A New Portable Device for 24-hour Recording of Ambulatory Intra-arterial Blood Pressure and Heart Rate in Hypertensive Patients Osamu TOCHIKUBO, M.D., Takashi OOTA, M.D., Naomichi MIYAZAKI, M.D., and Yoshihiro KANEKO, M.D. SUMMARY A new portable device (measuring 14 ~11 ~5cm, weighing 600g) has been developed for recording ambulatory intra-arterial blood pressure (BP) and R-R intervals on the electrocardiogram at 10sec intervals throughout a 24 hour period and a computer system for analysis of the 8,640 digital recordings of systolic blood pressure (SBP), diastolic blood pressure (DBP) and heart rate (HR). Using this device, SBP, DBP and HR recordings were obtained in 12 normotensive patients and 37 patients with essential hypertension (of whom 19 presented in WHO Stage I and 18 in WHO Stage II). The 24-hour BP variation (SD) increased with increasing 24 hour mean BP but there was no difference of statistical significance in SD/mean among the 3 groups. There was a good correlation (r=0.70 to 0.94) between SBP and DBP during 24 hours. The average of A in the linear regression equation, SBP=A ~DBP +B, was greatest for patients with WHO Stage I and the average of 24-hour HR was highest for this group. The results suggested that these patients were in a hyperdynamic cardiac state. The new portable SBP-DBP-HR recorder is easy and simple to use and has the advantage of being low-cost, and therefore will be useful in clinical applications. Additional Indexing Words: 24-hour blood pressure 24-hour heart rate Blood pressure recorder Essential hypertension WENTY-FOUR hour recording of intra-arterial blood pressure under unrestricted conditions provides valuable information required in the From the Second Department of Internal Medicine, School of Medicine, Yokohama City University, Yokohama, Japan. Address for reprint: Osamu Tochikubo, M.D., Second Department of Internal Medicine, School of Medicine, Yokohama City University, 3-46, Urafune-cho, Minami-ku, Yokohama 232, Japan. Received for publication January 7, 1986. 661
662 TOCHIKUBO, OOTA, MIYAZAKI, AND KANEKO Jpn. Heart J. September 1986 diagnosis and treatment of hypertension. In 1969, a small completely portable apparatus1) was developed, providing for continuous direct recording of intra-arterial blood pressure throughout 24 hours. With this apparatus, there was no need to restrict the movement of the subject during the procedure and no human intervention was needed except for checking the apparatus every 12 hours. The pressure measured with this original apparatus was recorded on bromide photographic paper, and it was necessary to read the arterial pressure directly from the paper. This apparatus2),3) has been further developed using a miniature multichannel magnetic tape recorder (Oxford Instrument Co., Ltd.). This new system basically consists of three units, a perfusion pump, a transducer and a miniature analogue tape recorder. The advantage of this system is that it can record not only blood pressure but also the electrocadiogram (ECG). However, the frequency response of the system as a whole is flat to 10Hz and resorts to a tape recorder storing analogue data.2),3) It therefore takes time to reproduce data and the system requires complicated manipulations and is costly. In the present study we developed a new portable recording device, using a transducer with an excellent frequency response as a pressure sensor and a specially developed miniature perfusion pump. This portable device provides for recording ambulatory intra-arterial blood pressure and heart rate directly in the digital memory under unrestricted conditions. Further, a computer system was also developed whereby data obtained by the portable recorder can be easily analyzed and printed out, both numerically and graphically. The present study was performed in hypertensive patients, and thus we were able to evaluate the apparatus from a clinical viewpoint. METHODS AND MATERIALS Apparatus: The 24-hour intra-arterial blood pressure (BP) and heart rate (HR) recording system consists of 4 units; (1) a portable memory unit, (2) a miniature perfusion pump, (3) an interface unit and (4) a computing unit (Fig.1). The pressure transducer used is a Gould Statham P50, driven and demodulated by an electrically isolated preamplifier. The pressure signal is filtered and fed to analogue "peak" and "bottom" detectors, which determine the systolic blood pressure (SBP) and diastolic blood pressure (DBP) values. The values of SBP, DBP and successive R-R intervals (msec) on the ECG after the pulse wave are converted to digital form at 10sec intervals by an analogue-
Vol.27 No.5 A NEW PORTABLE ARTERIAL PRESSURE RECORDER 663 Fig.1. Computer system for ambulatory 24-hour intra-arterial and heart rate analysis. Fig.2. Portable intra-arterial pressure and h eart rate recorder with di gital memory.
664 TOCHIKUBO, OOTA, MIYAZAKI, AND KANEKO Jpn. Heart J. September 1986 to-digital converter. The full pressure setting range of the system is 0 to 16,320msec. The processed signal is written into a 56k byte RAM (random access memory) in the microprocessor (8k byte ROM; read only memory) of the portable device powered by 4 small nickel-cadmium batteries giving a maximum recording time of 24 hours. The portable recording device measures 14 ~11 ~5cm (weighing 600g) and may be easily carried around by the patient. It is provided with an event mark switch which can be operated by the patient (Fig.2). The frequency response of the whole system is approximately flat to 100Hz. The amplifiers show a baseline variation of not more than 2% over 24 hours, with a negligible temperature drift between 0 and 40 Ž. To prevent blood coagulation in the indwelling catheter, each subject carries a specially designed miniature perfusion pump, measuring 10 ~5 ~3cm (weighing 250g), which continuously infuses into the catheter physiological saline containing heparin (5IU/ml) at a rate of 2ml/hour. Before recording Fig.3. Typical 24-hour blood pressure (BP) and heart rate (HR) trendgram patterns in a 45 year old female. SBP (systolic BP), DBP (diastolic BP) and HR were sampled at 10sec intervals and calculated mean values and +2SD of SBP, -2SD of DBP and }2SD of HR plotted every 4min. MBP=mean BP; PP=pulse pressure.
Vol.27 A NEW PORTABLE ARTERIAL PRESSURE RECORDER 665 No.5 SCATTER DIAGRAM Fig.4. Systolic and diastolic BP values during a 24-hour period (case shown in Fig.3) plotted by the computer. is begun, the blood pressure analogue signals and ECG waveforms are monitored by a microcomputer (PC-9801, NEC, Co., Ltd., Japan) and a penrecorder through SIO (Serial Input/Output) connectors, the system is calibrated with a mercury column and the starting time is recorded. After 24 hours of recording the recorder is detached and the keyswitch placed in a hold mode position. The stored data are then transferred to the microcomputer through SIO connectors (RS232c, NEC Co., Ltd.). Transfer takes approximately 5min, and the data are printed, both numerically and graphically. A full 24-hour graphic display is usually used, but an expanded mode is also available for greater detail. Further, the data are permanently stored on a floppy disk. The 24-hour blood pressure (SBP and DBP) and heart rate patterns are displayed and printed as mean }2SD (standard deviation) for every 4min (Fig.3). The 24-hour scatter diagram is plotted as SBP on the Y (vertical)-axis and DBP on the X (horizontal)-axis (Fig.4); the correlation (r) and regression coefficients (A) between SBP and DBP are calculated by least squares linear regression analysis, and the results are printed out. Measurement: A disposable teflon microcatheter (Plastimed Co., Ltd.,
666 TOCHIKUBO, OOTA, MIYAZAKI, AND KANEKO Jpn. Heart J. September 1986 length 6cm, internal diameter 0.6mm) is inserted percutaneously into a brachial artery using the Seldinger technique. The catheter is connected to the perfusion pump by the use of a special connector with a pressure monitoring tube (disposable Gritiflo diaphragm dome; Gould Model TA1017). The tube is anchored to the skin with strapping every 5cm up the arm to the front of the chest, where the transducer is attached at heart level. The perfusion pump and portable recorder are carried in a padded harness with a belt around the waist. The electrocardiogram lead system consists of dipolar electrodes with an earth electrode. One electrode is placed in the midline at the level of the second intercostal space and the other over the cardiac apex. Leads are held in the place by electrode disks and secured with surgical tape to minimize movement artifacts. Subjects: Recordings were obtained in 12 normotensives (3 women and 9 men, with a mean age of 40 }8 SD years), 37 patients with essential hypertension (18 women and 19 men, with a mean age of 46 }20 years). Informed consent detailing the investigational nature of the procedure was obtained in all subjects. The normotensive subjects were healthy volunteers with no history of heart disease, renal disorder and hypertension. The essential hypertensive group consisted of 19 patients (12 women and 7 men, with a mean age of 42 }7 years) in WHO Stage I group and 18 patients (6 women and 12 men, with a mean age of 48 }20 years) in WHO Stage II group. Data were assessed by analysis of variance (ANOVA) for the significance of differences among the 3 groups, and the difference between 2 groups was tested for significance by the unpaired t-test, with probabilities (p) of 0.05 or less taken as the level of significance. RESULTS 1) Variability of 24-hour BP and HR: The mean 24-hour SBP and DBP values were obtained for each subject, and the average of mean 24-hour values was calculated for each group. This average was greater, as expected, in the hypertensive groups than in the normotensive group. The SD of 24- hour SBP and DBP values were calculated for each subject, and then the average SD obtained for each group. It was again found that the average 24-hour SD was larger in the hypertensive groups than in the normotensive group. The coefficients of SBP and DBP variation (CV), obtained by dividing SD values by the mean, were tested for significance of difference, but no difference of statistical significance was noted among the 3 groups. Briefly,
Vol.27 No.5 A NEW PORTABLE ARTERIAL PRESSURE RECORDER 667 these facts suggest that the SD of BP increases with increasing mean, but SD/mean does not vary with each individual, that is, the pattern of blood pressure variation does not differ from group to group (Table I). 2) Variability of 24-hour HR: The mean 24-hour heart rate was obtained for each subject, and the average of mean 24-hour heart rates was determined for each group (Table I). The average of mean 24-hour heart rates was a little higher in the hypertensive group presenting in WHO Stage I than in the normotensive group (p<0.05), but the hypertensive group presenting in WHO Stage II showed no difference in mean heart rate from the normotensive group. 3) Correlation and regression coefficients between SBP and DBP: The correlation coefficient (r) of 24-hour SBP and DBP and the equation of linear regression between SBP and DBP (SBP=A ~DBP+B) were obtained for each subject. The mean values of r and A which were obtained for each Table I. Variability of 24-hour BP and HR Mean }SD. CV=coefficient of variation; SBP=systolic BP; HR=heart rate; ANOVA=analysis of variance; DBP=diastolic BP; BP=blood pressure. Table II. Correlation and Regression Coefficient between 24-hour SBP and DBP Mean }SD. ANOVA=analysis of variance. * p<0,05, ** p<0.01. WHO I vs NT or WHO II.
668 TOCHIKUBO, OOTA, MIYAZAKI, AND KANEKO Jpn. Heart J. September 1986 group indicated that there was a close correlation between SBP and DBP (with r ranging from 0.70 to 0.94). The average of A values was 1.13 } 0.19, with a spread of 0.76 to 1.50, showing comparative consistency. On a group basis, however, it varied. The mean value of A was higher for the WHO-Stage I group (1.28 }0.15) than for the WHO-Stage II group (1.09 } 0.13, p<0.05). Briefly, it may be said that SBP variability relative to DBP variation during the 24-hour period was greater in the WHO-I group than in the other 2 groups (Table II). DISCUSSION Ambulatory 24-hour intra-arterial blood pressure measurement with a portable device with a hard memory has thus far been performed either by the use of an ambulatory tape recorder of Oxford type1)-4) or by a telemetry which consists of transmitting blood pressure waves to a recorder via radio.5) The former method is excellent, but tape recorder operation is somewhat timeconsuming and the equipment is costly. As it is hard to obtain in Japan, it is not in practical use by the Japanese medical profession. The latter method is easier to employ and in fact, we developed our own system based on this method.5) The problem with telemetric systems, however, is that they are only effective within the reach of the transmitter used. To overcome this problem, we have developed a new system whereby continuously measured intra-arterial blood pressure and R-R intervals on ECG under unrestricted conditions are sampled at 10sec intervals and then 8,640 pairs of data on 24-hour SBP, DBP and HR are stored in RAM for later computer-aided analysis. Employing this newly developed system, circadian changes in blood pressure and heart rate were determined in patients with essential hypertension and normotensive subjects. As a result, it was revealed that there was no difference in rate of variation (CV) during 24-hour blood pressure between the normotensive and hypertensive groups. In other words, there was a quantitative difference in blood pressure (difference in absolute value of the average of 24-hour blood pressure) between the 2 groups, but there was no qualitative difference (difference in relative value, that is, circadian pattern in blood pressure). In hypertensive patients in WHO Stage I, however, the mean 24-hour HR and the increase in SBP relative to DBP were somewhat higher than in the other groups. [The value of A in the correlation equation SBP=A ~DBP+B was larger in the WHO-I hypertensive group. If pulse pressure is ƒ BP then ƒ BP=(A-1)DBP+B, since ƒ BP=SBP-DBP. In short, the variation in pulse pressure (ƒ BP) which is related to the stroke volume of the heart is larger for the change of DBP in the WHO-I hypertensive
Vol.27 A NEW PORTABLE ARTERIAL PRESSURE RECORDER 669 No.5 group.] This fact suggests that the heart has a greater influence on circadian variation in blood pressure in the WHO-I hypertensive group than in the WHO-II hypertensive group and the normotensive group. Our system which does not use a tape recorder or radio waves, is simple to operate and immune to noise. It provides for fast analysis of intra-arterial blood pressure and heart rate data obtained under unrestricted conditions around the clock. Further, by the use of a personal computer, this system can be built at a low price. Taken altogether, this system will lend itself to diagnosis and treatment in clinical practice. ACKNOWLEDGMENT The authors wish to express the gratitude to Mr. Ryoichi Saitoh of San-ei Service Co. Ltd. for the assistance in developing the apparatus. REFERENCES 1. Bevan AT, Honour AJ, Stott FD: Direct arterial pressure recording in unrestricted man. Clin Sci 36: 329, 1969 2. Littler WA, Honour AJ, Pugsley DJ, Sleight P: Continuous recording of direct arterial pressure in unrestricted patients. Circulation 51: 1101, 1975 3. Littler WA, Honour AJ, Sleight P, Stott FD: Continuous recording of direct arterial pressure and electrocardiogram in unrestricted man. Br Med J 3: 76, 1972 4. Millar-Craig MW, Hawes B, Whittington J: New system for recording ambulatory blood pressure in man. Med Biol 16: 111, 1978 5. Tochikubo O, Umemura S, Noda K, Kaneko Y: Variability of arterial blood pressure and classification of essential hypertension by multivariate statistical analysis. Jpn Circ J 45: 781, 1981