(Hypertension. 1995;26:307-314.)
© 1995 American Heart Association, Inc.
Articles |
Physiological Influences on Continuous Finger and Simultaneous Intra-arterial Blood Pressure
From the Cardiovascular Research Unit, Department of Cardiology, Royal North Shore Hospital, St Leonards (Sydney), NSW, Australia.
Correspondence to Prof Stephen N. Hunyor, Cardiovascular Research Unit, Department of Cardiology, Royal North Shore Hospital, St Leonards (Sydney), NSW 2065, Australia. E-mail crcctozemail.com.au.
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Abstract |
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Abstract Because of the clinical and experimental utility of continuous finger blood pressure measurements and the need for accuracy, we tested the performance of a new hydraulic device in 22 consecutive hypertensive subjects during physiological and pharmacological interventions. Ipsilateral brachial intra-arterial pressure was monitored during rest, Valsalva's maneuver, static handgrip, and mental arithmetic and after sublingual glyceryl trinitrate. In excess of 40 000 blood pressure values were analyzed. Average bias (intra-arterial minus finger blood pressure) was 8.2±17.0 mm Hg (mean±SD, P=NS) for systolic and 2.8±10.4 mm Hg (P=NS) for diastolic pressure. Two-way ANOVA of biases with subject and task factors showed a subject effect (P<.001). Intersubject and intrasubject standard deviations of bias were 13.8 and 9.8 mm Hg systolic and 8.7 and 5.7 diastolic, respectively. Linear drift (millimeters of mercury per minute) of finger pressure was greater (P<.001) for systolic than diastolic pressure during static exercise and math and after glyceryl trinitrate. Coefficients of determination for blood pressure ranged from 0.4±0.3 to 0.8±0.3 during the tasks. We conclude that (1) noninvasive finger blood pressure faithfully follows intra-arterial changes but with clinically relevant offsets, (2) this technique is best suited for assessing pressure changes, (3) physiological and pharmacological interventions do not consistently affect finger pressure accuracy, (4) many reports of finger blood pressure measuring devices are based on direct readings obtained with inadequate system response characteristics, and (5) the tested instrument falls short of the standards requirements (bias
5±8 mm Hg) for devices that measure
intermittently.
Key Words: bias • Valsalva's maneuver • blood pressure monitoring
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Introduction |
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TopAbstractIntroductionMethodsResultsDiscussionReferences |
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Volume-clamp plethysmography in the finger now provides a potential alternative to invasive arterial cannulation for continuous BP measurement.1 First proposed by Penaz,2 the technique has been implemented by several groups3 4 and also in the commercially available Finapres.5 The latter instrument has been compared to both IABP6 7 8 9 10 11 12 13 14 15 16 17 18 19 and arm-cuff BP values12 20 21 22 in various settings and found to have small average bias but large variability.13 21 23 24 25 26 27 We have used challenging physiological and pharmacological interventions to assess the application and accuracy of a prototype continuous, noninvasive, hydraulic FBP monitor compared with simultaneous ipsilateral brachial IABP in consecutive subjects using blinded observers. This instrument was initially developed3 28 to investigate BP biofeedback29 30 and is based on a pressurized finger cuff adjusted by a speaker coil mechanism controlled by a personal computer.31
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Methods |
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Twenty-two consecutive subjects in sinus rhythm (19 males) with treated hypertension (mean age, 57±12 years) gave informed consent for the study, which was approved by the Institutional Ethics Review Committee and carried out in accordance with institutional guidelines. The AAMI standard32 and Australian standard33 recommendations were followed with respect to age, numbers, and range of BP values. Seven subjects were aged 35 to 54 years; 14 were older and 1 was younger. Antihypertensive medications included vasodilators and/or calcium blockers (n=18), ß-receptor blockers (n=11), diuretics (n=3), and angiotensin-converting enzyme inhibitors (n=14), with some overlap.
With the use of the Seldinger technique a 110-mm polytetrafluoroethylene cannula (Seldicath 3F, Plastimed Laboratoire Pharmaceutique) was inserted with subjects under 2% lignocaine into the brachial artery 30 to 50 mm above the elbow crease. It was connected via 300-mm tubing to a high-fidelity pressure transducer (Physiological Pressure Transducer, Bentley Trantec Inc) that used a pressure monitoring kit (disposable dome, American Edwards Laboratories) and was in turn connected to a Sirecust 404-1 monitor (Siemens Aktiengesellschaft, Medical Engineering Group) that amplified and displayed the IABP values and waveforms. The monitor output was sampled by a custom-made 8-bit analog-to-digital convertor at a rate of 80 Hz. Before each study static calibration of the pressure transducer was performed with a standard mercury manometer. It was fixed at right atrial level and adjusted to atmospheric zero, which was regularly checked and corrected. The frequency response of the system was determined from the rise time after application of a step pressure. The damping coefficient of this system was 0.52±0.10, with an undamped resonant frequency of 23.9±5.9 Hz (range, 18 to 29.8 Hz).
FBP was measured with a CNIBP device that uses the principle of vascular unloading, implemented by the technique of finger volume clamping.2 The unit consists of an infrared finger photoplethysmograph (sensing at 940 nm) enveloped by an adjustable fluid-filled pressure cuff. CNIBP measurement is automatically initiated by stepwise adjustment of finger cuff pressure to achieve maximal plethysmographic finger pulsation from which a reference finger volume is derived. Cuff pressure then follows arterial pressure by clamping the finger volume to a constant value through a negative feedback loop designed to resist change in plethysmographic signal by continuous rapid adjustment of cuff pressure.3 An abridged algorithm occupying three heart beats adjusts the reference finger volume after every 50 heart beats to minimize drift.
Before the study the CNIBP monitor was calibrated with pressures up to 200 mm Hg. The middle finger was placed in the adjustable finger cuff with the infrared diodes positioned on the sides of the middle phalanx. Fluid-filled tubing connected to the finger cuff was attached to the subject at right atrial level (hydrostatic reference). This negated the effect of finger position and had a time constant of 0.7 second. SBP, mean BP, and DBP were detected in real time by custom-designed software operating on a Powermate I computer (NEC Information Systems, NEC Corp). Both IABP and FBP were recorded on computer disk and on a high-fidelity chart recorder (model ES1000, Gould Instruments). During measurements, seated subjects faced a videomonitor display of the CNIBP values and waveforms. One of two observers controlled either the FBP or IABP equipment during the study, each blinded to the other's result.
Subjects were kept warm with adequate clothing in a temperature-controlled room (21.5±1.5°C) during the study. If the optic plethysmogram signal was initially damped or distorted because of cold hands or finger movement, the finger was repositioned in the cuff and/or the hand was immersed in warm water for 2 to 3 minutes. Frequently, the signal improved spontaneously after several minutes, presumably because of resolution of local arterial constriction caused by the initial digital compression. BP values were compared during 20 minutes of sitting (REST) and subsequently during the performance of four tasks: (1) Valsalva's maneuver (VAL), during which an expiratory pressure of 30 mm Hg was used for 30 seconds; (2) static handgrip exercise (EXERC) to 30% of the maximal voluntary contraction for 4 minutes; (3) mental stress (MATH) as a harassed period of rapid serial subtraction of a two-digit from an initial four-digit number over approximately 2 to 3 minutes; and (4) administration of sublingual glyceryl trinitrate (TNG), 0.3 mg, with monitoring for approximately 10 minutes. The first three tasks were applied in random order after the REST segment. TNG was always given last. A 5-minute baseline period preceded each task.
Bias (IABP minus FBP) was calculated from beat-by-beat BP and is expressed as mean and SD of differences. We edited out noise-corrupted beats by excluding values at which the discrepancy exceeded 70 mm Hg (194 beats) and during self-adjustment of the CNIBP monitor for drift (339 beats). Beat-to-beat data were then reduced to more clinically meaningful five-beat averages for ANOVA and derivation of overall mean bias and SD. A two-way ANOVA with replication was used to further test for differences in accuracy across subjects and tasks. The corresponding components of variance and within- and between-subject SD were estimated from the expectations of the mean squares using the appropriate ANOVA model on the BP data averaged over five beats.
Agreement between IABP and FBP values was determined with the method of Bland and Altman,34 and a paired t test was used to test for significance of differences. The accuracy of the CNIBP monitor was further assessed by analyzing beat-to-beat BP data to (1) derive correlation coefficients for each task and the resulting r2 value and (2) calculate drift in bias between IABP and FBP by linear regression of pooled differences against time; the results were expressed as the average change of bias in millimeters of mercury per minute (per 30 seconds for Valsalva's maneuver).
A computer statistical package was used for the above analyses (Statistical Package for Interactive Data Analysis, SPIDA V6.0, Macquarie University, Sydney, Australia). Interpretation of repeated tests of significance used the recommendations of Hochberg and Benjamini35 with respect to large data sets.
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Results |
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Entry BP for the group was 150/85±19/14 mm Hg. A total of 43 567 data points were analyzed in the pressure range of 44 to 241/26 to 144 mm Hg (intra-arterial five-beat averages). All subjects had REST, EXERC, and MATH periods. One subject was unable to perform the Valsalva maneuver satisfactorily. Following a presyncopal episode in response to TNG in one early subject, five subsequent subjects with low resting BP (<120/69 mm Hg) did not receive the hypotensive agent. Fig 1 shows typical traces of beat-by-beat FBP and IABP during the tasks and illustrates that the essential characteristics of the IABP response were faithfully recorded by the finger monitor. The overall pooled five-beat clinically relevant average BP bias (IABP-FBP) was 8.2±17.0 mm Hg for SBP and 2.8±10.4 for DBP. These were not significantly different from zero (P>.05). The SBP and DBP biases in relation to the average of IABP and FBP values were also graphed according to Bland and Altman's34 recommendations for all tasks (Fig 2). There was no correlation between the FBP and reference IABP values (P=.08 for SBP and P=.20 for DBP). The coefficients of determination for the pooled five-beat average BP values were 0.82 (SBP) and 0.79 (DBP).
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Two-way ANOVA with replication of the five-beat averages for subject by task showed a large subject and a smaller task effect on both SBP and DBP bias (P<.001 for both) but with a strong interaction term (P<.001), indicating lack of consistency of effect from either task or subject. The relative percentage of contributions to variances of errors for SBP and DBP, respectively, were subject, 67% and 70%; task, 6% and 2%; interaction, 15% and 12%; and random error effects, 12% and 16%. The corresponding intersubject and intrasubject SD values for SBP biases were 13.8 and 9.8 mm Hg and for DBP, 8.7 and 5.7 mm Hg.
Average BP bias (millimeters of mercury), limits of agreement (mean bias±2 SD), coefficients of determination (r2), and drift in bias with time (millimeters of mercury per minute) calculated from beat-to-beat BP data are also reported separately for rest and the four tasks in Table 1. The positive bias for SBP was greater than that for DBP for all tasks except during VAL, when they were equal. However, biases were not significantly different from zero during any task except for SBP during EXERC (15.7±17.5 mm Hg, P<.01).
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Each subject's bias during rest and the four tasks is reported in Table 2. During rest and the tasks 10 to 12 subjects for SBP and 12 to 18 for DBP had a BP bias less than or equal to 10 mm Hg. The correlations between the CNIBP results and IABP (Table 1) documented closer correlations for SBP than DBP in all cases except during VAL. An estimate of drift in bias with time (changes in the difference between FBP and IABP in millimeters of mercury per minute), shown for each task in Table 1, was greater for SBP than DBP during the tasks, except during rest when they were the same. The drift was significantly different from zero only for SBP during EXERC, MATH, and TNG (P<.001 for all three), with values of 3.1±2.8, 3.5±4.9, and -0.9±1.2 mm Hg/min, respectively. This indicates a tendency for progressive underestimation of IABP by the finger device during EXERC and MATH and a reversal in this trend after TNG administration.
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Discussion |
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The ability to accurately monitor CNIBP and its changes with time and circumstances has many experimental and clinical applications. Overall, our CNIBP finger instrument follows intra-arterial changes faithfully, although large constant offsets occur in some subjects. A clinically relevant but not statistically significant overall bias of 8.2±17.0 mm Hg for SBP and 2.8±10.4 for DBP was seen. The intersubject and intrasubject SD values were greater for systolic (13.8 and 9.8, respectively) than for diastolic (8.7 and 5.7) bias. The cause of the large intersubject variability, although not addressed by this study, may be related to finger positioning within the cuff.31 In the absence of performance standards for CNIBP measuring devices, the AAMI standard32 and Australian standard33 recommendations developed for intermittent noninvasive instruments have usually been applied. In our study only the mean DBP bias (2.8 mm Hg) complied with the standards requirement. Several studies, mostly using the Finapres or its prototypes, have compared FBP with various reference methods, mostly radial intra-arterial values, during a variety of clinical states (Table 3). Some Finapres models11 13 27 meet the AAMI standard32 and Australian standard33 requirements with respect to mean bias (
5±8 mm Hg). Similar discrepancies between
intra-arterial and arm-cuff
readings12 36 suggest inherent limitations in intermittent
indirect BP measurement techniques.
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The present study has paid attention to the following design features: (1) "blinded" data acquisition, (2) use of consecutive subjects, (3) response characteristics of the direct pressure measuring system, and (4) analysis of BP tracking as well as "absolute" accuracy.
Comparison of IABP and FBP Measurements
We found that the FBP bias was neither greatly nor
consistently affected by the various tasks.
Eckberg37 has drawn attention to large variations in blood
flow during the Valsalva maneuver with the potential for creating
varying pressure gradients between the arm and hand. However, in the
present study despite the large changes in BP, autonomic activity,
and intrathoracic pressure, which occur with Valsalva's maneuver, the
FBP bias (5.0±19.1/5.0±9.8 mm Hg) during the
vasoconstrictive phase remained positive and comparable
to that of the resting state (9.5±15.6/3.7±9.8 mm Hg), in agreement
with the findings of Parati et al.17
Simultaneous FBP and IABP values have been reported during handgrip using the final 10 seconds of the test compared with a 20-second baseline period.17 We found a somewhat larger change in bias compared with the REST phase when analyzing the entire 4 minutes of exercise (6.2±1.3 mm Hg SBP and 1.1±1.1 DBP), which may relate to finger movement within the cuff.
Nitroglycerin exerts a "balanced" vascular action, resulting in venous pooling. Because of the limitation of venous capacitance in digits, the venous outflow pressure during volume-clamp plethysmography will approach arterial levels so that venous volume changes may compete with those arising from the arterial segment. We found that after sublingual nitroglycerin IABP was only slightly underestimated, whereas Parati et al17 found a directionally opposite change in bias.
We found that correlations between CNIBP and IABP were higher for SBP than for DBP during all tasks except Valsalva's maneuver (Table 1). Overall, 82% of SBP and 79% of DBP variation in IABP was predicted by FBP, in agreement with other studies.5 21 Friedman et al41 reported higher r2 values of 0.98 (SBP) and 0.85 (DBP) during head-up tilt. The small drift in bias seen at rest could be attributed to lack of finger movement within the cuff. During all physiological interventions drift was greater for SBP than DBP. Drift in FBP compared with intra-arterial levels has also recently been addressed with the use of time series analysis38 and spectral and sequence analysis.42 In the latter, specific frequency-domain and time-domain components of BP and pulse interval variability were properly assessed by FBP in most cases, although low-frequency spectral powers of SBP were overestimated.
Factors Contributing to Discrepancies in IABP and FBP
Values
Published studies show inconsistent magnitude, polarity,
and variance of discrepancies between FBP and IABP values. Generally,
FBP underestimates both SBP and DBP, possibly because of instrument
factors or anatomic differences. We have previously shown that finger
insertion and positioning within the cuff may be the most important
sources of variability.31
Finger vasoconstriction may also selectively affect FBP,9 11 19 but Wesseling et al39 found that FBP could still be reliably measured even during severe vasoconstriction. Although we did not measure the fingertip vasoconstriction status of our patients, the findings of Wesseling et al imply that the arteries in the cuffed finger are spared the vasoconstriction. The variability of results in the literature may arise from inadequate attention to characteristics of the intra-arterial measuring system. For faithful reproduction of an arterial waveform at a heart rate of 120 beats per minute (2 Hz), the fundamental frequency of the system should be greater than 20 Hz.43 We achieved a system frequency response of 23.9±5.9 Hz with a damping ratio of 0.52±0.10, avoiding artifactual amplification of SBP. A damping ratio of 0.52±0.10 placed our system within the response range (0.2 to 1.1 at 24 Hz) suggested as the minimum requirement by Gardner.44 Of the 18 studies listed in Table 3 that used an intra-arterial reference for validation, only 7 document both the frequency response and damping ratio of the IABP system,5 12 14 18 27 36 40 and only 25 40 with adequate frequency response also had a damping ratio close to or in the range suggested by Gardner44 (0.15 to 1.18 at 25 Hz). Because of the variable and often inadequate response characteristics of the intra-arterial systems used, the comparability of results from published validation studies deserves to be seriously questioned.
With existing technological capabilities, CNIBP monitors cannot be optimally compared with IABP systems. An ipsilateral cannula, particularly in the radial artery, may interfere with pressure waveform transmission to the finger, whereas comparison with the contralateral arm creates a systematic anatomic difference possibly varying with physiological status. Also, pulse amplification toward the periphery from summation with reflected waves45 results in higher SBP levels, whereas mean BP and DBP diminish because of the effect of resistance to flow. The lower SBP at the finger in the present study indicates either a general pressure drop predominating at the smaller arteries of the periphery, perhaps caused by volume clamping interfering with the reflection mechanism of pulse amplification, or an underestimation of SBP by the CNIBP instrument. However, demonstration of SBP overestimation by finger pressure with the use of spectral analysis42 suggests pulse amplification at this level. We conclude that noninvasive beat-to-beat FBP measured by volume-clamp plethysmography tracks BP levels reliably but has clinically relevant offsets under diverse physiological states and on a background of different vasoactive antihypertensive therapies. Current capabilities do not allow partitioning of this "inaccuracy" into device and anatomic components. Most reports of FBP measurement accuracy are based on comparison with direct readings obtained with inadequate system response characteristics. There are no current standards for performance of CNIBP measuring devices, and only a few such monitors comply with AAMI standards developed for instruments that measure intermittently.
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Selected Abbreviations and Acronyms |
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Acknowledgments |
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This research was supported by the National Health and Medical Research Council, National Heart Foundation, North Shore Heart Research Foundation, and Government Health Employees Medical Research Fund. We wish to thank Dr Anastasia Mihailidou and Judith Timpson for assistance in preparation of this article.
Received March 17, 1995; first decision March 22, 1995; accepted April 24, 1995.
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References |
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S. N. Hunyor, R. J. Henderson, S. K. L. Lal, N. L. Carter, H. Kobler, M. Jones, R. W. Bartrop, A. Craig, and A. S. Mihailidou Placebo-Controlled Biofeedback Blood Pressure Effect in Hypertensive Humans Hypertension, June 1, 1997; 29(6): 1225 - 1231. [Abstract] [Full Text]
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