Jos J. Settels

Nexfin noninvasive continuous blood pressure validated against Riva-Rocci/Korotkoff.

BACKGROUNDnThe Finapres methodology offers continuous measurement of blood pressure (BP) in a noninvasive manner. The latest development using this methodology is the Nexfin monitor. The present study evaluated the accuracy of Nexfin noninvasive arterial pressure (NAP) compared with auscultatory BP measurements (Riva-Rocci/Korotkoff, RRK).nnnMETHODSnIn supine subjects NAP was compared to RRK, performed by two observers using an electronic stethoscope with double earpieces. Per subject, three NAP-RRK differences were determined for systolic and diastolic BP, and bias and precision of differences were expressed as median (25th, 75th percentiles). Within-subject precision was defined as the (25th, 75th percentiles) after removing the average individual difference.nnnRESULTSnA total of 312 data sets of NAP and RRK for systolic and diastolic BP from 104 subjects (aged 18-95 years, 54 males) were compared. RRK systolic BP was 129 (115, 150), and diastolic BP was 80 (72, 89), NAP-RRK differences were 5.4 (-1.7, 11.0) mm Hg and -2.5 (-7.6, 2.3) mm Hg for systolic and diastolic BP, respectively; within-subject precisions were (-2.2, 2.3) and (-1.6, 1.5) mm Hg, respectively.nnnCONCLUSIONnNexfin provides accurate measurement of BP with good within-subject precision when compared to RRK.

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.

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.

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 plus/minus 1.06 to plus/minus 0.5 l/min after flow correction. In group B, the scatter decreased similarly and the limits of agreement also became plus/minus 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.

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 hyperadrenergic episodes. We conclude that sympathetic overactivity in tetauns temporarily overrules a functionally intact baroreflex leading to severe blood pressure instability with episodes of hypertension.

Machine-learning Algorithm to Predict Hypotension Based on High-fidelity Arterial Pressure Waveform Analysis

What We Already Know about This Topic The ability to predict intraoperative hypotension may advance the ability to prevent hypotension-associated complications effectively The extent to which advanced waveform analysis of invasive arterial lines may provide meaningful forewarning remains unknown What This Article Tells Us That Is New A machine-learning algorithm based on thousands of arterial waveform features can identify an intraoperative hypotensive event 15 min before its occurrence with a sensitivity of 88% and specificity of 87% Further studies must evaluate the real-time value of such algorithms in a broader set of clinical conditions and patients Background: With appropriate algorithms, computers can learn to detect patterns and associations in large data sets. The authors’ goal was to apply machine learning to arterial pressure waveforms and create an algorithm to predict hypotension. The algorithm detects early alteration in waveforms that can herald the weakening of cardiovascular compensatory mechanisms affecting preload, afterload, and contractility. Methods: The algorithm was developed with two different data sources: (1) a retrospective cohort, used for training, consisting of 1,334 patients’ records with 545,959 min of arterial waveform recording and 25,461 episodes of hypotension; and (2) a prospective, local hospital cohort used for external validation, consisting of 204 patients’ records with 33,236 min of arterial waveform recording and 1,923 episodes of hypotension. The algorithm relates a large set of features calculated from the high-fidelity arterial pressure waveform to the prediction of an upcoming hypotensive event (mean arterial pressure < 65 mmHg). Receiver-operating characteristic curve analysis evaluated the algorithm’s success in predicting hypotension, defined as mean arterial pressure less than 65 mmHg. Results: Using 3,022 individual features per cardiac cycle, the algorithm predicted arterial hypotension with a sensitivity and specificity of 88% (85 to 90%) and 87% (85 to 90%) 15 min before a hypotensive event (area under the curve, 0.95 [0.94 to 0.95]); 89% (87 to 91%) and 90% (87 to 92%) 10 min before (area under the curve, 0.95 [0.95 to 0.96]); 92% (90 to 94%) and 92% (90 to 94%) 5 min before (area under the curve, 0.97 [0.97 to 0.98]). Conclusions: The results demonstrate that a machine-learning algorithm can be trained, with large data sets of high-fidelity arterial waveforms, to predict hypotension in surgical patients’ records.

Explanation Of Short-tern Blood Pressure Responses Needs Baromodulation

term blood pressure variability has periods of a few seconds to many hours. The presence of a fast, powerful baro- reflex is in contradiction to this varia- bility. To resolve this paradox the baro- modulation hypothesis was postulated. It is modulation of baroreflex loop gain or sensitivity only by inhibition or potentiation of baroreceptor afferent ac- tivity. The relation between blood pres- sure variability and baromodulation was studied in a model of the baroreflex controlled circulation. It was concluded that combinations of proper random sympa- thetic, parasympathetic and periodic baro- modulation components can explain the blood pressure variability in normal sub- jects. The key concept is thus a time- variable baroreflex gain or sensitivity.