Physiological influences on continuous finger and simultaneous intra-arterial blood pressure.

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 standard requirements (bias < or = 5 +/- 8 mm Hg) for devices that measure intermittently.

Continuous, non-invasive volume-clamp blood pressure: determinants of performance.

OBJECTIVE: To test a prototype hydraulic, non-invasive, continuous finger blood pressure monitor based on the volume-clamp principle for procedure-related factors likely to influence precision. DESIGN: The influence of these factors was determined by repeatability of finger blood pressure measurement and the relationship to contralateral arm-cuff blood pressure. METHODS: Repeated blood pressure measurements from three different fingers were made in 60 subjects following re-initialization of the device and re-insertion of the finger into the cuff. Repeatability was assessed in relation to simultaneous arm-cuff readings. Drift in arm-finger discrepancy was measured over a 1-h period. Finger diameter, drug therapy and presence of peripheral vascular disease were correlated with arm-finger blood pressure difference. RESULTS: Repeatability coefficients (twice the SD of the arm-finger difference) across device re-initialization were large, but similar to parallel repeated arm blood pressure determinations: 17.6 and 17.1 mmHg for systolic blood pressure (SBP) and 13.9 and 13.6 mmHg for diastolic blood pressure (DBP), respectively. Withdrawing and re-inserting the finger reduced repeatability substantially, with a 50% increase in repeatability coefficient. A trend towards a progressive 9-mmHg increase was observed in overestimation of SBP over the 1-h period. Mean +/- SD pooled arm-finger blood pressure differences were -10.8 +/- 14.6 mmHg for SBP and 4.5 +/- 9.4 mmHg for DBP. Blood pressure measured in different fingers was similar on average, with repeatability no poorer than for re-insertion of the same finger. The presence of peripheral vascular disease in 15 subjects correlated with a smaller arm-finger difference for DBP. CONCLUSIONS: Variations in positioning of the finger within the cuff influences blood pressure measurement during volume-clamp plethysmography, reducing its precision. Finger SBP exceeds brachial auscultatory readings and has similar precision.

Quantitative photoplethysmography: Lambert-Beer law or inverse function incorporating light scatter.

Finger blood volume is commonly determined from measurement of infra-red (IR) light transmittance using the Lambert-Beer law of light absorption derived for use in non-scattering media, even when such transmission involves light scatter around the phalangeal bone. Simultaneous IR transmittance and finger volume were measured over the full dynamic range of vascular volumes in seven subjects and outcomes compared with data fitted according to the Lambert-Beer exponential function and an inverse function derived for light attenuation by scattering materials. Curves were fitted by the least-squares method and goodness of fit was compared using standard errors of estimate (SEE). The inverse function gave a better data fit in six of the subjects: mean SEE 1.9 (SD 0.7, range 0.7-2.8) and 4.6 (2.2, 2.0-8.0) respectively (p < 0.02, paired t-test). Thus, when relating IR transmittance to blood volume, as occurs in the finger during measurements of arterial compliance, an inverse function derived from a model of light attenuation by scattering media gives more accurate results than the traditional exponential fit.

Relevance of the waterfall phenomenon in continuous finger cuff blood pressure measurements.

1. Finger cuff blood pressure monitors that provide continuous presentation of arterial pressure at the finger are available. 2. The operation of such instruments is based on 'clamping' arterial volume under a pressurized encircling cuff applied to a digit. Blood volume is detected by measuring infra-red transmission across the finger. 3. The compressed veins adopt cuff pressure and combine with the resistance of the peripheral vascular bed at the finger tip to control the blood flow. This results in waterfall behaviour in the veins under the cuff. 4. With the cuff set just 5 mmHg below the undisturbed arterial pressure, adequate flow to the distal finger is maintained. 5. Two potential sources of error affect the finger cuff blood pressure method. These are related to the increase in systolic pressure associated with moving peripherally and the variable value of occluding transmural pressure, both of which depend on muscular tone.

Sympathetic muscle nerve activity during sleep in man.

Muscle sympathetic activity (MSA) was recorded in the peroneal nerve during sleep in 14 sleep-deprived healthy subjects. Continuous noninvasive recordings of finger blood pressure were obtained in 7 subjects. In light sleep (stage 2 sleep) the number of sympathetic bursts/min decreased to 90 +/- 8% (mean +/- SEM) and total MSA (= burst/min x mean burst area) to 89 +/- 5% of the awake value (P less than 0.05, n = 14). In deep sleep (stage 3-4) total MSA decreased further, to 71 +/- 8% of the awake value (n = 5). There was no close correlation between variations of depth of sleep and variations of sympathetic activity but during continuously deepening sleep MSA decreased progressively with time. In stage 2 sleep, high amplitude K complexes were accompanied by short-lasting increases of sympathetic activity. Since these increases of MSA were not preceded by decreases of diastolic blood pressure, which is known to evoke increased sympathetic nerve traffic in muscle nerves, we suggest that K complex related increases of MSA are signs of arousal which elicit both cortical EEG phenomena and activation of cerebral sympathetic centres. During desynchronized (REM) sleep, total MSA increased to 124 +/- 12% of the value in awake state (n = 5). The increases occurred mainly in short irregular periods, often related to rapid eye movements and there was an inverse relationship between the duration of the desynchronized sleep and the increase of total MSA. Our findings are similar to the data obtained in animal experiments and may partly explain changes of blood pressure during synchronized and desynchronized sleep reported previously in man.

Physical characteristics and clinical evaluation of a new disposable fibreoptic transducer-tipped catheter system.

A new disposable fibreoptic transducer-tipped catheter manometer system was evaluated to assess its accuracy, stability of accuracy under prolonged simulated intra-arterial conditions, and dynamic characteristics. Maximum errors observed in the measurement of static pressure using a sample of five catheters (with one display unit) were 2 mmHg at 0 mmHg reference pressure, 2 at 20 mmHg, 4 at 40 mmHg, 4 at 100 mmHg and 9 at 200 mmHg. An immersion artifact caused a shift in baseline of up to 2 mmHg. Exposure of the transducer to 24 hours of simulated intra-arterial conditions (pulsatile pressure at 40 degrees C) resulted in errors of up to 7 mmHg for pressures up to 100 mmHg, and 11 mmHg for 200 mmHg, which were largely attributable to a drift in baseline pressure (up 6 mmHg by 24 hours). Consistent overestimation by the system suggested inappropriate gain setting within the display unit which, however, is not user-adjustable. The system exhibited uniform frequency response up to 33 Hz.

Voluntary blood pressure control using continuous systolic blood pressure biofeedback.

1. Ability to alter blood pressure (BP) acutely with continuous systolic (SBP) BP biofeedback was assessed in volunteers using a new non-invasive finger BP monitor. 2. Reliability of finger BP measurement was demonstrated in six hypertensive subjects (21-60 years), by beat-to-beat comparison with brachial intra-arterial BP over 90 min. Wide variation of BP was achieved by physiological manoeuvres. Mean error of finger BP was -3.0/-2.2 mmHg with intra-subject s.d. of 7.2/5.4 mmHg. 3. Thirteen normotensives (21-51 years) were paid to undergo 30 trials of SBP biofeedback in six sessions over 3 weeks. The SBP trend was displayed on a monitor with appropriately 'shaped' targets; each trial consisted of BP-raising and -lowering periods of 45 and 90 s respectively with intervening 45 s baselines. 4. Nine subjects raised BP, on analysis of the last 10 trials, by an average of 18.8 mmHg while five of the 13 successfully lowered BP by an average of 10.0 mmHg. BP lowering was best achieved by diminishing respiratory rate and depth, and muscular relaxation. 5. Demonstration of large BP reductions in five of 13 normotensives using strategies applicable to longer training sessions warrants further investigations in hypertensive subjects, focusing on mediating mechanisms and transfer of effect beyond the laboratory.

Voluntary blood pressure control by biofeedback: relationship to psychological characteristics.

1. Psychological characteristics were studied in 25 hypertensives (mean and standard deviation of blood pressure 150/95 +/- 12/5 mmHg), who received blood pressure (BP) biofeedback (BFB). Personality factors and success in BFB-BP modifying ability were correlated and the predictive value of psychological factors was estimated. 2. Questionnaires consisted of a Locus of Control of Behaviour (LCB) scale, the Spielberger state trait anxiety inventory and the Framingham Type A personality inventory. 3. BP was monitored continuously from the finger by volume clamp plethysmography during eight BFB sessions, each with three trials of raising, ignoring and lowering systolic blood pressure (SP). 4. SP was raised/lowered by 12 +/- 11/6 +/- 9 mmHg and heart rate (HR) increased by 10 +/- 3.9/+ 1 +/- 6.1. Ten subjects were able to lower SP by greater than or equal to 5 mmHg (15 +/- 7.5) and raise it by 17 +/- 11. The others achieved no decrease in SP and were also less successful at raising (8 mmHg, P = 0.04). 5. Changes in LCB and trait anxiety correlated with DP rise, whereas type A and pre-study state anxiety correlated with rising HR. Lowering of SP correlated weakly with change in LCB (r = 0.47, P = 0.06). 6. Combinations of psychological factors had independent predictive value for BP and HR change: trait anxiety (P = 0.03) and change in LCB (P = 0.009) with rise in diastolic blood pressure (DP); type A (P = 0.009), pre-study LCB (P = 0.02) and pre-study state anxiety (P = 0.01) with HR rise.(ABSTRACT TRUNCATED AT 250 WORDS)