Effect of altering vasoactivity on the measurement of finger blood pressure

The effect of anesthetic drug choice on accuracy of high-definition oscillometry in laterally recumbent horses

OBJECTIVE

To determine the accuracy of high-definition oscillometry (HDO) for arterial pressure measurement during injectable or inhalation anesthesia in horses.

STUDY DESIGN

Prospective, clinical study.

ANIMALS

Twenty-four horses anesthetized for procedures requiring lateral recumbency.

METHODS

Horses were premedicated with xylazine, and anesthesia induced with diazepam-ketamine. Anesthesia was maintained with xylazine-ketamine-guaifenesin combination [TripleDrip (TD; n = 12) or isoflurane (ISO; n = 12)]. HDO was used to obtain systolic (SAP), mean (MAP) and diastolic (DAP) arterial pressures, and heart rate (HR) using an 8-cm-wide cuff around the proximal tail. Invasive blood pressure (IBP), SAP, MAP, DAP and HR were recorded during HDO cycling. Bland-Altman analysis for repeated measures was used to compare HDO and IBP for all measurements. The generalized additive model was used to determine if means in the differences between HDO and IBP were similar between anesthetic protocols for all measurements.

RESULTS

There were >110 paired samples for each variable. There was no effect of anesthetic choice on HDO performance, but more variability was present in TD compared with ISO. Skewed data required log-transformation for statistical comparison. Using raw data and standard Bland-Altman analysis, HDO overestimated SAP (TD, 3.8 ± 28.3 mmHg; ISO, 3.5 ± 13.6 mmHg), MAP (TD, 4.0 ± 23.3 mmHg; ISO, 6.3 ± 10.0 mmHg) and DAP (TD, 4.0 ± 21.2 mmHg; ISO, 7.8 ± 13.6 mmHg). In TD, 26-40% HDO measurements were within 10 mmHg of IBP, compared with 60-74% in ISO. Differences between HDO and IBP for all measurements were similar between anesthetic protocols. The numerical difference between IBP and HDO measurements for SAP, MAP and DAP significantly decreased as cuff width:tail girth ratio increased toward 40%.

CONCLUSION AND CLINICAL RELEVANCE

More variability in HDO occurred during TD. The cuff width:tail girth ratio is important for accuracy of HDO.

Postprandial hypotension in clinical geriatric patients and healthy elderly: prevalence related to patient selection and diagnostic criteria

The aims of this study were to find out whether Postprandial hypotension (PPH) occurs more frequently in patients admitted to a geriatric ward than in healthy elderly individuals, what the optimal interval between blood pressure measurements is in order to diagnose PPH and how often it is associated with symptoms.The result of this study indicates that PPH is present in a high number of frail elderly, but also in a few healthy older persons. Measuring blood pressure at least every 10 minutes for 60 minutes after breakfast will adequately diagnose PPH, defined as >20 mmHg systolic fall, in most patients. However with definition of PPH as >30 mmHg systolic fall, measuring blood pressure every 10 minutes will miss PPH in one of three patients. With the latter definition of PPH the presence of postprandial complaints is not associated with the existence of PPH.

Simultaneous application of differential servo-oscillometry and volume-clamp plethysmography for continuous non-invasive recording of the finger blood pressure response to a hand postural change

Continuous blood pressure recordings obtained by the volume-clamp method (Finapres 2300 BP monitor, Ohmeda, USA) were compared with measurements made by using the modified oscillometric method (UT9201 beat-to-beat monitor, University of Tartu, Estonia) in normal subjects during a hand postural change. Changes in the local blood pressure were induced in 11 healthy volunteers by passive elevation of the hand by 40 cm and lowering it again after one minute. We did not reveal a statistically significant difference between the group-averaged responses of the oscillometric finger mean blood pressure measurement device and those of Finapres during this test. The difference UT9201 minus Finapres equalled -1.3 +/- 3.1 mm Hg (p = 0.15). Blood pressure responses were similarly tracked by both beat-to-beat finger blood pressure monitors despite the considerable variability in the shape of the subjects' individual responses.

Non-constrained monitoring of systolic blood pressure on a weighing scale

In this study, we developed a novel technique for estimating non-constrained and cuffless blood pressure (BP) that was based on electrocardiogram (ECG) and ballistocardiogram (BCG). The BCG was non-invasively measured using a common electronic weighing scale when a subject was standing on it. The ECG was measured using three different methods: on the chest using Ag/AgCl electrodes, on the hands using dry electrodes and on the feet also using dry electrodes. For a BP correlated parameter, a time interval parameter, which was defined as the time difference between the ECG R-peak and BCG J-peak, was employed for evaluating and estimating beat-to-beat BP. Under a BP varying experiment with a Valsalva manoeuvre, the R-J intervals were extracted at every beat cycle and a systolic blood pressure (SBP) estimation equation was established using linear regression analysis for each subject. In the case of feet delivered ECG (F-ECG), an ensemble average technique synchronized at the BCG J-peak point was applied to extract the ECG signal from the feet. The performance of the proposed method was evaluated using Finapres, a non-invasive blood pressure measurement system, as a reference BP signal, and a scatter plot was used to find the regression line between the reference values and estimated BPs. A moving-window averaging technique was applied to remove the high-frequency noise in the R-J intervals and was applied to enhance the accuracy of the SBP estimation. For all individuals, the estimated SBP was similar to the measured SBP with a reliable correlation, which makes the proposed method suitable for use in a home healthcare system to monitor blood pressure on a weighing scale at the same time as measuring weight.

Sympathetic peripheral vasoconstriction may be measured using an artifact of the Finapres volume clamp technique

OBJECTIVES

The objective of this work was to test the hypothesis that the Finapres underestimates blood pressure during sympathetic peripheral vasoconstriction.

METHODS

Measurements were made simultaneously with two Finapres devices and one radial artery tonometer during the onset of periodic lower body negative pressure in healthy volunteers. The Finapres is believed to underestimate blood pressure during sympathetic peripheral vasoconstriction, but tonometry is not considered to be influenced. A lower blood pressure recording was therefore expected from the Finapres during the induced sympathetic vasoconstriction. To test the association with autonomic activity the time course of the difference between the two measurement techniques was compared with the induced change in heart rate.

RESULTS

In averaged results from 10 volunteers the Finapres and tonometer both showed a drop in blood pressure with the vacuum onset. A significantly larger drop was recorded by the Finapres. The result is consistent with an underestimate of blood pressure by the Finapres during the autonomic stimulation. The time course of the difference between the two measures of blood pressure follows the induced changes in heart rate, providing further evidence that the differences relate to autonomic activity.

CONCLUSION

Measurement of the difference may be a convenient method for monitoring the neurological component of peripheral vasoactivity. It is argued that the difference is insensitive to peripheral vasoactivity mediated by local endothelial or myogenic mechanisms.

Angiotensin II decreases the renal MRI blood oxygenation level-dependent signal

Acute experimental reduction of renal blood flow decreases the renal blood oxygenation level-dependent (BOLD) MRI signal in animals. Angiotensin II also reduces renal blood flow, but the ability of BOLD MRI to dynamically detect this response has not yet been investigated in humans. Six healthy male volunteers underwent an individual dose-finding study to identify the intravenous doses of angiotensin II, norepinephrine, and sodium nitroprusside necessary to induce a 15-mm Hg peak mean arterial blood pressure change. MRI studies followed within 3 weeks, when angiotensin II (8.8+/-1.4 ng/kg), norepinephrine (52+/-12 ng/kg), and sodium nitroprusside (2.0+/-0.3 microg/kg) were given twice in an unblocked, randomized sequence while imaging experiments were performed on a 1.5-T Siemens Sonata. A multiecho echo-planar imaging sequence was used to acquire T2* maps with a temporal resolution of 1 respiratory cycle. Averaged over a renal cortex dominated region of interest, angiotensin II caused a shortening of T2* between 6% and 10%. Sodium nitroprusside and norepinephrine, although of equal potency concerning blood pressure responses, did not alter the renal BOLD signal. The renal BOLD response to angiotensin II appeared with short onset latency (as early as 10 seconds after peripheral intravenous angiotensin II bolus administration) suggesting that this response is a consequence of altered perfusion rather than increased renal oxygen consumption. The methods described here are suitable to assess renal responsiveness to angiotensin II and may, thus, be of great value in human hypertension research.

Recording of dynamic arterial compliance changes during hand elevation

Finger arterial compliance has been studied on the beat-to-beat basis by using the digital arterial pressure and volume waveforms and performing measurements at zero transmural pressure during arm elevation in 11 volunteers. Continuous non-invasive finger blood pressure was measured by applying the Finapres monitor and the finger volume pulses were recorded by the UT9201 physiograph by using the photoplethysmographic principle of registration. Estimation of the beat-to-beat finger arterial compliance is based on (i) the recorded volume and pressure wave amplitudes (Vpulse and Ppulse) and (ii) on the calculation of the slope of the pressure-volume relationship from the first derivatives dV/dt and dP/dt of the recorded volume and pressure pulses near the point of the maximum slope. The results of the study demonstrate that the applied two methods similarly (correlation coefficient r = 0.97) describe the changes of the beat-to-beat compliance during hand elevation test. At the same time the second estimate was 18% higher than the first one (P = 0.003).

Effect of gastric distension on cardiovascular parameters: gastrovascular reflex is attenuated in the elderly

Stretching the stomach wall in young healthy subjects causes an increase in muscle sympathetic nerve activity and in blood pressure, the gastrovascular reflex. We compared healthy elderly subjects with healthy young subjects to find out whether the gastrovascular reflex attenuates in normal ageing and we studied whether there was a difference in autonomic function or gastric compliance that could explain this possible attenuation. Muscle sympathetic nerve activity, finger blood pressure and heart rate were continuously recorded during stepwise isobaric gastric distension using a barostat in eight healthy young (6 men and 2 women, 27 +/- 3.2 years, mean +/-s.e.m.) and eight healthy elderly subjects (7 men and 1 woman, 76 +/- 1.5 years). Changes in cardiac output and total peripheral arterial resistance were calculated from the blood pressure signal. The baseline mean arterial pressure and muscle sympathetic nerve activity were higher in the elderly group (both P < 0.05) and muscle sympathetic nerve activity increase during the cold pressor test was lower in the elderly group (P = 0.005). During stepwise gastric distension, the elderly subjects showed an attenuated increase in muscle sympathetic nerve activity compared to the young subjects (P < 0.01). The older group tended to show a higher increase in mean arterial pressure (P = 0.08), heart rate (P = 0.06) and total peripheral arterial resistance (P = 0.09) The cardiac output rose slightly in both groups without significant difference between groups. The fundic compliance did not differ between groups. We conclude that stepwise gastric distension caused an increase in muscle sympathetic nerve activity in both groups, but the increase in the elderly was attenuated.

Portapres and differential oscillometric finger blood pressure changes during deep breathing test in the assessment of BRS index

The aim of this study was to investigate changes in the finger blood pressure during a deep breathing test (DB) and to find out whether the mean blood pressure might be used as a substitute for the systolic pressure in calculations of baroreflex sensitivity from data derived from the DB test. Continuous beat-to-beat finger pressure was recorded by the volume clamp method (Portapres model 2 monitor). In addition, the mean arterial pressure was recorded by the modified oscillometric method (UT9201 beat-to-beat monitor, University of Tartu, Estonia). Fifteen healthy volunteers, aged 25-56 years, were studied. The amplitude of respiration-linked oscillations in the Portapres systolic (Psyst) and mean blood pressure (Pmean) was 22.2 +/- 8.8 and 16.6 +/- 5.8 mmHg, respectively. There was no significant difference between the amplitudes of induced changes in Pmean recorded by the two devices: the amplitude of oscillations in the mean blood pressure recorded by the differential oscillometric monitor was 16.0 +/- 5.9 mmHg. The amplitude of oscillations in Psyst correlated significantly with the amplitude of oscillations in Pmean recorded either by Portapres or by UT9201 (r=0.95 and 0.98, respectively). The high correlation between the amplitudes of oscillations in mean and systolic blood pressure allows to conclude that mean arterial pressure changes during a DB test might be used instead of systolic pressure changes in calculation of the ratio of changes in pulse interval to changes in blood pressure, which is considered to reflect baroreflex sensitivity.