIVE mass of fluid fn ? 10
- 20 Hz (4 tubing no bubbles) Add a small
bubble ? fn lt 8 Hz. RESULT higher BP frequencies
amplified To avoid overshoot push fn into high
region
HOW DO I DO THAT ???
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How to affect fn and ?
Stiffer tubing reduces capacitance.From Bruner
JMR, Handbook of blood pressure monitoring. PSG,
Littleton MA, p. 65
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How to increase fn and ?

• Practically, fluid type fixedHence, use to ?
• stiff tubing both
• small radius ?
• short length fn
• NOTE long tubing degrades system fidelity

From Millers Anesthesia, 6th ed, p. 1201
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Underdamped tracing
From Mark JB. Atlas of cardiovascular
monitoring, Churchill Livingstone, New York,
1998, p 107
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Large Abdominal Aortic Aneurysm
fn ? 20 Hz, underdamped
From Bruner JMR, Handbook of blood pressure
monitoring. PSG, Littleton MA, p.81
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Outline

• What is blood pressure?
• Physiology of blood pressure
• Invasive measurement techniques
• Arterial Cannulation
• Noninvasive techniques

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Arterial Cannulation

• Ulnar artery is LARGER than radial
• Allens test gt5 sec is abnormal
• Beware of false positives hand position
• Brachial artery abundant collaterals
• Axillary artery flush precautions
• Complications
• Hematoma nerve compression
• Infection stopcocks (16 contaminated)
• Thrombosis

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Thrombosis of Radial Arteries
From Bedford RD, Wollman H. Anesthesiology 1973
38228
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Recanalization of Radial Arteries
From Bedford RD, Wollman H. Anesthesiology 1973
38228
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Outline

• What is blood pressure?
• Physiology of blood pressure
• Invasive measurement techniques
• Arterial Cannulation
• Noninvasive techniques

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BP Occlusion Techniques

• Palpation
• Flush
• Ultrasound
• Oscillotonometry
• Oscillometry
• Auscultation
• Plethysmography

COMMON LIMITATION Cuff width must match limb
girth
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PALPATION

• Cuff that encircles the limb
• Palpate peripheral pulse
• Limitations
• SYSTOLIC pressure only
• Highly operator dependent
• Rapid deflation ? less precision
• Slower heart rate ? less precision

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Palpation is BIASED (v. invasive)

• Bias a systematic error
• UNDERESTIMATES systolic pressure
• At 120 mmHg, palpation ? 90
• BIAS increases as BP increases

Source van Berger FH et al. Circulation 1954
10 481
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Flush Technique

• Return of color to blanched extremity
• Pediatric use
• Recorded as mean pressure
• Sources of error
• Rapid cuff deflation
• Anemia, edema, vasoconstriction
• Moving subject
• Operator dependent

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Ultrasound Technique

• Flow detection beam along artery
• whoosh begins at systolic
• Arterial wall motion detection
• Systolic BP at 1st sound
• Diastolic BP when 2 sounds merge
• Small children
• Superseded by oscillometric techniques

From Stegall et al. J Appl Physiol 1968 25 793
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Oscillotonometry
Mode A During inflation Pressure in UC Aneroid
at A
Mode B During deflation Pulsations on LC Aneroid
at B
Cannot reverse UC and LC
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Oscillometry a single cuff
Systolic _at_max d/dt(amplitude)Diastolic _at_max
d/dt(amplitude)
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Oscillometry a single cuff
Systolic _at_max d/dt(amplitude) Diastolic _at_max
d/dt(amplitude)
From Ramsey M. J Clin Monit 1979 756
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Oscillometry

• Microprocessor automation ? cuff and monitor
  paired
• Different algorithms for S,M,D per manufacturer
• Convenient and reliable ? popular
• Correlations with invasive pressure vary by study
• Dysrhythmias can fool algorithms

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Auscultation 5 phases
From Geddes LA. The direct and indirect
measurement of blood pressure. Year Book,
Chicago, 1970.
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Auscultation 5 phases
Described by Korotkoff, 1905 Origins of sounds
still unclear Disagreement persists about
diastole
Phase I snapping sound SYSTOLIC Phase II
murmurs ? IGNORE Phase
IIIthumping Phase IV muffling and Phase
V silence ? DIASTOLIC
Hutton T, Prys-Roberts C. Monitoring in
anaesthesia and intensive care. Saunders, London,
1994, p113.
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Plethysmography

• Pressure in upper arm cuff when
• Pulsatile flow detected on pulse oxs
  plethysmographic trace.
• Important that device not revert to pulse
  search mode in interim
• Accuracy (v. invasive) similar to other indirect
  methods

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Effect of cuff size

• Von Recklinghausen, 1901
• 5-cm width v. 12-cm width
• Smaller cuff ? higher pressure reading
• Pressure not linear with internal diameter

From Blitt CD, Hines RL Monitoring in
anesthesia and critical care medicine, Churchill,
New York, 1995, p. 118
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Indirect v. Direct SYSTOLIC BP
From Mark JB. Atlas of cardiovascular monitoring.
Churchill Livingstone, New York, 1998, p. 86
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Cuff-related issues

• I.V. / pulse ox are not available
• Skin avulsion
• Petechiae
• Tourniquet effect
• Nerve compression
• apply well above elbow joint

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Review

• BP is like an elephant
• Pressure wave faster than flow
• Direct measurement transducer
• Resonance
• Damping
• Occlusion techniques

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References

• Bruner JMR. Handbook of Blood Pressure
  Monitoring. PSG Publishing, Littleton,
  Massachusetts, 1978.
• Hutton P, Prys-Roberts C. Monitoring in
  Anaesthesia and Intensive Care. Saunders, London,
  1994, p 105-144
• Mark JM. Atlas of Cardiovascular Monitoring.
  Churchill Livingstone, New York, 1998, p 81-126
• Szocik JF, Barker SJ, Tremper KK Fundamental
  Principles of Monitoring Instrumentation. In
  Millers Anesthesia, 6th ed, Elsevier, 2005, p
  1191-1226
• Cliffe P. Transducers for the measurement of
  pressure. In Scurr C, Feldman S. Scientific
  Foundations of Anaesthesia. Year Book, Chicago,
  1982, p 40-52
• Bedford RF, Shah NK. Blood pressure monitoring.
  Invasive and noninvasive. In Blitt CD, Hines RL.
  Monitoring in Anesthesia and Critical Care
  Medicine, 3rd ed., Churchill Livingstone, New
  York, 1995, p 95-130
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