1 Department of Anesthesiology and 2 Department of Cardiothoracic Surgery, Wake Forest University School of Medicine, Medical Center Boulevard, Winston-Salem, NC 27157, USA. 3 Department of Medicine, University of New South Wales, St Vincents Hospital, Sydney, Australia
*Corresponding author. E-mail: apauca{at}wfubmc.edu. Reprints will not be available from the author
Accepted for publication: December 23, 2003
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Abstract |
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Methods. We compared simultaneously recorded radial and aortic pressures from 21 anaesthetized adult patients using identical fluid-filled pressure measuring systems.
Conclusions. The second radial pressure peak agreed with that in the aorta within a mean of 0.6 (SD 1.5) mm Hg. The difference between DP in the aorta and radial artery was 1.4 (2) mm Hg. The radialaortic SP and pulse pressure differences were 5.9 (7.6) and 7.3 (7.6) mm Hg, respectively. These results confirm that when the radial artery pressure wave shows a first and second, or only a second systolic shoulder/peak (on the right side of the pressure wave), the second represents the maximal ascending aortic SP, and that the radial and aortic DP are equivalent, even in older hypertensive patients.
Br J Anaesth 2004; 92: 6517
Keywords: age factors; arterial pressure, hypertension; heart, radial systolic pressure; heart, systolic pressure differences
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Introduction |
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Using identical pressure systems to record pressure in the aorta and radial artery, we assessed agreement between aortic pressure and the second deflection of pressure in the radial artery, and the agreement between DP in the radial artery and aorta in older5 or hypertensive patients.
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Methods |
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In this study, the terms elderly or older refer to patients over 60 yr. Above this age, SP continues to increase in both normal and hypertensive patients, while DP declines.5 Consequently, therapeutic decreases in SP might enhance this age-related decrease in DP in elderly patients. To assess this possibility, we used the ratio of DP to SP, which expresses DP as a fraction of SP.
Statistics
We used the paired t-test to compare radial and aortic second systolic peaks, SP, DP, pulse and mean arterial pressures. P<0.05 was considered significant. We plotted the difference between radial and aortic pressure against the arithmetic mean of these values to display graphically both the size of the difference between the radial and aortic pressures measured and their maximal differences (BlandAltman plot).11 All data are expressed as mean (SD).
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Results |
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Discussion |
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These data also confirm that the agreement between the aortic and radial DP in normotensive patients6 persists in older hypertensives. Thus, the large overestimation of directly measured brachial DP by cuff sphygmomanometry (18 (12) mm Hg in patients with isolated systolic hypertension15) is not because of lower DP in the radial artery, but the inability of non-invasive pressure measuring systems to measure true DP in these patients.9
Representation of the aortic SP by the second radial systolic peak implies that radial and aortic SP are equivalent when the second radial systolic peak is equal to or greater than the first. Thus, the radial and aortic SP were equal in 23 of the 45 patients (51%) before, and in 8 of 21 (38%) during anaesthesia. However, this study was not designed to assess the effect of age or treatment on the aortic and radial SP, so agreements of these estimates are provisional.
Estimates of the aortic SP and DP made from the radial artery pressure wave allow rapid assessment of the effect of reducing arterial pressure on aortic pressures in older hypertensive patients, in whom aging and hypertension increase the pulse pressure5 16 by increasing SP and decreasing DP. Arterial pressure reduction by a treatment may reduce pressures to a point where the reduction of DP (a major coronary perfusion pressure factor) is relatively greater than the reduction of SP4 9 (left ventricle work load). In the radial artery, the aortic pulse pressure is the difference between the second systolic deflection and DP, therefore observing it before and early on during anaesthesia would reveal whether the DP is greater than the pulse pressure for any SP, as it normally is in young8 and middle-aged6 normotensive adults, and what effect anaesthesia has on such a relationship. We incidentally noted in this study that aortic pulse pressure was greater than DP in more than 50% of patients before anaesthesia, and that anaesthesia improved this relationship by decreasing the pulse pressure in most patients (Fig 1, panels A, B and D), but had the opposite effect in three patients. The present findings may help in evaluating the effects of anaesthesia (or any vasoactive treatment) on the true aortic pulse pressure by comparing the DP/SP ratio produced by such treatment with known values in normotensive adults.6 8 This ratio estimates DP as a fraction of SP and is 0.66 for awake normotensive adults aged 35 (11) yr8 and 0.62 for normotensive, anaesthetized patients aged 54 (12) yr.6 In Figure 1 and Table 3, this ratio was 0.48 at the beginning of anaesthesia, increased to above 0.56 during anaesthesia, and reached 0.64 when assessed in the aorta. However, in three of the 45 patients, it decreased from 0.48 to 0.30.4. Although there are several possible causes of low DP in older hypertensives (low heart rate, central venous pressure and/or stroke volume),2 4 17 in these three patients a heart rate below 50 beats min1 seemed to be the reason.
Analysis of the radial artery waveform alone has usually been disappointing;18 however, simultaneous observation of radial and aortic pressure waves has shown that the late radial systolic deflection, which appears at age 40 yr and continues increasing to reach the height of the first peak by the eighth decade,7 is closely associated with aortic SP.4 Aging and hypertension increase aortic SP by increasing the second aortic systolic deflection19 20 and, coincidentally, the height of the second peripheral systolic peak also increases.1 4 Also, in older patients with arterial degeneration,19 the aortic pressure wave reaches peripheral arteries with minimal distortion. Thus, there are persuasive reasons for the presence of the second systolic shoulder/peak on the radial pressure wave in older hypertensive awake patients. In fact, the late tidal wave (second systolic peak) was recorded for the first time in hypertensive patients in 187221 using a mechanical sphygmographpolygraph applied on the radial artery at the wrist.
Recently, second systolic peaks from the left ventricle,22 ascending aorta and peripheral arteries have been recorded with great fidelity using fluid-filled clinical catheter systems with frequency response of 1112 Hz and damping coefficient of 0.15.19 23 Additionally, diastolic and the second systolic shoulder features of the radial pressure pulse are included within 48 Hz.24 25 In contrast, signals with a high frequency content such as rate of rise, first peak and incisura of pressure waves at heart rates of 120 beats min1 require pressure measuring systems with a frequency response above 20 Hz.10 22 26 Therefore, we maximized the frequency response of the systems used in this study to record undistorted aortic pressure waves, which retain greater details than clinically used radial pressure, as can be seen in Figures 1 and 2. There are clinical states when the second radial systolic deflection is poorly defined or absent in awake patients.20 However, the most common cause of blurred or hidden second radial systolic deflection is pressure wave amplification caused by the observer, for example by changing a pressure wave displayed on four separate channels to full-screen display where all pressures show on the same scale. This manoeuvre increases the amplitude of the displayed signal from 100 mm Hg cm1 to 25 mm Hg cm1, which straightens the rises and falls of the pressure wave, reduces the separation between the first and second systolic peaks when they are at similar levels, and changes the second shoulder into a convexity when the second shoulder is lower than the first. This manoeuvre would have turned the second radial artery shoulder in Figures 1 and 2 into poorly defined convexities. This effect arises whether microtip1 27 or high-pressure fluid-filled catheter18 systems are used. We verified this distorting effect by amplifying digitized pressure waves from a previous study 24 times.12
A practical approach to identifying second radial systolic peaks is to consider the SP wave, from its foot to the incisura, as three fractions. The first systolic peak is located on the first fraction (0.080.12 s from the foot of the wave), and the second on the third fraction on the right (approximately 0.25 s from the foot of the pressure wave).
Limitations
In this study, the pressure waves were compared after a long period of anaesthesia and administration of a large dose of heparin,13 which erased the second radial systolic peak in more than half the initially sampled group. This was unavoidable because this was the only chance to record radial and aortic pressure waves concurrently with minimal changes in clinical management. Use of paper recorder instead of A/D conversion did not permit recording radial artery pressures at 25 mm s1 for long periods of time in all patients because of the large amount of paper needed. However, these limitations did not hinder the evaluation of aortic SP and DP by the second radial systolic peak/shoulder and the radial diastolic pressure, respectively.
Clinical implications
At present, the incidence of isolated systolic hypertension is as high as 54% at 5059 yr, and 87% of those aged 60 years and over; diastolic hypertension prevails in a small number of hypertensives below 50 yr of age.16 Systolic hypertension is more difficult to control in older than in younger patients.28 Thus, most treated older hypertensives may present for anaesthesia and major surgery with SP above 140 mm Hg and DP below 80 mm Hg (85% of 45 in the present group). Reduction of SP by anaesthesia to a desirable clinical level (about 120 mm Hg) is usually easy, but can cause a low DP (e.g. below 40 mm Hg), which, if ignored, could lead to myocardial ischaemia in elderly hypertensives.29 However, the present findings indicate that invasive radial artery pressure monitoring provides SP and DP measurements in close agreement with those in the aorta of older hypertensives. Thus, anaesthetists can accurately estimate aortic SP and DP using the radial artery waveform, unlike epidemiologists and other clinicians who have to guess, by sphygmomanometry, whether an arterial pressure of 140/70 mm Hg is 140/70 or 140/30 mm Hg in the aorta.15 Whether reduction of arterial pressure from 160/
80 to
120/
40 mm Hg by anaesthesia is the cause of perioperative myocardial ischemia30 or of adverse outcomes from coronary bypass surgery31 in patients with isolated systolic hypertension remains to be investigated.
In summary, we found that the second radial artery systolic peak, when visible, represents the maximal aortic SP, and that the radial and aortic DP agree in hypertensives and elderly patients.
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