non-invasive device, Finapres, which measures blood pressure continuously in the finger, was evaluated in 14 ... technique of blood pressure measurement.
Cardiovascular Research, 1988 , 22, 390-397
Continuous non-invasive blood pressure monitoring: reliability of Finapres device during the Valsalva manoeuvre BEN PM IMHOLZ,* GERT A VAN MONTFRANS, * JOS J SETTELS,+ GERARD M A VAN DER HOEVEN ,+ JOHN M KAREMAKER ,t WOUTER WIELING* From the Departments of*Internal Medicine and t Physiology, University ofAmsterdam; and the :f:TNO Biomedical Instrumentation Research Unit , A cademic Medical Centre , Amsterdam , The Netherlands .
Because of the inherent risks of intra-arterial blood pressure monitoring a new non-invasive device, Finapres , which measures blood pressure continuously in the finger, was evaluated in 14 hypertensive and one normotensive subject. Brachia! intra-arterial and finger pressures were compared during a control period and a subsequent Valsalva manoeuvre. Visually, blood pressures measured by Finapres faithfully reproduced the intra-arterial recordings in all subjects. From each pressure signal beat to beat systolic, diastolic, and mean blood pressure values and their differences were obtained and the time course of the response and its characteristic features were analysed. During the control period the Finapres measurements were lower than intra-arterial systolic, mean , and diastolic pressures (mean(SD) 1(9 .6), 9(6.8), and 4(6.1) mmHg respectively). During the response to the Valsalva manoeuvre the brachial-finger pressure differences showed limited deviation from those during the control period; median differences were at most 6 mmHg occurring late during the intrathoracic strain period and 7 mmHg during the post-release blood pressure overshoot. In general , the Finapres device reproduced intra-arterial patterns faithfully. This device appears to offer a reliable alternative to intra-arterial blood pressure monitoring. ABSTRACT
The present availability of a fully automated instrument, Finapres (Ohmeda Monitoring Systems , Englewood, Colorado , USA), that allows the continuous non-invasive measurement of arterial pressure may be the long awaited step forward in the technique of blood pressure measurement. 1 It is based on a method reported by Penaz in 1973 2 and has been developed by the Netherlands Organization for Applied Scientific Research , TNO - Biomedical Instrumentation. 3-S Another development using the Address for correspondence and reprints to : Dr B PM Imholz , Department of Internal Medicine, Academic Medical Centre , Room F4-257 , Meibergdreef 9 , 1105 AZ Amsterdam , The Netherlands .
Key words: continuous finger arterial pressure; Valsalva manoeuvre; hypertensive subjects; non-invasive blood pressure monitoring; intra-arterial blood pressure . Submitted 18 December 1987 Accepted 5 February 1988
same method of Penaz has been reported by Y amakoshi and colleagues .6 7 Previous evaluation studies of the Finapres device either involved anaesthetised patients to assess the usefulness of the device in the operating room 3 s- 11 or used early prototypes. 12 No study has compared blood pressures on a consecutive beat to beat basis . In this study we assessed the performance of a TNO model 4 Finapres , which is now commercially available as the Ohmeda 2300 Finapres BP monitor. We compared finger arterial pressures recorded with Finapres with simultaneous intra-arterial pressures recorded in a group of awake subjects. Since in cardiovascular and physiological laboratories there is a particular interest in the study of cardiovascular reflexes, we determined whether or not Finapres reproduced intra-arterial pressures accurately and reliably on a beat to beat basis during the large and instantaneous blood pressure changes induced by the Valsalva manoeuvre .
390 .
Reliability of Finapres during Valsalva manoeuvre
Methods PRJNCJPLES OF OPERATION
The Finapres method and technical details have been described elsewhere .3- 5 Briefly , the instrument uses the volume clamp method of Penaz. 2 13 The blood volume under an inflatable finger cuff is measured with an infrared photoplethysmograph , which is mounted inside the cuff. The finger cuff is connected to a small box (the front end) attached to the hand (fig 1). The box houses a fast proportional pneumatic valve, a pressure transducer, and the electronics for the plethysmograph. The blood volume as seen by the plethysmograph is clamped to a setpoint value by appropriately adjusting cuff pressure in parallel with intra-arterial pressure by means of an electropneumatic servosystem. The servosystem has a bandwidth of at least 40 Hz , typically 60 Hz depending on finger cuff size. 3 The volume clamp setpoint is regularly adjusted to keep the pressure difference across the arterial wall, the transmural pressure, at zero . 3 5 At zero transmural pressure , cuff pressure equals intra-arterial pressure , which is then determined
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indirectly by measuring cuff pressure . At this cuff pressure , the veins under the cuff are collapsed. The arteries under the cuff are not fully occluded but remain open at one third to one half their normal diameter. Thus during systole there is inflow to, and during diastole outflow from , the finger tip distal to the Finapres cuff, causing oxygenation in the fingertip to remain at an acceptable value. 14 SUBJECTS
The study group consisted of 19 patients with hypertension and one normotensive volunteer. The subjects had been selected on account of their arm circumference and level of blood pressure to participate in a study comparing the accuracy of auscultatory blood pressure measurements using different cuff sizes. 15 In the present study data from three of the patients were excluded owing to procedural errors and from two because of severe artefacts in the finger arterial pressure during the manoeuvre caused by strong bending of the cuffed finger. The results presented are those of 15 subjects (six men and nine women) with a mean age of 50 (25-61) years and a mean body weight of 97 (61-140)
.B FIG I TNO Finapres model 4. The device is connected to a small box, strapped to the back of the hand. The finger cuff is connected to the front end of the box.
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BP M lmholz, GA van Montfrans, J J Settels, GM A van der Hoeven, J M Karemaker, W Wieling
kg . Mean resting auscultatory blood pressure was 159/94 mmHg, ranging from 112/64 to 204/122 mmHg. Eleven of the remaining 14 hypertensive patients were taking antihypertensive medication. As an inclusion criterion for the study the mean of six consecutive auscultatory systolic and diastolic pressures in the right and left arms had to agree to within 10 mmHg. These comparisons were made by two observers measuring blood pressure simultaneously in both arms using their own stethoscopes. The two cuffs were connected to a single random zero sphygmomanometer (Gelman Hawksley, Lancing, UK). After the first three measurements the observers changed positions and performed another three measurements. Intra-arterial blood pressure (IAP) was measured by means of an 11 cm long Teflon cannula with an internal diameter of 1 mm, which was inserted using the Seldinger technique into the brachia! artery of the non-dominant arm. The cannula was attached via a 10 cm long polyethylene tube to a Hewlett-Packard 1290A transducer and monitor (Hewlett Packard, California, USA). The transducer was strapped to the mid upper arm. The system was continuously flushed, and the resonance frequency, measured in situ using the tapping method, was on average 26 Hz, range 11-40 Hz. Since blood flow to the finger may be reduced by a cannula in the proximal brachial artery, 11 16 finger arterial pressure (FIN AP) was recorded from the middle finger mid-phalanx of the opposite hand. A difference in height between the Finapres cuff and the IAP transducer causes hydrostatic pressure differences between the two signals. During the measurements the subjects were therefore asked to keep their cuffed finger at the level of the IAP transducer. In our patient group, which included obese subjects, this sometimes proved difficult in practice. In an early stage of the study we therefore decided to record the difference in height between the Finapres cuff and the IAP transducer continuously using a hydrostatic height measurement system consisting of a liquid filled tube with the open end strapped to the cuffed finger and the other end, which was connected to a pressure transducer, attached to the upper arm next to the IAP transducer. On subsequent analyses the height difference could thus be corrected for by subtracting the hydrostatic pressure from the finger pressures. All signals and an event marker were recorded on an eight channel Hewlett Packard FM instrumentation recorder and simultaneously on a strip chart. Subjects were placed in a recumbent position, after which auscultatory blood pressures were recorded from the non-cannulated arm, requiring approximately 90 min to perform. 15 The subjects were then asked to perform a Valsalva manoeuvre in the
semirecumbent position according to the method of Levin. 17 A mouthpiece was connected via suitable tubing to a manometer, and the subjects were instructed to maintain an expiratory pressure of 40 mmHg during 10 s. A small bore needle was inserted into the tubing to prevent the subjects from maintaining manometer pressure by closing the glottis. SIGNAL AND DATA ANALYSIS
Upon playback the periods of 30 s before and 40 s after the onset of the Valsalva manoeuvre were selected. Both blood pressure signals were AID converted at a sample rate of 200 Hz and input to a PDPl l/44 computer system. A signal analysis program was used to derive automatically the beat to beat systolic, diastolic, and mean pressures from both signals. As ·diastolic value, the end diastolic pressure just before the beginning of the next upstroke was recorded even
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FIG 2 Two recordings ofthe Valsalva manoeuvre, showing a classical (a) and an atypical (b) manoeuvre. /AP = intra-arterial pressure ( mmHg), F/NAP = finger arterial pressure(mmHg). ThearrowsmarJceda,b,c,d, ande indicate the five instants used to compute the stylised Valsalva curves: a=largest /AP systolic value during phase I, b=smallest /AP diastolic value during phase II, c=last /AP beat during phase II, d=smallest /AP diastolic value during phase Ill, e =largest/AP systolic overshoot during phase IV.
Reliability of Finapres during Valsalva manoeuvre
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Systolic, diastolic, and mean intra-arterial (/AP ) pressures (mmHg) and Finapres minus /AP differences (FIN - /AP ) averaged over the 30 s control period . Values are mean or mean(SD )
TABLE
Systolic
M ean
Diastolic
Case No
/AP
FIN - /AP
/AP
FIN - /AP
/AP
FIN - /AP
I
203 164 192 145 2 17 201 173 151 168 184 137 161 178 204 120 173(27 .6)
12( 3 .0) - 1(2. 5) 6(2 .7) 6(1.4) - 14(8. 9) -4(3 .3) 6(3.3 ) 2(2 .6) 2(3 .0) 3(5.4) 1(2 .6) -16(3 .3) - 22(4 .8) -8 (3 .6) 7(3 . 1) - 1(9 .6)
151 122 142 105 149 148 127 11 3 125 127 110 110 138 157 95 128(19. 0)
- 11 ( 1.2) - 10( 1.7) - 11 (0 .9) 1(0 .6) - 17(4 . 1) - 10(2. 0) I ( 1.6) - 5( 1.8) 2( 1.5) - 18( 1.4) - 3(1. 0) - 13( 1. 3) - 19(2. 0) - 10(2 .0) - 8(2. 5) - 9(6 .8)
119 94 107 80 105 112 92 85
- 6( 1. 3) -5 (2 . 1) - 9( 1.0) 2(1 . l ) - 10(5 .0) - 8( 1.8) 7( 1.9) - 1( 1.8) 7( 1.8) - 12( 1.6) 1(2 .2) - 5( 1.5) -12(1 .8) - 7(2 .4) -9(3 .0) -4(6 . 1)
2 3 4 5 6 7 8 9 10 II
12 13 14 15 Group mean(SD)
when lower pressures were present earlier in diastole , as occurred frequently in both signals during the strain period of the Valsalva manoeuvre. Individual beat to beat trend charts and scattergrams were plotted for the entire 70 s period for visual inspection. Only systolic , mean, and diastolic pressures were used for all further analyses. To obtain control values for each subject , the mean , standard deviation , and range were computed for IAP, FIN AP, and their differences (FIN AP- IAP) over the 30 s period just before the Valsalva manoeuvre. For the group comparisons of the response to the Valsalva manoeuvre, the individual recordings were synchronised at the onset of the strain period. Group means of the responses of the IAP and FIN AP signals as well as the FIN AP - IAP signal were computed and plotted at 1 s intervals. To detect possible systematic deviations during the Valsalva manoeuvre , corrected difference signals were obtained by subtracting the control FINAP-IAP differences from the FINAP-IAP signal . Patterns in the corrected difference signals were tested for randomness by the Kruskal-Wallis one way analysis of variance by ranks. The median and the 10th and 90th percentile of the corrected difference signals were computed and plotted. Finally, in those subjects who produced a classical Valsalva response , 18 the various peaks and troughs were identified and marked as shown in fig 2. After the mean values were subtracted for the control period as before , the blood pressure excursions at these instants were pooled producing stylised Valsalva responses for both IAP and FIN AP. The differences between FIN AP and IAP excursions at the defined instants were tested using Wilcoxon ' s matched pair signed Iank test.
96
89 88 80 107 123 78 97(14.4)
Results
Recordings of both continuous blood pressure signals in the one normotensive subject and · in one hypertensive patient are shown in fig 2, demonstrating the similarity between non-invasive finger and invasive brachial pressures . This similarity was present in all subjects . BEAT TO BEAT ANALYSIS
Individual IAP and FINAP beat to beat values and their differences for systolic , mean and diastolic pressures, 250
200
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AG 3 Systolic( diastolic (• )and mean(*) intra-arterial and finger arterial (FINAP) control pressures for all /5 subjects.
BP M lmholz, GA van Montfrans , J J Settels, GM A van der Hoeven , J M Karemaker , W Wieting
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I Strain I 200 180 200
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1000 Hz) proportional air valve. The system bandwidth ranges from 40 to 100 Hz, depending on cuff size, and has a slew rate of 10 000 mmHg·s- 1 . A microcomputer controls the instrument, finding a proper setpoint and loop gain in about 10 s. During the course of the measurement the correctness of the setpoint is periodically checked and adjusted by a small software expert system, which calibrates the physiological state of the finger arteries. This system compensates for changes in finger size, packed cell volume, colour of blood, smooth muscle tone, and red cells washed out of the microcirculation under the cuff and is considered mandatory for reliable longer term measurements. 3 5 The accuracy and precision of the Finapres are the subject of the present study. Its waveform quality may be judged from fig 2.
