Continuous cardiac output monitoring in humans by invasive and noninvasive peripheral blood pressure waveform analysis.

We present an evaluation of a novel technique for continuous (i.e., automatic) monitoring of relative cardiac output (CO) changes by long time interval analysis of a peripheral arterial blood pressure (ABP) waveform in humans. We specifically tested the mathematical analysis technique based on existing invasive and noninvasive hemodynamic data sets. With the former data set, we compared the application of the technique to peripheral ABP waveforms obtained via radial artery catheterization with simultaneous thermodilution CO measurements in 15 intensive care unit patients in which CO was changing because of disease progression and therapy. With the latter data set, we compared the application of the technique to noninvasive peripheral ABP waveforms obtained via a finger-cuff photoplethysmography system with simultaneous Doppler ultrasound CO measurements made by an expert in 10 healthy subjects during pharmacological and postural interventions. We report an overall CO root-mean-squared normalized error of 15.3% with respect to the invasive hemodynamic data set and 15.1% with respect to the noninvasive hemodynamic data set. Moreover, the CO errors from the invasive and noninvasive hemodynamic data sets were only mildly correlated with mean ABP (rho = 0.41, 0.37) and even less correlated with CO (rho = -0.14, -0.17), heart rate (rho = 0.04, 0.19), total peripheral resistance (rho = 0.38, 0.10), CO changes (rho = -0.26, -0.20), and absolute CO changes (rho = 0.03, 0.38). With further development and successful prospective testing, the technique may potentially be employed for continuous hemodynamic monitoring in the acute setting such as critical care and emergency care.     

Assessment of dynamic changes in cerebral autoregulation.

Cerebral autoregulation (CA) is a control mechanism that adjusts cerebral vasomotor tone in response to changes in arterial blood pressure (ABP) to ensure a nearly constant cerebral blood flow. Patient treatment could be optimized if CA monitoring were possible. Whereas the concept of static CA assessment is simply based on comparison of mean values obtained from two stationary states (e.g., before and after a pressure change), the evaluation of dynamic CA is more complex. Among other methods, moving cross-correlation analysis of slow waves in ABP and cerebral blood flow velocity (CBFV) seems to be appropriate to monitor CA quasi-continuously. The calculation of an "instantaneous transfer function" between ABP and CBFV oscillations in the low-frequency band using the Wigner-Ville distribution may represent an acceptable compromise in time-frequency resolution for continuous CA monitoring.     

Dynamic cerebral autoregulation under sinusoidal gravitational loading

Dynamic cerebral autoregulation (CA) has been studied previously using spectral analysis of oscillations in arterial blood pressure (ABP) and cerebral blood flow velocity (CBFV). The dynamics of the CA can be modeled as a high-pass filter. The purpose of this study is to compare CA of blood pressure oscillations induced by gravitational loading to CA during resting conditions. We subjected twelve healthy subjects to repeated sinusoidal head-up (0 degrees - 60 degrees) tilts at several set frequencies (0.07 to 0.25 Hz) on a computer controlled tilt table while we recorded ABP (Finapres) and CBFV (transcranial Doppler ultrasound). We fitted the data sets to a high-pass filter model and computed an average time constant (T). Our results show similar phase leads of CBFV to ABPbrain in the rest recording and in sinusoidal tilting, in the studied frequency range. The transfer function gain of the resting spectra increased with increasing frequency, the gain of the tilting spectra did not. Fitting the phase responses of both data sets to a high pass filter model yielded similar time constants.     

Statistical criteria for using short-term measurements as an index of 24-hour mean arterial pressure in unanesthetized unrestrained dogs

This study assessed the statistical validity of short time-interval measurements as estimators of true 24 hour mean arterial pressure in unanesthetized, unrestrained dogs. 24 hour intra-arterial pressure recordings were obtained using a stable FM telemetry system. The 24 hour pressure measurements approximated a normal distribution whose variance was inversely related to the selected averaging interval. Given the variance of a normal distribution one can calculate the 95% confidence interval for any single random measurement. Conversely the number of random samples necessary to be within a prescribed confidence interval can be determined. In this study, the 95% confidence interval for a single, random 30 minute arterial pressure average was calculated to be 11.2 mmHg. Only 4.8 +/- 1.4% of 480 individual 30 minute arterial pressure measurements fell beyond this confidence interval. These outlying values were distributed throughout the 24 hour period. The data suggest that randomly chosen short time-interval measurements may be a valid index of true 24 hour mean pressure if the average variance of a population is known and confidence intervals are defined.     

The BpTRU automatic blood pressure monitor compared to 24 hour ambulatory blood pressure monitoring in the assessment of blood pressure in patients with hypertension

Background

Increasing evidence suggests that ABPM more closely predicts target organ damage than does clinic measurement. Future guidelines may suggest ABPM as routine in the diagnosis and monitoring of hypertension. This would create difficulties as this test is expensive and often difficult to obtain. The purpose of this study is to determine the degree to which the BpTRU automatic blood pressure monitor predicts results on 24 hour ambulatory blood pressure monitoring (ABPM).

Methods

A quantitative analysis comparing blood pressure measured by the BpTRU device with the mean daytime blood pressure on 24 hour ABPM. The study was conducted by the Centre for Studies in Primary Care, Queen's University, Kingston, Ontario, Canada on adult primary care patients who are enrolled in two randomized controlled trials on hypertension. The main outcomes were the mean of the blood pressures measured at the three most recent office visits, the initial measurement on the BpTRU-100, the mean of the five measurements on the BpTRU monitor, and the daytime average on 24 hour ABPM.

Results

The group mean of the three charted clinic measured blood pressures (150.8 (SD10.26) / 82.9 (SD 8.44)) was not statistically different from the group mean of the initial reading on BpTRU (150.0 (SD21.33) / 83.3 (SD12.00)). The group mean of the average of five BpTRU readings (140.0 (SD17.71) / 79.8 (SD 10.46)) was not statistically different from the 24 hour daytime mean on ABPM (141.5 (SD 13.25) / 79.7 (SD 7.79)). Within patients, BpTRU average correlated significantly better with daytime ambulatory pressure than did clinic averages (BpTRU r = 0.571, clinic r = 0.145). Based on assessment of sensitivity and specificity at different cut-points, it is suggested that the initial treatment target using the BpTRU be set at <135/85 mmHG, but achievement of target should be confirmed using 24 hour ABPM.

Conclusion

The BpTRU average better predicts ABPM than does the average of the blood pressures recorded on the patient chart from the three most recent visits. The BpTRU automatic clinic blood pressure monitor should be used as an adjunct to ABPM to effectively diagnose and monitor hypertension.     

Biobehavioral treatment of essential hypertension: a group outcome study

In a group outcome and follow-up study of 77 patients with essential hypertension, significant reductions were seen in systolic and diastolic blood pressure (BP) and in hypotensive medication requirement. A multimodality biobehavioral treatment was used which included biofeedback-assisted training techniques aimed at teaching self-regulation of vasodilation in the hands and feet. Of the 54 medicated patients, 58% were able to eliminate hypotensive medication while at the same time reducing BP an average of 15/10 mm Hg. An additional 19 (35%) of the medicated patients were able to cut their medications approximately in half while reducing BP by 18/10 mm Hg. The remaining 4 (7%) medicated patients showed no improvement in either BP or medication requirement. Similar reductions in BP were seen in initially unmedicated patients. Seventy percent of the 23 unmedicated patients achieved average pressures below 140/90 mm Hg, with an additional 22% of these patients making clinically significant reductions in pressure without becoming normotensive, and with 8% unsuccessful at lowering pressures to a clinically significant extent. Follow-up data available on 61 patients over an average of 33 months indicated little regression in these results with 51% of the total patient sample remaining well-controlled off medication, an additional 41% partially controlled, and 8% unsuccessful in lowering either medications and/or blood pressures to a clinically significant extent.     

THE TOLERANCE-HYPERBARIC TEST: A CHRONOBIOLOGIC APPROACH FOR IMPROVED DIAGNOSIS OF HYPERTENSION

Blood pressure (BP) displays predictable large-amplitude circadian variability. Thus, the identification and the proper definition of hypertension are highly ambiguous when based on single time-unspecified measurements. One way to deal with such variability in the diagnosis of hypertension is to replace the commonly used constant limits of BP by a time-specified reference interval based on the normal circadian BP rhythm assessed by ambulatory BP monitoring (ABPM). A proper reference limit can be constructed, for instance, as a tolerance interval computed for every specific time interval throughout the 24 h. Once such a threshold (given by the upper limit of the tolerance interval) is constructed, a hyperbaric index (HBI) can be computed by numerical integration of the total area of any given patient's BP profile above threshold. The HBI plus the duration of excess within the 24h day serves as nonparametric endpoints for assessing hypertension. Both retrospective and prospective evaluation of this tolerance-hyperbaric test validate its high sensitivity and specificity in the diagnosis of hypertension. We describe the theory of the HBI as well as a newly created dedicated software program that automatically derives the tolerance intervals from a reference database of normotensive subjects and calculates the HBI and other potentially valuable parameters based on data obtained by ABPM. The establishment of time-qualified tolerance limits and the assessment of the extent and timing of BP elevation represents a valuable tool for the more accurate diagnosis of hypertension as well as means of gauging response to treatment.     

Biobehavioral treatment of essential hypertension: A group outcome study

In a group outcome and follow-up study of 77 patients with essential hypertension, significant reductions were seen in systolic and diastolic blood pressure (BP) and in hypotensive medication requirement. A multimodality biobehavioral treatment was used which included biofeedback-assisted training techniques aimed at teaching self-regulation of vasodilation in the hands and feet. Of the 54 medicated patients, 58% were able to eliminate hypotensive medication while at the same time reducing BP an average of 15/10 mm Hg. An additional 19 (35%) of the medicated patients were able to cut their medications approximately in half while reducing BP by 18/10 mm Hg. The remaining 4 (7%) medicated patients showed no improvement in either BP or medication requirement. Similar reductions in BP were seen in initially unmedicated patients. Seventy percent of the 23 unmedicated patients achieved average pressures below 140/90 mm Hg, with an additional 22% of these patients making clinically significant reductions in pressure without becoming normotensive, and with 8% unsuccessful at lowering pressures to a clinically significant extent. Follow-up data available on 61 patients over an average of 33 months indicated little regression in these results with 51% of the total patient sample remaining well-controlled off medication, an additional 41% partially controlled, and 8% unsuccessful in lowering either medications and/or blood pressures to a clinically significant extent.This research was partially supported by grant HL-32136. The Authors wish to thank Sarah Bremer for her assistance in preparing this article.     

Withdrawal of long-term diuretic medication in elderly patients: a double blind randomised trial.

OBJECTIVES: About 20% of elderly people use long-term diuretic medication, but there is doubt whether prolonged diuretic medication on such a large scale is necessary. We performed a study to assess what proportion may successfully be withdrawn from diuretic therapy. DESIGN: Double blind randomised controlled trial with six month follow up. SETTING: General practice. SUBJECTS: 202 patients taking long-term diuretics without manifest heart failure or hypertension. INTERVENTIONS: Patients were allocated to either placebo (withdrawal group, n = 102) or continuation of diuretic treatment (control group, n = 100). MAIN OUTCOME MEASURE: Occurrence of clinical conditions requiring diuretic therapy based on fixed criteria. RESULTS: During follow up diuretic therapy was required in 50 patients in the withdrawal group and 13 in the control group (risk difference 36%; 95% confidence interval 22% to 50%). Heart failure was the most frequent cause of prescribing diuretic therapy (n = 25). Cessation of diuretic therapy caused a mean increase in systolic blood pressure of 13.5 (9.2 to 17.8) mm Hg and in diastolic pressure of 4.6 (1.9 to 7.3) mm Hg. CONCLUSION: Withdrawal of long-term diuretic treatment in elderly patients leads to symptoms of heart failure or increase in blood pressure to hypertensive values in most cases. Any attempt to withdraw diuretic therapy requires careful monitoring conditions, notably during the initial four weeks.     

The effect of the phase of prestimulus alpha activity on the averaged visual evoked response

The relationship between the latencies and amplitudes of the N1 and P2 components of the averaged visual evoked potential (EP) and the phase of the alpha activity immediately preceding the time of the stimulus, has been investigated in 7 male subjects. Low intensity flashes, delivered randomly between 2 and 6 whole seconds, were used as the stimuli. The phase angle of the EEG at the moment of stimulation was computed for all trials containing more than 100 μV² of prestimulus alpha power. The single trials were grouped into 8 classes on the basis of the phase angle value, and averaged EPs for each individual were computed from these groups. In addition, averaged EPs were computed in 3 ways: (1) a grand average consisting of all artifact-free trials, (2) an ‘alpha average’ consisting of all trials containing more than 100 μV² of prestimulus alpha power, and (3) a ‘non-alpha average’ consisting of all trials with less than 100 μV² of prestimulus alpha power. Each of these 3 averages were cross-correlated with the phase-selective averages. It was found that the particular N1 component assessed in this experiment may possibly be entrained alpha activity, and that the measured P2 component is not an alpha process, yet it is influenced by the amount of prestimulus alpha activity.     

Limitations of Stroke Volume Estimation by Non-Invasive Blood Pressure Monitoring in Hypergravity

Background

Altitude and gravity changes during aeromedical evacuations induce exacerbated cardiovascular responses in unstable patients. Non-invasive cardiac output monitoring is difficult to perform in this environment with limited access to the patient. We evaluated the feasibility and accuracy of stroke volume estimation by finger photoplethysmography (SVp) in hypergravity.

Methods

Finger arterial blood pressure (ABP) waveforms were recorded continuously in ten healthy subjects before, during and after exposure to +Gz accelerations in a human centrifuge. The protocol consisted of a 2-min and 8-min exposure up to +4 Gz. SVp was computed from ABP using Liljestrand, systolic area, and Windkessel algorithms, and compared with reference values measured by echocardiography (SVe) before and after the centrifuge runs.

Results

The ABP signal could be used in 83.3% of cases. After calibration with echocardiography, SVp changes did not differ from SVe and values were linearly correlated (p<0.001). The three algorithms gave comparable SVp. Reproducibility between SVp and SVe was the best with the systolic area algorithm (limits of agreement −20.5 and +38.3 ml).

Conclusions

Non-invasive ABP photoplethysmographic monitoring is an interesting technique to estimate relative stroke volume changes in moderate and sustained hypergravity. This method may aid physicians for aeronautic patient monitoring.     

The tolerance-hyperbaric test: a chronobiologic approach for improved diagnosis of hypertension

Blood pressure (BP) displays predictable large-amplitude circadian variability. Thus, the identification and the proper definition of hypertension are highly ambiguous when based on single time-unspecified measurements. One way to deal with such variability in the diagnosis of hypertension is to replace the commonly used constant limits of BP by a time-specified reference interval based on the normal circadian BP rhythm assessed by ambulatory BP monitoring (ABPM). A proper reference limit can be constructed, for instance, as a tolerance interval computed for every specific time interval throughout the 24 h. Once such a threshold (given by the upper limit of the tolerance interval) is constructed, a hyperbaric index (HBI) can be computed by numerical integration of the total area of any given patient's BP profile above threshold. The HBI plus the duration of excess within the 24h day serves as nonparametric endpoints for assessing hypertension. Both retrospective and prospective evaluation of this tolerance-hyperbaric test validate its high sensitivity and specificity in the diagnosis of hypertension. We describe the theory of the HBI as well as a newly created dedicated software program that automatically derives the tolerance intervals from a reference database of normotensive subjects and calculates the HBI and other potentially valuable parameters based on data obtained by ABPM. The establishment of time-qualified tolerance limits and the assessment of the extent and timing of BP elevation represents a valuable tool for the more accurate diagnosis of hypertension as well as means of gauging response to treatment.     

Circadian Time Structure of Cardiovascular Characteristics in Human Pregnancy

Conventional time-unspecified single measurements of blood pressure and heart rate may be misleading because they may be influenced, among other factors, by the patient's emotional state, position, diet, and external stimuli. All of these effects depend on the stages of a (mathematical) spectrum of rhythms and trends with age. The evaluation of predictable variability in blood pressure and heart rate by (a) the use of fully ambulatory devices, and (b) chronobiologic data processing, assesses early cardiovascular disease risk, e.g., in pregnancy. We have used this approach to quantify changes in 24-h synchronized (circadian) characteristics of cardiovascular variables in two consecutive pregnancies of a clinically healthy woman. Blood pressure and heart rate were automatically monitored, with few interruptions, at I-h intervals, each time for at least 48 consecutive h, and for a total of 76 days of monitoring in each pregnancy. Circadian parameters of those circulatory variables were computed for each single day of measurement by the least-squares fit of a 24-h cosine curve. Regression analysis of parameters thus obtained revealed patterns of variation of circadian-rhythm-adjusted means and amplitudes with gestational age. In both pregnancies, the predictable variability of the circadian-rhythm-adjusted mean of blood pressure can be approximated by a second-order polynomial model on gestational age: a steady linear decrease in systolic, diastolic, and mean arterial blood pressure up to the 22nd week of pregnancy is followed by an increase in blood pressure up to the day of delivery. This longitudinal study confirms and extends to ambulatory everyday life conditions the predictable pregnancy-associated variability in blood pressure and heart rate and also allows the establishment of prediction and confidence limits for cardiovascular parameters in a healthy pregnancy.     

Circadian Time Structure of Cardiovascular Characteristics in Human Pregnancy

Conventional time-unspecified single measurements of blood pressure and heart rate may be misleading because they may be influenced, among other factors, by the patient's emotional state, position, diet, and external stimuli. All of these effects depend on the stages of a (mathematical) spectrum of rhythms and trends with age. The evaluation of predictable variability in blood pressure and heart rate by (a) the use of fully ambulatory devices, and (b) chronobiologic data processing, assesses early cardiovascular disease risk, e.g., in pregnancy. We have used this approach to quantify changes in 24-h synchronized (circadian) characteristics of cardiovascular variables in two consecutive pregnancies of a clinically healthy woman. Blood pressure and heart rate were automatically monitored, with few interruptions, at I-h intervals, each time for at least 48 consecutive h, and for a total of 76 days of monitoring in each pregnancy. Circadian parameters of those circulatory variables were computed for each single day of measurement by the least-squares fit of a 24-h cosine curve. Regression analysis of parameters thus obtained revealed patterns of variation of circadian-rhythm-adjusted means and amplitudes with gestational age. In both pregnancies, the predictable variability of the circadian-rhythm-adjusted mean of blood pressure can be approximated by a second-order polynomial model on gestational age: a steady linear decrease in systolic, diastolic, and mean arterial blood pressure up to the 22nd week of pregnancy is followed by an increase in blood pressure up to the day of delivery. This longitudinal study confirms and extends to ambulatory everyday life conditions the predictable pregnancy-associated variability in blood pressure and heart rate and also allows the establishment of prediction and confidence limits for cardiovascular parameters in a healthy pregnancy.     

Circulating endothelin parallels arterial blood pressure during sleep in healthy subjects

OBJECTIVE: To characterize plasma endothelin 1 (ET-1) and arterial blood pressure (ABP) time courses during the first complete non-rapid eye movement (NREM)-REM sleep cycle in healthy subjects, together with plasma renin activity (PRA) and plasma atrial natriuretic peptide (ANP). METHODS: Heart rate (HR), intra-arterial blood pressure and sleep electroencephalographic activity were recorded continuously during the night in eight healthy 20-28-year-old males. Blood was sampled every 10 min during their first complete sleep cycle for simultaneous measurements of plasma ET-1, PRA and ANP. RESULTS: Circulating ET-1 demonstrated significant variations during the sleep cycle (p<0.0001) that paralleled those of ABP (p<0.05) and HR (p<0.005), with a minimum during NREM sleep and a maximum during REM sleep. ET-1 time course opposed that of PRA which increases during NREM sleep and decreases during REM sleep (p<0.0005). Plasma ANP did not demonstrate systematic variation in relation with the sleep cycle. CONCLUSION: Circulating ET-1, which parallels variations of ABP, may participate in ABP regulation during sleep in healthy subjects, in association with the renin-angiotensin system.     

Circadian periodicity of cerebral blood flow revealed by laser-Doppler flowmetry in awake rats: relation to blood pressure and activity

Cardiovascular parameters such as arterial blood pressure (ABP) and heart rate display pronounced circadian variation. The present study was performed to detect whether there is a circadian periodicity in the regulation of cerebral perfusion. Normotensive Sprague-Dawley rats (SDR, approximately 15 wk old) and hypertensive (mREN2)27 transgenic rats (TGR, approximately 12 wk old) were instrumented in the abdominal aorta with a blood pressure sensor coupled to a telemetry system for continuous recording of ABP, heart rate, and locomotor activity. After 5-12 days, a laser-Doppler flow (LDF) probe was attached to the skull by means of a guiding device to measure changes in brain cortical blood flow (CBF). After the animals recovered from anesthesia, measurements were taken for 3-4 days. The time series were analyzed with respect to the midline estimating statistic of rhythm (i.e., mean value of a periodic event after fit to a cosine function), amplitude, and acrophase (i.e., phase angle that corresponds to the peak of a given period) of the 24-h period. The LDF signal displayed a significant circadian rhythm, with the peak occurring at around midnight in SDR and TGR, despite inverse periodicity of ABP in TGR. This finding suggests independence of LDF periodicity from ABP regulation. Furthermore, the acrophase of the LDF was consistently found before the acrophase of the activity. From the present data, it is concluded that there is a circadian periodicity in the regulation of cerebral perfusion that is independent of circadian changes in ABP and probably is also independent of locomotor activity. The presence of a circadian periodicity in CBF may have implications for the occurrence of diurnal alterations in cerebrovascular events in humans.     

Contribution of arterial blood pressure and PaCO2 to the cerebrovascular responses to motor stimulation

Motor stimulation induces a neurovascular response that can be detected by continuous measurement of cerebral blood flow (CBF). Simultaneous changes in arterial blood pressure (ABP) and Pa(CO(2)) have been reported, but their influence on the CBF response has not been quantified. Continuous bilateral recordings of CBF velocity (CBFV), ABP, and end-tidal CO(2) (ET(CO(2))) were obtained in 10 healthy middle-aged subjects at rest and during 60 s of repetitive, metronome-controlled (1 Hz) elbow flexion. A multivariate autoregressive-moving average model was adopted to quantify the relationship between beat-to-beat changes in ABP, breath-by-breath ET(CO(2)), and the motor stimulus, represented by the metronome on-off signal (inputs), and the CBFV response to stimulation (output). All three inputs contributed to explain CBFV variance following stimulation. For the ipsi- and contralateral hemispheres, ABP explained 20.3 ± 17.3% (P = 0.0007) and 19.5 ± 17.2% (P = 0.01) of CBFV variance, respectively. Corresponding values for ET(CO(2)) and metronome signals were 22.0 ± 24.2% (P = 0.008), 24.0 ± 24.1% (P = 0.037), 32.7 ± 22.5% (P = 0.0015), and 43.2 ± 25.1% (P = 0.013), respectively. Synchronized population averages suggest that the initial sudden change in CBFV was largely due to ABP, while the influence of ET(CO(2)) was more erratic. The component due to elbow flexion showed a well-defined pattern, with rise time slower than the main CBFV change but reaching a stable plateau after 15 s of stimulation. Identifying and removing the influences of ABP and Pa(CO(2)) to motor-induced changes in CBF should lead to more robust estimates of neurovascular coupling and better understanding of its physiological covariates.     

Confirming the diagnosis of mild hypertension.

Patients with newly found raised blood pressure are known to have lower pressures at subsequent measurements even when not treated. A study was undertaken to determine the extent to which (a) the number of follow-up measurements and (b) the duration of the intervals between them contributed to this fall in pressure. In 42 general practices 110 patients were identified as having for the first time a diastolic pressure (phase V) greater than 90 and less than 110 mm Hg. Both diastolic and systolic pressures were appreciably lower when measured at return visits when compared with the first measurement. The systolic pressure dropped appreciably in the intervals between the first and the second visits and again between the second and third visits. The diastolic pressure fell appreciably only between the first and second visits. The duration of the interval between visits was not associated with a fall in either systolic or diastolic pressure, but the number of measurements was. This pattern of fall in pressure was not affected by the patient''s age or sex. From these results we conclude that patients with newly identified blood pressures that are mildly raised should be seen at two further visits before a decision about treatment is made. The timing of these follow-up visits is not crucial.     

Comparative Evaluation of Heart Rate Variability Based on the Data of ECG and Blood Pressure Measurements

The development of new approaches to the assessment of heart rate variability (HRV) is an important problem, since HRV reflects the functioning of cardiovascular control and is affected by various diseases. The purpose of this study was to evaluate the informative value of statistical and spectral HRV parameters calculated from pulse interval (PI) data of blood pressure as compared with those calculated from RR-interval data of electrocardiograms (ECG). We recorded ECG in conscious rats using skin adhesive electrodes simultaneously with blood pressure signal obtained through a catheter in the femoral artery. It has been found that the PI sequence can be used to calculate the statistical HRV indices that describe the HRV at time intervals about 1 min or longer, but statistical indices of the PI and RR intervals may differ in the analysis of beat-tobeat variations. The power spectra of the RR intervals and PI coincide in the low-frequency region, including the band of baroreflex cardiac rhythm oscillation. However, they can differ in the high-frequency region (at respiration frequency and above).     

Estimation of changes in instantaneous aortic blood flow by the analysis of arterial blood pressure

The purpose of this study was to introduce and validate a new algorithm to estimate instantaneous aortic blood flow (ABF) by mathematical analysis of arterial blood pressure (ABP) waveforms. The algorithm is based on an autoregressive with exogenous input (ARX) model. We applied this algorithm to diastolic ABP waveforms to estimate the autoregressive model coefficients by requiring the estimated diastolic flow to be zero. The algorithm incorporating the coefficients was then applied to the entire ABP signal to estimate ABF. The algorithm was applied to six Yorkshire swine data sets over a wide range of physiological conditions for validation. Quantitative measures of waveform shape (standard deviation, skewness, and kurtosis), as well as stroke volume and cardiac output from the estimated ABF, were computed. Values of these measures were compared with those obtained from ABF waveforms recorded using a Transonic aortic flow probe placed around the aortic root. The estimation errors were compared with those obtained using a windkessel model. The ARX model algorithm achieved significantly lower errors in the waveform measures, stroke volume, and cardiac output than those obtained using the windkessel model (P < 0.05).