Arterial Pressure Signals Research Articles

Spectral analysis of systemic arterial pressure and heart rate signals as a prognostic tool for the prediction of patient outcome in the intensive care unit

To evaluate the applicability of changes in spectra of systemic arterial pressure and heart rate signals in the prediction of patient outcome in an adult intensive care unit (ICU). To compare the prognostic predictability of this method with the Acute Physiology and Chronic Health Evaluation II (APACHE II) scoring system. Prospective data collection from 52 ICU patients. Adult ICU at a large, university-affiliated, medical center. Consecutive patients who were admitted to the adult ICU due to noncardiac emergencies, and who remained for at least 2 days. None. The demographic data, diagnosis, and survival data were recorded for each patient enrolled in this study. For the period between admission and 24 hrs before discharge, the APACHE II score was tabulated daily. Likewise, continuous, on-line, and real-time spectral analysis of systemic arterial pressure and heart rate signals was carried out every day for at least 30 mins at 2200 to 2400 hrs. The averaged power density values during this 30-min recording period of the high-frequency (0.15 to 0.4 Hz), low-frequency (0.08 to 0.15 Hz), and very low-frequency (0.016 to 0.08 Hz) components of systemic arterial pressure and heart rate signals were subsequently computed. Systemic vascular resistance index and cardiac index were also determined daily. We observed a trend of changes in the spectral components of systemic arterial pressure and heart rate signals in patients who eventually survived (n = 25) or died (n = 27). Progressive increases in the power density values of both the low-frequency and very low-frequency components of systemic arterial pressure and heart rate signals appeared to be related to recovery. Conversely, progressive decreases in the power density values of these spectral components was indicative of deterioration and fatality. The predicted outcome based on the trend of changes in the low-frequency and very low-frequency components of systemic arterial pressure and heart rate signals correlated positively with daily APACHE II scores. No direct correlation, however, was indicated by mean systemic arterial pressure, heart rate, systemic vascular resistance index, and cardiac index. We also confirmed that the differential trend of spectral changes in patients who survived or died was not due to circadian rhythm, nor alterations in the responsiveness of the blood vessels to intravenous infusion of dopamine. Power spectral analysis of systemic arterial pressure and heart rate signals offers a reasonable means of monitoring acute, critically ill patients, and may be used as an alternative prognostic tool for the prediction of patient outcome in the ICU.

Detection of dicrotic notch in arterial pressure signals.

A novel algorithm to detect the dicrotic notch in arterial pressure signals is proposed. Its performance is evaluated using both aortic and radial artery pressure signals, and its robustness to variations in design parameters is investigated. Most previously published dicrotic notch detection algorithms scan the arterial pressure waveform for the characteristic pressure change that is associated with the dicrotic notch. Aortic valves, however, are closed by the backwards motion of aortic blood volume. We developed an algorithm that uses arterial flow to detect the dicrotic notch in arterial pressure waveforms. Arterial flow is calculated from arterial pressure using simulation results with a three-element windkessel model. Aortic valve closure is detected after the systolic upstroke and at the minimum of the first negative dip in the calculated flow signal. In 7 dogs ejection times were derived from a calculated aortic flow signal and from simultaneously measured aortic flow probe data. A total of 86 beats was analyzed; the difference in ejection times was -0.6 +/- 5.4 ms (means +/- SD). The algorithm was further evaluated using 6 second epochs of radial artery pressure data measured in 50 patients. Model simulations were carried out using both a linear windkessel model and a pressure and age dependent nonlinear windkessel model. Visual inspection by an experienced clinician confirmed that the algorithm correctly identified the dicrotic notch in 98% (49 of 50) of the patients using the linear model, and 96% (48 of 50) of the patients using the nonlinear model. The position of the dicrotic notch appeared to be less sensitive to variations in algorithm's design parameters when a nonlinear windkessel model was used. The detection of the dicrotic notch in arterial pressure signals is facilitated by first calculating the arterial flow waveform from arterial pressure and a model of arterial afterload. The method is robust and reduces the problem of detecting a dubious point in a decreasing pressure signal to the detection of a well-defined minimum in a derived signal.

Selective activation of vasomotor component of SAP spectrum by nucleus reticularis ventrolateralis in rats.

We evaluated the contribution of the rostral nucleus reticularis ventrolateralis (NRVL) to the vasomotor component in the spectrum of systemic arterial pressure (SAP) signals by quantifying the transfer function between electrical stimulation of this medullary nucleus and the SAP response. Sprague-Dawley rats anesthetized with pentobarbital sodium, paralyzed with pancuronium, and mechanically ventilated were used. Broad-band stimulation of the NRVL with computer-generated rectangular current pulses (10-50 microA, 1 ms), at a mean spike rate of 50 pulses/s and randomized modulation frequency of 0-3 Hz, elicited a site-specific and intensity-related pressor response. Intriguingly, the corresponding autospectrum of SAP signals exhibited prevailing power density only in the lower frequency range (0-0.8 Hz). This low-pass response characteristic was confirmed by the observation that 90% of the total magnitude of transfer function between NRVL stimulation and SAP response concentrated between 0 and 0.6 Hz. The magnitude of NRVL-SAP transfer function was significantly reduced by phentolamine or prazosin but appreciably enhanced by yohimbine. We conclude that the NRVL may contribute to the very-low (0-0.25 Hz)- and low (0.25-0.8 Hz)-frequency components of the SAP spectrum, which are belived to reflect sympathetic modulation on vasomotor activity via alpha-adrenergic neurotransmission.

Correction for respiration artifact in pulmonary blood pressure signals of ventilated patients.

To develop an algorithm that corrects pulmonary artery pressure signals of ventilated patients for the respiration artifact. The algorithm should test the validity of the pulmonary pressure signal and differentiate between the cyclic respiration artifact and true measurement artifacts. The shape of each pulmonary pressure beat is described by eight characteristic features, including mean pressure value and the systolic and diastolic timing and pressure values. The features are corrected for the respiration artifact by fitting them in a least-squares sense on the first and second harmonics of the ventilator frequency. The corrected features are used by a signal validation algorithm, which adds a validity flag to each pressure beat. The validation algorithm rejects pressure beats with sudden changes in their shape but adapts itself when the changes persist. The performance of the correction and validation technique was evaluated using pulmonary artery pressure signals of 30 patients who were scheduled for open heart surgery. The algorithm correctly recognized as invalid data those pressure signals disturbed by coagulation, surgical manipulations, or flushes of the pressure line. The algorithm marked on average 77 +/- 11% of the pulmonary pressure beats as valid. The validation algorithm marked sufficient pressure beats as valid to update a trend display every 5 sec. The correction algorithm enabled the validation algorithm to differentiate between true measurement artifacts and the respiration artifact.

Auto- and cross-spectral analysis of cardiovascular fluctuations during pentobarbital anesthesia in the rat.

We applied auto- and cross-spectral analysis of systemic arterial pressure (SAP) and heart rate (HR) signals to quantify the effects of pentobarbital sodium on short-term cardiovascular fluctuations in adult, male Sprague-Dawley rats. Intravenous administration of pentobarbital, delivered as a bolus injection (5, 10, or 20 mg/kg) or continuous infusion (10, 20, or 40 mg.kg-1.h-1), elicited only mild hypotension and tachycardia. This was accompanied by a dose-related depression of the very low (0-0.25 Hz) and low (0.25-0.8 Hz)-frequency components of both SAP and HR signals and high (0.8-2.4 Hz)-frequency component of HR signals. Cross-spectral analysis of SAP and HR signals during intravenous infusion of pentobarbital revealed a maintained coherence in the high-frequency range, together with a gradual and dose-related decrease in magnitude of transfer function and baro-receptor reflex sensitivity. Stable plasma concentration and all hemodynamic parameters were observed during 120 min of infusion at 20 mg.kg-1.h-1. Under this dosing condition, autonomic blockade by phentolamine, propranolol, or atropine still evoked discernible but differential reductions in the SAP and HR spectral components. Our data suggest that continuous intravenous administration of pentobarbital at 20 mg.kg-1.h-1 offers maintained anesthesia while preserving the capacity of cardiovascular regulation.

Estimation of left ventricular function in patients with a left ventricular assist device.

Patients with profound left ventricular (LV) dysfunction supported by a left ventricular assist device (LVAD) may have recovery of LV function, but current methods for assessing this are difficult or not readily available. A new method to estimate LV performance was tested in 6 patients on LVAD (age, 47 +/- 12 years). Simultaneous measures of LV cross-sectional area (substitute for volume) using transesophageal echocardiographic automated border detection and femoral arterial pressure (substitute for LV ejection pressure) were recorded during brief periods on and off LVAD. During the LVAD off period, femoral arterial pressure and LV area signals were combined on-line to generate arterial pressure-LV area loops. The slope of the linear regression line connecting the end systolic points generated during LV filling after LVAD off was used as an estimate of end systolic elastance denoted by Ees. The average Ees was 3.8 +/- 3.1 mmHg/cm2. These values are significantly lower than those of 13 control patients (22.3 +/- 15.4 mmHg/cm2). This method of estimating Ees may be clinically useful in assessing LV recovery in patients on LVAD after extended support.

Continuous, on-line, real-time spectral analysis of SAP signals during cardiopulmonary bypass

We communicated the application of continuous, on-line, real-time power spectral analysis of systemic arterial pressure (SAP) signals during cardiopulmonary bypass when the heart was functionally but reversibly disconnected from the blood vessels. Based on observations from 15 cases of successfully completed coronary artery bypass grafting procedures, we found that the very low (0.00-0.08 Hz), low (0.08-0.15 Hz)-, high (0.15-0.25 Hz)-, and very high (0.80-1.60 Hz) frequency components of SAP signals exhibited differential changes before, during, and after cardiopulmonary bypass. In particular, the very low-frequency component, which purportedly represents the contribution of vasomotor activity to SAP, presented only a mild decrease in power during hypothermic cardioplegia. Interestingly, the total peripheral resistance also manifested only a slight reduction during the same period. On the other hand, the low-, high-, and very high frequency components were essentially eliminated. These results unveiled an active role for the blood vessels in the maintenance of SAP during cardiopulmonary bypass, possibly as a result of a maintained vasomotor tone as reflected by the sustained very low frequency component of the SAP signals.

Detection of loss of cerebral vascular tone by correlation of arterial and intracranial pressure signals

With the use of a laboratory model, arterial and intracranial pressure signals were obtained under conditions of intact regulation of cerebral blood flow and massive dilation. During elevated intracranial pressure and intact regulation, positive pressure inhalation appears to briefly occlude venous flow into the cranial sinuses during inspiration. As a result, the intracranial pressure and arterial pressure signals are not similar. In contrast, when maximal dilation causes failure of regulation of cerebral blood flow, the intracranial pressure signal is approximately proportional to the arterial pressure signal. Comparison of the cross-correlation function derived from the intracranial and arterial pressure signals to the autocorrelation function of the arterial signal reveals that the two correlation functions are: 1) different during intact regulation and 2) nearly identical during dilation induced failure of regulation of cerebral blood flow.

Power spectral analysis of electromyographic and systemic arterial pressure signals during fentanyl-induced muscular rigidity in the rat

We have measured electromyographic (EMG) and systemic arterial pressure (SAP) signals during fentanyl-induced muscular rigidity in adult male Sprague-Dawley rats anaesthetized initially with ketamine 120 mg kg-1 i.p. during controlled ventilation. Fentanyl 100 micrograms kg-1 i.v. induced significant increase in EMG activity, recorded from the sacrococcygeus dorsi lateralis muscle. Power spectral analysis revealed that this was produced by an increase in the root mean square and a decrease in the mean power frequency values of the signals, signifying recruitment and synchronous activation of motor units. Together with transient hypotension and bradycardia, power spectral analysis of the SAP signals demonstrated a reduced but maintained power density of the frequency components that represent respiratory, baroreceptor and vasomotor activities. All these effects were only demonstrated unequivocally in rats maintained by i.v. infusion of ketamine until 10 min before the administration of fentanyl. We conclude that analysis of the temporal alterations in the spectral components of the EMG and SAP signals in rats during mechanical ventilation provides a sensitive method of measuring fentanyl-induced muscular rigidity and the accompanying alterations in haemodynamic variables.

Power spectral analysis of the electroencephalographic and hemodynamic correlates of propofol anesthesia in the rat: Intravenous infusion

Based on the tail-flick response to noxious thermal stimuli, we determined in the present study that effective antinociception could be achieved in adult male Sprague-Dawley rats 15 min after intravenous infusion of propofol at 60 mg/kg/h. Simultaneous power spectral analysis of the electroencephalographic (EEG) and systemic arterial pressure signals further revealed a concomitant depression of the activity of all EEG frequency bands (δ, θ, α, β), alongside hypotension, negative inotropic and chronotropic actions, and attenuated baroreceptor reflex and vasomotor activity. These effects were congruent with a plasma concentration of propofol in the arterial blood of 1.70 ± 0.13 μg/ml, as determined by high-performance liquid chromatography.

Continuous, on-line, real-time spectral analysis of systemic arterial pressure signals

We present in this communication an algorithm that offers continuous, on-line, real-time analysis of the power spectrum (CORAPS) of systemic arterial pressure (SAP) signals. Based on direct fast Fourier transform of raw SAP signals into power spectral density, it is suitable for the delineation of acute SAP spectral changes over time after physiological or pharmacological perturbations. Thus it is an improvement over most current methods, which only generate the spectra off-line or from intermittent data. Furthermore, all executions can be accomplished economically with a general-purpose personal computer. We demonstrated and quantified vivid sequential alterations in the low-frequency components (< 3 Hz) of the SAP spectrum, which purportedly reflect the influence of vasomotor, baroreceptor, and respiratory activity on SAP during pentobarbital administration or transient hypertension. Three window options incorporated into the algorithm further improved the frequency or temporal resolution and discrimination of the spectral changes during abrupt hypertension or hypotension. Thus our CORAPS algorithm may be useful as an on-line monitor for acute hemodynamic changes.

Interpretation of the Laser Doppler Flow Signal from the Liver of the Rat

It has been proposed that the laser Doppler flow (LDF) signal from the surface of the rat liver is almost exclusively a measure of hepatic arterial and not of total liver blood flow and therefore that LDF is not a suitable technique for the measurement of blood flow in the hepatic microcirculation. The objective of the present study was twofold: (i) to establish that liver blood flow is homogeneously distributed and (ii) to assess the behavior of the LDF signal during changes in hepatic perfusion. When 31Cr-labeled microspheres were injected into the portal vein ( n = 12), no significant differences in the relative flow (cpm/lobe to cpm/liver) to each of the liver lobes were found nor was there any difference in the ratio of flow to the outer 1-2 mm of lobe as compared to that to the "core" of the liver. Temporary occlusion of the hepatic artery and the portal vein caused ∼13% ( n = 7, P < 0.001) and ∼74% ( n = 7, P < 0.001) fall in LDF signal, respectively. Diversion of flow from the anterior to the posterior lobes ( n = 5) caused a 97.9 ± 21.1% (SD, P < 0.001) rise in LDF signal in the posterior lobes. Zero-flow LDF signal was found to represent 13.0 ± 4.1% of maximum. Hemorrhage (in 1.5-ml aliquots) was associated with a fall in mean arterial pressure (MAP) and LDF signals. A linear relationship between MAP and the LDF signal ( r > 0.9) was found. Reinfusion of blood caused both MAP and the LDF signal to return to normal. We conclude that (i) blood flow in rat liver is homogeneously distributed; (ii) the LDF signal from the liver surface responds in a manner predicted by conventional theories of hepatic hemodynamics during alteration, either independent or combined, in hepatic arterial and portal venous blood flow; and (iii) LDF may be used to measure relative changes in hepatic perfusion but problems associated with zero-flow signal and intersite variability preclude its quantification in absolute flow units.

Power spectral analysis of the electroencephalographic and hemodynamic correlates of propofol anesthesia in the rat: Intravenous bolus administration

Based on the tail-flick response to noxious thermal stimuli, we determined in the present study that the minimal effective antinociceptive dose of propofol in adult, male Sprague-Dawley rats, given in an intravenous bolus manner, was 10 mg/kg. Simultaneous power spectral analysis of the electroencephalographic (EEG) and systemic arterial pressure signals further revealed a concomitant transient suppression of the EEG activity, primarily in the ϑ and σ bands, alongside minor hypotensive and negative inotropic and chronotropic actions, but with maintained vasomotor tone. These alterations followed a time course that paralleled the plasma concentration of propofol in the arterial blood, as detected by high-performance liquid chromatography.

Simultaneous, continuous, on-line and real-time spectral analysis of multiple physiologic signals by a personal-computer-based algorithm.

We presented in this communication a computer algorithm that offers the advantage of simultaneous analysis of the power spectrum of multiple physiologic signals, including systemic arterial pressure, single-neuronal and electroencephalographic signals, on a continuous, on-line and real-time basis. It provided sensitive definition, discrimination and quantification (graphic and digital) of the various frequency components in the spectrum. There was also great flexibility in selecting the physiologic signal(s) to be analyzed, the choice of results of analysis to be displayed for each physiologic parameter and the alteration of sensitivity and baseline of each event channel. Furthermore, it included window options to improve the frequency or temporal resolution and discrimination of the spectral changes. Most importantly, all these desirable features can be accomplished economically with a general purpose personal computer.

Expert control of the arterial blood pressure during surgery

During and after many surgical procedures the patient's arterial blood pressure must be artificially decreased to a lower than normal level. Although there are alternatives, infusion of the drug sodium nitroprusside (SNP) is frequently the preferred technique to achieve this controlled hypotension. The fast action of the drug and the danger of a too low pressure make manual control of the SNP infusion flow rate, even if done by an expert, a difficult and demanding task. This is mainly due to an occasional large unpredictable variability over time of the patient's sensitivity to SNP, and to the fact that a multitude of other factors also influence the arterial pressure. Due to these and several other causes, current automatic controllers cannot handle all cases equally well. A new expert system based SNP controller was designed to perform well for all patients, regardless of their characteristics. It monitors and adjusts its own performance, employing a number of heuristics derived from a careful study of the properties of the arterial pressure signal, the effects of SNP and other clinical provocations on the arterial pressure, and the ways in which expert clinicians manually manage the SNP infusion. Expert systems technology allows the new controller to access and employ this type of expert medical knowledge, resulting in expert-level performance. The controller was tested on 30 patients undergoing cardiac surgery, both before, during and after bypass. It was safe, needed little attention, and performed well in all cases.

The Inhibitory Action of Halothane on Reflex Constriction in Mesenteric Capacitance Veins

Potent inhalational anesthetics depress autonomic reflex responses at multiple sites. Most studies emphasize cardiac chronotropic changes and changes in systemic blood pressure. Recently, active reflex venoconstriction of 500-1,000 microns O.D. mesenteric veins has been demonstrated. In the current study, the effects of halothane on the reflex responses of similar mesenteric veins were measured. Mesenteric vein diameter and intravenous pressure were measured in 500-1,000 microns O.D. veins from the mesentery of segments of terminal ileum externalized in situ from 27 New Zealand white rabbits anesthetized with alpha-chloralose. Mean arterial pressure was measured with femoral arterial cannulation, and heart rate was determined from the arterial pressure signal. In a separate group of six animals, sympathetic efferent nerve activity was measured from a postganglionic splanchnic nerve. Reflex venoconstriction and increases in mean arterial pressure and mesenteric vein pressure in response to bilateral carotid occlusion were attenuated by 0.5% and 1% inhaled halothane but not by superfusate equilibrated with 3% halothane. Decreases in mesenteric vein diameter and increases in mesenteric vein pressure in response to celiac ganglion stimulation were unaffected by both 0.5% inhaled halothane and superfusate equilibrated with 5% halothane. The bilateral carotid occlusion reflex-mediated increase in sympathetic efferent nerve activity was depressed by both 0.5% and 1% inhaled halothane. The effect of inhaled halothane on prestimulation baseline vein diameter was inconsistent. Superfusate equilibrated with 5% but not 3% halothane caused baseline venodilation. These results suggest a mechanism whereby control of venous tone is inhibited by halothane proximal to the postganglionic neuron. This could involve central or ganglionic inhibition.

Pulsatile vs. mean component of baroreflex compensation for posthemorrhage hypotension.

We studied the influence of pulsatile pressure and mean arterial pressure signals on the restoration of arterial pressure after 10% hemorrhage in seven anesthetized dogs. After transection of the aortic nerve, a quick 10% hemorrhage was repeated under four different sinus conditions: condition 1, carotid sinus pressure depulsated and fixed at a level equal to the prehemorrhage level (no feedback); condition 2, pulsatile component of aortic pressure fed back to the carotid sinus with a fixed mean pressure (pulsatile feedback); condition 3, depulsated mean aortic pressure fed back (mean pressure feedback); condition 4, both pulsatile and mean pressure fed back (pulsatile plus mean component feedback). The restoration of arterial pressure in condition 2 was not significantly different from that in condition 1, but there was greater restoration in conditions 3 and 4. At 1.5 min posthemorrhage, the open-loop gains calculated from the restoration values were nearly zero for the pulsatile feedback only, 2.8 +/- 0.8 for mean arterial pressure feedback, and 1.5 +/- 0.3 for pulsatile and mean pressure feedback. These results indicate that the pulsatile component of the carotid baroreflex contributes minimally to the restoration of arterial pressure after 10% hemorrhage in the anesthetized dog.

Three-element model for total systemic circulation: Emphasis on the accuracy of parameter estimates

In this study, the accuracy achievable in the parameter estimates of a three-element linear model for the systemic vascular bed is considered. The model neglects inertial effects and includes only three elements representing arterial compliance, peripheral resistance and venous compliance, in agreement with recent sensitivity investigations. Parameter estimation starts from arterial and right atrial pressure signals generated by a closed-loop simulator of the cardiovascular circulation and corrupted with normal noise to account for measurement errors. In this way, the influences of a wide variety of circulatory conditions were investigated. The results achieved give evidence that arterial compliance is generally well estimated, while venous compliance is more variable, particularly at high peripheral resistance where measured signals appear to be less sensitive to this parameter. However, presence of cardiac disease, such as heart failure and valvular stenosis has minimal influence on compliance estimates. These results suggest that this simple model can be conveniently applied even under noisy conditions.

A real-time pressure algorithm

A digital computer algorithm is described for real-time detection of critical points in the arterial pressure signal. Two externally selected parameters allow for filtering without loss of high frequency components. The accuracy of the algorithm was tested on 1005 cycles. Worse case a verage error was 2 mm Hg for diastolic pressure, 4 mm Hg for peak systolic pressure, and 11 msec in the placement of the dicrotic notch, Stability of the algorithm was shown by detection of 1004 of the 1005 cycles with two false positive cycles. A relative error bound is given for the Kouchoukos pulse contour formula for the estimate of stroke volume. The bound is a function of the notch placement relative error.

Computer detection of distortion in arterial pressure signals.

An application of the mathematical concept of curvature to the detection of distortion in arterial pressure signals is described. Curvature is defined and examples of digitally filtered pressure tracings show the relationship between curvature and the degree of damping. Even though curvature is also a function of the cardiovascular system producing the signal, the average sum of curvature can generally distinguish control signals from signals distorted by moderate amounts of blood or air in the catheter systems. Selected pressure signals with their associated sums of curvature show the properties and limitations of curvature for discriminating between the changes in the cardio-vascular system and distortion of pressure signals.