Continuous non-invasive finger arterial pressure monitoring reflects intra-arterial pressure changes in children undergoing cardiac surgery.

BACKGROUND: Continuous non-invasive measurement of finger arterial pressure (FAP) is a reliable technology in adults. FAP is measured with an inflatable cuff around the finger and simultaneously converted to a reconstructed brachial artery pressure waveform (reBAP) by the Nexfin™ device. We assessed the adequacy of a prototype device (Nexfin-paediatric), designed for a paediatric population, for detecting rapid arterial pressure changes in children during cardiac surgery. METHODS: Thirteen anaesthetized children with a median age of 11 months (2 months-7 yr) undergoing congenital cardiac surgery were included in the study. reBAP and intra-arterial pressure (IAP) were recorded simultaneously during the surgical procedure. To assess the accuracy of reBAP in tracking arterial pressure changes, the four largest IAP variations within a 5 min time interval were identified from each procedure. These variations were compared offline with reBAP during a 10 s control period before and a 10 s period after an arterial pressure change had occurred. RESULTS: In 10 out of 13 children, a non-invasive arterial pressure recording could be obtained. Therefore, recordings from these 10 children were eligible for further analysis, resulting in 40 data points. The correlation coefficient between reBAP and IAP in tracking mean arterial pressure (MAP) changes was 0.98. reBAP followed changes in IAP with a mean bias for systolic, diastolic arterial pressure, and MAP of 0.0 mm Hg (sd 5.8), 0.1 (sd 2.8), and 0.19 (sd 2.7), respectively. CONCLUSIONS: The prototype device closely follows arterial pressure changes in children. However, in a considerable number of attempts, obtaining a signal was time-consuming or unsuccessful. This technique seems promising but requires further technical development.

Beat-to-Beat Measurement of Cardiovascular Effects of a Single Subcutaneous Dose of Terbutaline in Healthy Subjects.

OBJECTIVE: To evaluate the cardiovascular effects over time of a single subcutaneous (SC) dose of terbutaline 0.75mg in young healthy volunteers using continuous, beat-to-beat monitoring of cardiovascular effects. DESIGN AND METHODS: Nine healthy young volunteers were administered a SC dose of terbutaline sulphate 0.75mg. Cardiovascular effects were continuously monitored over 2 hours using Finapres and Modelflow technology. Blood was drawn at several timepoints for determination of the plasma terbutaline concentration. RESULTS: The peak plasma concentration of terbutaline was 17.3 ± 4.5 (µg/L at 28.9 ± 12.5 minutes after SC administration. Changes in cardiovascular parameters were observed very quickly, with increases in stroke volume (16.7 ±8.9%), cardiac output (46.0 ± 22.6%), systolic blood pressure (15.1 ± 11.6%) and heart rate (48.1 ± 15.7%) at 9.3 ± 3.8, 16.9 ± 4.8, 21.3 ± 8.9 and 49.7 ± 16.4 minutes, respectively. In five of eight subjects a very rapid (at 9.6 ± 3.7 minutes) drop in diastolic blood pressure (9.8 ± 5.1 %) was observed, while total peripheral resistance decreased maximally by 33.5 ± 7.9% at 18.9 ± 7.1 minutes. CONCLUSIONS: The magnitude of the cardiovascular effects again stresses the need for cautious use of SC terbutaline in patients with a history of cardiovascular disease. The time-course of some of the cardiovascular effects of a SC dose of terbutaline in relation to terbutaline plasma concentrations was unexpected and suggests direct ß2-adrenoreceptor-mediated effects on the heart. Further investigations using both selective and nonselective ß-receptor antagonists are needed to unravel the complex interactions of ß2-receptor-mediated terbutaline-induced effects and cardiovascular reflex mechanisms.

Mechanisms underlying the impairment in orthostatic tolerance after nocturnal recumbency in patients with autonomic failure.

In the present study, we have assessed in patients with neurogenic orthostatic hypotension the haemodynamics underlying the reduced tolerance to standing after prolonged recumbency at night. In 10 patients with neurogenic orthostatic hypotension (age 33-68 years), of which seven were being treated with fludrocortisone and/or sleeping in the 12 degrees head-up tilt position, 24 h continuous non-invasive finger blood pressure was recorded by a Portapres device. Beat-to-beat blood pressure, heart rate, stroke volume, cardiac output and total peripheral vascular resistance obtained by pulse contour analysis were assessed during 5 min of standing in the evening (at 22.30 hours) and in the morning (at 06.30 hours). On average, the inverse of the normal 24 h blood pressure profile was found, with a large diversity in blood pressure profiles among patients. Supine blood pressure values were similar, but standing blood pressure values were lower in the morning than in the evening (P<0.01). This resulted from larger falls in stroke volume and cardiac output upon standing in the morning compared with the evening, while total peripheral resistance did not change. There was no relationship between the decrease in body weight during the night (mean 0.9 kg; range 0.2-1.6 kg) and the evening-morning difference in standing blood pressure. We conclude that, in patients with neurogenic orthostatic hypotension, the impaired tolerance to standing in the morning is due to larger falls in stroke volume and cardiac output. Not only nocturnal polyuria, but also a redistribution of body fluid, are likely mechanisms underlying the pronounced decreases in stroke volume and cardiac output after prolonged recumbency at night.

A comparison of cardiac output derived from the arterial pressure wave against thermodilution in cardiac surgery patients.

In three clinical centres, we compared a new method for measuring cardiac output with conventional thermodilution. The new method computes beat-to-beat cardiac output from radial artery pressure by simulating a three-element model of aortic input impedance, and includes non-linear aortic mechanical properties and a self-adapting systemic vascular resistance. We compared cardiac output by continuous model simulation (MF) with thermodilution cardiac output (TD) in 54 patients (18 female, 36 male) undergoing coronary artery bypass surgery. We made three or four conventional thermodilution estimates spread equally over the ventilatory cycle. In 490 series of measurements, thermodilution cardiac output ranged from 2.1 to 9.3, mean 5.0 litre min(-1). MF differed +0.32 (1.0) litre min(-1) on average with limits of agreement of -1.68 and +2.32 litre min(-1). Differences decreased when the first series of measurements in a patient was used to calibrate the model. In 436 remaining series, the mean difference became -0.13 (0.47) litre min(-1) with limits of agreement of -1.05 and +0.79 litre min(-1). When consecutive measurements were made, the change was greater than 0.5 litre min(-1), on 204 occasions. The direction of change was the same with both methods in 199. The difference between the methods remained near zero during surgery suggesting that a single calibration per patient was adequate. Aortic model simulation with radial artery pressure as input reliably monitors changes in cardiac output in cardiac surgery patients. Before calibration, the model cannot replace thermodilution, but after calibration the model method can quantitatively replace further thermodilution estimates.

Why use Finapres or Portapres rather than intra-arterial or intermittent non-invasive techniques of blood pressure measurement?

In the clinic, blood pressure is measured almost exclusively using non-invasive intermittent techniques, of which the auscultatory (Riva-Rocci/Korotkoff, RRK) and the computerized oscillometric method are most often used. However, both methods only provide a momentary value. In addition, the accuracy is hampered by phenomena such as cuff response and white coat hypertension, thus providing artefactually increased values. The vascular unloading technique of Penáz together with the Physiocal criteria of Wesseling provide reliable, non-invasive and continuous estimates of blood pressure. This technique is thus an alternative to the invasive intra-arterial measurements in many cases, without the risks and ethical questions inherent to invasive measurements. Since the pressure waveform is available continuously, computations such as pulse contour and Modelflow cardiac output, spectral analysis and baroreflex sensitivity provide further information on the dynamics of the cardiovascular system on a beat-to-beat basis, similar to intra-arterial measurements.

Single injection thermodilution. A flow-corrected method.

BACKGROUND: Application of the Stewart-Hamilton equation in the thermodilution technique requires flow to be constant. In patients in whom ventilation of the lungs is controlled, flow modulations may occur leading to large errors in the estimation of mean cardiac output. METHODS: To eliminate these errors, a modified equation was developed. The resulting flow-corrected equation needs an additional measure of the relative changes of blood flow during the period of the dilution curve. Relative flow was computed from the pulmonary artery pressure with use of the pulse contour method. Measurements were obtained in 16 patients undergoing elective coronary artery bypass surgery. In 11 patients (group A), pulmonary artery pressure was measured with a catheter tip transducer, in a partially overlapping group of 11 patients (group B), it was measured with a fluid-filled system. For reference cardiac output we used the proven method of four uncorrected thermodilution estimates equally spread over the ventilatory cycle. RESULTS: A total of 208 cardiac output estimates was obtained in group A, and 228 in group B. In group B, 48 estimates could not be corrected because of insufficient pulmonary artery pressure waveform quality from the fluid-filled system. Individual uncorrected Stewart-Hamilton estimates showed a large variability with respect to their mean. In group A, mean cardiac output was 5.01 l/min with a standard deviation of 0.53 l/min, or 10.6%. After flow correction, this scatter decreased to 5.0% (P < 0.0001). With no bias, the corresponding limits of agreement decreased from +/- 1.06 to +/- 0.5 l/min after flow correction. In group B, the scatter decreased similarly and the limits of agreement also became +/- 0.5 l/min after flow correction. CONCLUSION: It was concluded that a single thermodilution cardiac output estimate using the flow-corrected equation is clinically feasible. This is obtained at the cost of a more complex computation and an extra pressure measurement, which often is already available. With this technique it is possible to reduce the fluid load to the patient considerably.

Computation of aortic flow from pressure in humans using a nonlinear, three-element model.

We computed aortic flow pulsations from arterial pressure by simulating a nonlinear, time-varying three-element model of aortic input impedance. The model elements represent aortic characteristic impedance, arterial compliance, and systemic vascular resistance. Parameter values for the first two elements were computed from a published, age-dependent, aortic pressure-area relationship (G. J. Langewouters et al. J. Biomech. 17:425-435, 1984). Peripheral resistance was predicted from mean pressure and model mean flow. Model flow pulsations from aortic pressure showed the visual aspects of an aortic flow curve. For evaluation we compared model mean flow from radial arterial pressure with thermodilution cardiac output estimations, 76 times, in eight open heart surgical patients. The pooled mean difference was +7%, the SD 22%. After using one comparison per patient to calibrate the model, however, we followed quantitative changes in cardiac output that occurred either during changes in the state of the patient or subsequent to vasoactive drugs. The mean deviation from thermodilution cardiac output was +2%, the SD 8%. Given these small errors the method could monitor cardiac output continuously.

Cardiovascular instability and baroreflex activity in a patient with tetanus.

In a patient with tetanus we tested the hypothesis that the hyperadrenergic cardiovascular instability might be due to impairment of the baroreceptor reflex by the tetanus toxin. Baroreflex sensitivity assessed with the phenylephrine method was found to be normal. Changes in arterial pressure correlated inversely with relative changes in plasma volume but not with plasma catecholamine levels. There were both extreme hypo- and hyper-adrenergic episodes. We conclude that sympathetic overactivity in tetanus temporarily overrules a functionally intact baroreflex leading to severe blood pressure instability with episodes of hypertension.

Continuous cardiac output monitoring by pulse contour during cardiac surgery.

Most pulse contour methods are unreliable under changing haemodynamic conditions, because no corrections are made for pressure-dependent compliance and reflections of pressure waves. The pulse contour method of Wesseling includes such corrections. Four thermodilution measurements equally spread over the ventilatory cycle were used to calibrate and evaluate this pulse contour method. We designed a prototype incorporating a combination of the thermodilution method and pulse contour method and evaluated its potential for monitoring patients undergoing coronary bypass graft operation. Eight to 12 times during the operation, cardiac output was estimated by pulse contour and by thermodilution. The results were compared: the linear regression between the methods was COpc = 0.3 + 0.94. COth, (r = 0.94). The standard deviation for the difference between the methods against the mean of the methods was 10.6%. We concluded that the corrected pulse contour method estimates cardiac output accurately, even when heart rate, blood pressure, and total peripheral resistance change substantially.

Non-invasive continuous finger blood pressure measurement during orthostatic stress compared to intra-arterial pressure.

STUDY OBJECTIVE: The aim of the study was to evaluate whether invasive blood pressure responses to orthostatic stress can be replaced by non-invasive continuous finger blood pressure responses. DESIGN - Intrabrachial and Finapres blood pressures were simultaneously measured during passive head up tilt and during active standing from the supine or sitting position in healthy volunteers. SUBJECTS: Subjects (n = 11) were healthy males aged 22-40 years, mean age 30 years. MEASUREMENTS AND MAIN RESULTS: Beat to beat values of systolic, diastolic, and mean arterial pressures were calculated for both methods of measurement and the differences compared during a 30 s control period before, and during the first 120 s after, the change of posture. In the control period the difference between finger pressure and intra-arterial pressure was +2 (SD11) mm Hg for systolic, -4(8) mm Hg for mean, and -3(7) mm Hg for diastolic pressure. Compared to these control differences, the average finger minus intra-arterial pressure differences for mean and diastolic pressures during the three orthostatic manoeuvres were always within the range -6 to +6 mm Hg. The average systolic deviations were larger, ranging from -10 to +11 mm Hg. Despite these differences the characteristic changes in intra-arterial pressure during the initial phase (first 30 s after change of posture), as well as during the early steady state phase (1-2 min after the change of posture) were well reflected by finger pressure measurements in all subjects. CONCLUSIONS: For the assessment of the continuous blood pressure responses to orthostatic stress similar information can be obtained with non-invasive Finapres blood pressure recordings as with invasive recordings.

An adequate strategy for the thermodilution technique in patients during mechanical ventilation.

The application of the thermodilution method in conditions associated with variations in blood flow implies a misuse of the Stewart Hamilton equation. Therefore, we studied the reliability of the thermodilution method for the estimation of mean cardiac output (CO) during mechanical ventilation in patients (n = 9). Variation of the injection moment in the ventilatory cycle elicited a cyclic variation of CO estimates. This variation was not the same for all patients neither in phase nor in amplitude. Therefore, no specific phase in the ventilatory cycle could be selected for an accurate estimation of mean CO. Averaging CO estimates randomly distributed in the ventilatory cycle led to an improvement of accuracy with the square root of the number of observations. The averaging of CO estimates spread equally over the ventilatory cycle led to a much better result, e.g., the variation in the average of two estimates equally spread in the ventilatory cycle was similar to the variation in the average of four random estimates. We conclude that averaging of 3 or 4 estimates spread equally over the ventilatory cycle is an adequate strategy to estimate mean cardiac output in patients reliably.

Comparison of Finapres non-invasive beat-to-beat finger blood pressure with intrabrachial artery pressure during and after bicycle ergometry.

To evaluate the accuracy of continuous non-invasive blood pressure measurements in the finger during exercise, Finapres blood pressures of six normotensive healthy males were measured during increasing levels of bicycle exercise, using simultaneously registered ipsilateral intrabrachial artery pressures as a reference. At rest, finger systolic blood pressure was higher and finger diastolic and mean arterial pressures were lower than the corresponding intrabrachial pressures in five of the six subjects. During exercise, average finger diastolic and mean arterial pressures did not differ further from these intrabrachial pressures, but finger systolic pressure increased considerably more than the direct systolic pressure, exceeding it by 26 +/- 20 mmHg (mean +/- s.d.) at maximal exercise. This latter finding potentially limits the use of finger blood pressure measurements during exercise.

Continuous non-invasive blood pressure monitoring: reliability of Finapres device during the Valsalva manoeuvre.

Because of the inherent risks of intra-arterial blood pressure monitoring a new non-invasive device, Finapres, which measures blood pressure continuously in the finger, was evaluated in 14 hypertensive and one normotensive subject. Brachial intra-arterial and finger pressures were compared during a control period and a subsequent Valsalva manoeuvre. Visually, blood pressures measured by Finapres faithfully reproduced the intra-arterial recordings in all subjects. From each pressure signal beat to beat systolic, diastolic, and mean blood pressure values and their differences were obtained and the time course of the response and its characteristic features were analysed. During the control period the Finapres measurements were lower than intra-arterial systolic, mean, and diastolic pressures (mean(SD) 1(9.6), 9(6.8), and 4(6.1) mmHg respectively). During the response to the Valsalva manoeuvre the brachial-finger pressure differences showed limited deviation from those during the control period; median differences were at most 6 mmHg occurring late during the intrathoracic strain period and 7 mmHg during the post-release blood pressure overshoot. In general, the Finapres device reproduced intra-arterial patterns faithfully. This device appears to offer a reliable alternative to intra-arterial blood pressure monitoring.

Acute dysautonomia associated with Hodgkin's disease.

A patient is described with acute dysautonomia associated with Hodgkin's disease. Testing of cardiovascular reflex control showed that this patient had a rare manifestation of autonomic cardiovascular neuropathy, namely intact parasympathetic heart rate control in combination with a sympathetic postganglionic lesion affecting the control of the vascular tree.

Effects of peripheral vasoconstriction on the blood pressure in the finger, measured continuously by a new noninvasive method (the Finapres).

The authors determined whether vasoconstriction alters the ability of a noninvasive method (Finapres) of continuously measuring arterial blood pressure in the finger to function accurately. They compared the response of the Finapres to blood pressures determined simultaneously by an oscillometric technique (Dinamap) in six anesthetized patients. Vasoconstriction was detected from a photoelectric plethysmogram, which was recorded continuously from an adjacent finger. Vasoconstriction was defined as a decrease in amplitude to less than half of its highest value in one and the same patient. From the 378 paired blood pressure readings obtained in this study, 51% took place in such a vasoconstricted state. The authors found that diastolic and mean blood pressures in the finger were, on the average, 9 mmHg below those in the upper arm and that the systolic pressure was 7 mmHg above that in the upper arm. The authors concluded that the Finapres keeps functioning well during peripheral vasoconstriction and is a recommendable method to monitor arterial blood pressure in the finger.

Effects of peripheral vasoconstriction on the measurement of blood pressure in a finger.

Using noninvasive techniques only, the fall in mean pressure and the pulse amplification between brachial and finger arterial pressure were measured in six anaesthetised female subjects during surgery. Brachial pressure was measured every 2 min with an oscillometric technique. Finger pressure was measured continuously using an arterial volume clamp method. In addition changes in the degree of peripheral vasoconstriction were established on an adjacent finger with a photo reflection plethysmograph. On the average finger mean pressure is 10 mmHg below brachial pressure. The difference tends to decrease with increasing constriction. The change in the difference between full constriction and maximal dilatation is 8 mmHg. The average finger to brachial pulse amplitude ratio changes from 110% at maximal dilatation to 170% at full constriction. Finger systolic pressure overshoot is responsible for the pulse wave amplification. On the average it is + 7 mmHg and ranges between maximal dilatation and full constriction over 26 mmHg. The standard error deviation on the volume clamp method could be established at 5% for mean pressure, about equal to that of the oscillometric technique in the literature.

Evaluation of the Penàz servo-plethysmo-manometer for the continuous, non-invasive measurement of finger blood pressure.

Blood pressure in the finger was measured by a servo-plethysmomanometer constructed after the design of Penàz, which uses the principle of the unloaded arterial wall. The device contains a photoelectric plethysmograph mounted in an inflatable cuff and an electro-pneumatic transducer to control air pressure in the cuff via a servosystem. Comparison of simultaneous measurements of intra-arterial pressure in the brachial artery was performed on 33 patients suspected of having hypertension. In 12 patients evaluation of the technique could not be carried out due to technical failures or distorted blood pressure wave forms. Results of the remaining 21 patients show a mean underestimation of intra-arterial blood pressure by finger cuff blood pressure of 0.8 kPa (6 mm Hg), both for systolic and diastolic levels. The scatter range of the difference is from 1.9 to -3.5 kPa for systolic and 0.1 to -2.5 kPa for diastolic values. It appears that, although not all technical problems are solved, the Penàz servo-plethysmo-manometer is potentially an elegant method by which to arrive at the fully calibrated wave form of blood pressure in a finger in a non-invasive and continuous fashion.