Download Carotid Artery Tonometry: Pros and Cons

commentary
Carotid Artery Tonometry: Pros and Cons
Michael F. O’Rourke1
Palpation of the arterial pulse is one of the oldest and most
basic parts of the physical examination, and is classically
undertaken at the radial site. Paintings of medical practice
over centuries have come to typify the physician conversing
with the patient and in physical contact with the finger(s) over
the wrist in a gesture that combines humanity and trust, greeting, comfort and reassurance. To adequately feel the pulse, the
physician needs exert enough pressure to flatten the arterial
wall so as to gauge the pressure within.1 The physician will not
detect the pulse if he/she presses so lightly that the artery is
not deformed, or presses so heavily that the artery is occluded.
The physics of this process involves applanation (flattening),
and so, sensing the pressure within the artery by flattening
a small part of the arterial wall under the sensor2—which in
this case comprises the Paccinian corpuscles in the physician’s fingers over the flattened segment of artery. The pulse
feels stronger if arterial pressure within is high or if the radial
artery is large so that more Paccinian corpuscles are activated.
Instruments to measure the arterial pulse at the wrist
were introduced first by Marey in Paris then extended
by Mahomed in England and taken up for clinical use by
Mackenzie, Lewis and others3–5 for clinical practice. At
the turn of the 19th century the Dudgeon sphygmogram
(Figure 1) was widely used in specialist clinical and even
general clinical practice. Sensors for recording pulses in the
neck were introduced by Mackenzie, Lewis, Wiggers,3–5 and
others to record pulsations attributable to venous as well as
arterial pulsations. These combined a tambour (like a stethoscope head) and air-filled tubing connected to a stylus which
moved up and down with the carotid and jugular pulsations,
tracing a pulse waveform with the stylus moving on smoked
paper over a drum which was rotated by a clockwork motor.
Improvements on this purely mechanical technique were
used to obtain pressure wave tracings from the carotid arteries for measurement of systolic time intervals as indices
of left ventricular function in more recent times,6 and for
distinguishing the different effects of predominantly large
arterial vasodilators (such as nitrates and calcium-channel
blockers), and arteriolar vasodilators (such as hydralazine
and alpha-adrenergic blockers).7
Applanation tonometry has long been used by ophthalmologists on the eyeball to measure intra-ocular pressure
in patients with suspected glaucoma. This technique flattens
(applanates) a small part of the eyeball under a tiny sensor.
It was first applied to measurement of the arterial pulse by
Drzweicki et al.2 and then popularized with introduction of
the very precise applanation tonometer by Huntley Millar as
a derivative of his catheter tip intra-arterial sensor.8 Utility
and accuracy of this for noninvasive determination of the
arterial pulse was established by Kelly et al.8, and the instrument is now widely used in clinical practice and research
to measure arterial waveforms, and arterial stiffness from
determination of pulse wave velocity (PWV) over segments
of artery. A scientific statement on arterial stiffness from the
American Heart Association has just been released.9 This
relies heavily on use of applanation tonometry for measurement of aortic stiffness, and its direct and indirect effects
from change in timing and amplitude of wave reflection.
Use of applanation tonometry in the neck over the carotid
artery remains central to measurement of arterial stiffness
as “aortic” carotid/femoral PWV (cf-PWV) since it depends
on measurement of wave speed along the wall of the arterial
segment through identification of a feature (usually the wave
foot) which is not affected by wave reflection. Through identification of the same feature on the femoral artery and from
the time delay and distance between carotid and femoral
sites, one can measure cf-PWV. The most recent9 and earlier10
consensus documents identify this as the most useful clinical
measure of arterial stiffening with age, and an independent
predictor of cardiovascular events. Use of carotid tonometry
for measurement of cf-PWV requires gentle pressure on the
carotid artery in the neck, just sufficient to identify its most
obvious feature—a sudden upstroke which identifies the
beginning of left ventricular ejection into the aorta.
Carotid tonometry is now less favored for pulse wave
analysis than in the past. It is difficult to be certain of obtaining adequate applanation since the artery can move freely
under the sensor, and needs to be stabilized by pressure on
other neck structures by the operator. Artifact is common—
as from respiration and the procedure is uncomfortable to
many.11 Further it carries the possibility of activating baroreceptors, so leading to reflex changes in heart rate and arterial
pressure. It also carries the risk of dislodging carotid arterial
plaque or thrombus. Other problems with carotid tonometry arise from uncertainty in its calibration from conventional brachial cuff and upper limb tonometry. Anomalous
Correspondence: Michael F. O’Rourke ([email protected]).
1St. Vincent’s Clinic, University of New South Wales, Victor Chang Cardiac
Research Institute, Darlinghurst, Australia. Initially submitted August 25, 2015; date of first revision September 15,
2015; accepted for publication November 23, 2015; online publication
December 18, 2015.
296 American Journal of Hypertension 29(3) March 2016
doi:10.1093/ajh/hpv194
© American Journal of Hypertension, Ltd 2015. All rights reserved.
For Permissions, please email: [email protected]
Commentary
Figure 1. Dudgeon sphygmogram as used by Sir James Mackenzie circa 1900.
results12 are attributable to failure of applanation at the brachial as at the carotid sites.13–15 The original European10 and
the recent AHA9 consensus statements do not recommend
tonometry for waveform analysis at either the brachial or
carotid sites. Waveforms derived therefrom have not been
shown to predict risk of cardiovascular events. The favored
site for applanation tonometry remains the same (radial) site
which has been used over eons past.1,3,5
Carotid tonometry and potential problems with this
are reviewed in the present issue of American Journal of
Hypertension16 on the basis of a study by an experienced nurse
in 26 patients, and with attention to time delays and changes in
arterial pressure and heart rate. Findings confirm that carotid
applanation tonometry as used for determining cf-PWV can
be used with little change in heart rate or arterial pressure,
whether used sequentially or simultaneously at the carotid or
femoral site. A study such as this cannot however address the
more serious issues of transient asystole and fitting (which the
writer has seen) or of transient contra lateral hemiplegia (of
which the writer has been informed). These problems are likely
to be seen only in a large cohort and may not be reported. They
are possible and need be considered as a reason for approaching carotid tonometry with caution, for using radial artery
tonometry where possible for generation of central aortic
pressure, and never applying pressure simultaneously on both
carotid arteries as mentioned in this article. I would advise use
of carotid tonometry with gentle pressure only for measurement of carotid femoral PWV, and for checking unusual central pressure waves generated from the radial waves, and not
for routine measurement of central pressure waveforms. DISCLOSURE
Michael O’Rourke is a founding director of AtCor
Medical Pty Limited, manufacturer of systems for analyzing
the arterial pulse and Aortic Wrap Pty Limited, developer of
devices to improve aortic distensibility, and a consultant to
Novartis and to Merck.
REFERENCES
1. O'Rourke MF, Avolio AP, Kelly RP. The Arterial Pulse. Lea & Febiger:
Baltimore, 1992.
2. Drzewiecki GM, Melbin J, Noordergraaf A. Arterial tonometry: review
and analysis. J Biomech 1983; 16:141–152.
3. Mackenzie J. The Study of the Pulse: Arterial, Venous, and Hepatic, and
the Movements of the Heart. Young J. Pentland: Edinburgh, 1902.
4. Lewis T. Diseases of the Heart. MacMillan: London, 1934, p. 49.
5. Wiggers C. The Pressure Pulses in the Cardiovascular System. Longman:
London, 1928.
6. Weissler AM, Hussmave WS, Schoenfeld CV. Systolic time intervals in
heart failure in man. Circulation 1968; 37:149–159.
7.Safar ME, Toto-Moukouo JJ, Bouthier JA, Asmar RE, Levenson JA,
Simon AC, London GM. Arterial dynamics, cardiac hypertrophy, and
antihypertensive treatment. Circulation 1987; 75(1 pr 2):I156–I161.
8. Kelly RP, Hayward CS, Ganis J, Daley JE, Avolio AP, O'Rourke MF.
Non-invasive registration of the arterial pressure pulse waveform using
high-fidelity applanation tonometry. J Vasc Med Biol 1989; 1:142–149.
9. Townsend RB, Wilkinson IB, Schiffrin EL, , Avolio AP, Chirinos JA,
Cockcroft JR, Heffernan KS, Lakatta EG, McEniery CM, Mitchell
GF, Najjar SS, Nichols WW, Urbina EM, Weber T; American Heart
Association Council on Hypertension. Recommendations for improving and standardizing vascular research on arterial stiffness: a scientific
statement from the American Heart Association. Hypertension 2015;
66:698–722.
10. Laurent S, Cockcroft J, Van Bortel L, Boutouyrie P, Giannattasio C,
Hayoz D, Pannier B, Vlachopoulos C, Wilkinson I, Struijker-Boudier
H; European Network for Non-invasive Investigation of Large Arteries.
Expert consensus document on arterial stiffness: methodological issues
and clinical applications. Eur Heart J 2006; 27: 2588–2605.
11. Nichols WW, O’Rourke MF, Vlachopoulos C. McDonald’s Blood Flow in
Arteries, 6th edn. Arnold Hodder: London, 2011.
12. Verbeke F, Segers P, Heireman S, Vanholder R, Verdonck P, Van Bortel
LM. Noninvasive assessment of local pulse pressure: importance of brachial-to-radial pressure amplification. Hypertension 2005; 46:244–248.
13. O’Rourke MF, Adji A, Hoegler S. Calibration of non-invasively recorded
upper limb pressure waves. Hypertension 2005; 46:e15–e16.
American Journal of Hypertension 29(3) March 2016 297
Commentary
14. O’Rourke MF, Adji A. Guidelines on guidelines: focus on isolated systolic hypertension in youth. J Hypertens 2013; 61:649–654.
15. Adji A, O’Rourke MF. Brachial artery tonometry and the Popeye phenomenon: explanation of anomalies in generating central from upper
limb pressure waveform. J Hypertens 2012; 30:1540–1551.
298 American Journal of Hypertension 29(3) March 2016
16.Spronck B, Delhaas T, Op ‘T Roodt J, Reesink JDA, Carotid artery
applanation tonometry does not cause significant baroreceptor activation. Am J Hypertens, in press.