Ilja Guelen

Quantification of Wave Reflection in the Human Aorta From Pressure Alone: A Proof of Principle

Wave reflections affect the proximal aortic pressure and flow waves and play a role in systolic hypertension. A measure of wave reflection, receiving much attention, is the augmentation index (AI), the ratio of the secondary rise in pressure and pulse pressure. AI can be limiting, because it depends not only on the magnitude of wave reflection but also on wave shapes and timing of incident and reflected waves. More accurate measures are obtainable after separation of pressure in its forward (Pf) and reflected (Pb) components. However, this calculation requires measurement of aortic flow. We explore the possibility of replacing the unknown flow by a triangular wave, with duration equal to ejection time, and peak flow at the inflection point of pressure (FtIP) and, for a second analysis, at 30% of ejection time (Ft30). Wave form analysis gave forward and backward pressure waves. Reflection magnitude (RM) and reflection index (RI) were defined as RM=Pb/Pf and RI=Pb/(Pf+Pb), respectively. Healthy subjects, including interventions such as exercise and Valsalva maneuvers, and patients with ischemic heart disease and failure were analyzed. RMs and RIs using FtIP and Ft30 were compared with those using measured flow (Fm). Pressure and flow were recorded with high fidelity pressure and velocity sensors. Relations are: RMtIP=0.82RMmf+0.06 (R2=0.79; n=24), RMt30=0.79RMmf+0.08 (R2=0.85; n=29) and RItIP=0.89RImf+0.02 (R2=0.81; n=24), RIt30=0.83RImf+0.05 (R2=0.88; n=29). We suggest that wave reflection can be derived from uncalibrated aortic pressure alone, even when no clear inflection point is distinguishable and AI cannot be obtained. Epidemiological studies should establish its clinical value.

Validation of brachial artery pressure reconstruction from finger arterial pressure

Objective Measurement of finger artery pressure with Finapres offers noninvasive continuous blood pressure, which, however, differs from brachial artery pressure. Generalized waveform filtering and level correction may convert the finger artery pressure waveform to a brachial waveform. An upper-arm cuff return-to-flow measurement may be used to calibrate the blood pressure on an individual basis. We tested these corrective methods as implemented in the Finometer device. Methods Intrabrachial artery pressure (BAP) and finger artery pressures were recorded simultaneously in 37 cardiac patients, aged 41–83 years, who underwent a cardiac catheterization procedure. Finger artery pressures were compared after waveform filtering and level correction and after an additional return-to-flow calibration. Measurements were performed in supine and sitting positions. Accuracy and precision were considered clinically acceptable if the mean and standard deviation of the return-to-flow intrabrachial artery pressure (reBAP)–BAP differences were smaller than 5 ± 8 mmHg (Association for the Advancement of Medical Instrumentation requirements). Results Finger artery systolic, diastolic and mean pressures for the group differed from that of intrabrachial artery pressure by −10 ± 13, −12 ± 8 and −16 ± 8 mmHg, respectively. After waveform filtering and level correction the filtered level corrected arterial pressure differed by −1 ± 11, −0 ± 7 and −2 ± 7 mmHg. After individual calibration, reBAP differed by 3 ± 8, 4 ± 6 and 3 ± 5 mmHg. Comparable results were found in the sitting position but only when the supine return-to-flow calibration was used. Conclusion Reconstruction of intrabrachial artery pressure from finger artery pressure with waveform filtering and level correction reduces the pressure differences substantially, with diastolic and mean within Association for the Advancement of Medical Instrumentation requirements. After one supine return-to-flow calibration, all pressure differences meet the requirements. Return-to-flow calibration should not be repeated in sitting position.

Individualization of transfer function in estimation of central aortic pressure from the peripheral pulse is not required in patients at rest.

Central aortic pressure gives better insight into ventriculo-arterial coupling and better prognosis of cardiovascular complications than peripheral pressures. Therefore transfer functions (TF), reconstructing aortic pressure from peripheral pressures, are of great interest. Generalized TFs (GTF) give useful results, especially in larger study populations, but detailed information on aortic pressure might be improved by individualization of the TF. We found earlier that the time delay, representing the travel time of the pressure wave between measurement site and aorta is the main determinant of the TF. Therefore, we hypothesized that the TF might be individualized (ITF) using this time delay. In a group of 50 patients at rest, aged 28-66 yr (43 men), undergoing diagnostic angiography, ascending aortic pressure was 119 +/- 20/70 +/- 9 mmHg (systolic/diastolic). Brachial pressure, almost simultaneously measured using catheter pullback, was 131 +/- 18/67 +/- 9 mmHg. We obtained brachial-to-aorta ITFs using time delays optimized for the individual and a GTF using averaged delay. With the use of ITFs, reconstructed aortic pressure was 121 +/- 19/69 +/- 9 mmHg and the root mean square error (RMSE), as measure of difference in wave shape, was 4.1 +/- 2.0 mmHg. With the use of the GTF, reconstructed pressure was 122 +/- 19/69 +/- 9 mmHg and RMSE 4.4 +/- 2.0 mmHg. The augmentation index (AI) of the measured aortic pressure was 26 +/- 13%, and with ITF and GTF the AIs were 28 +/- 12% and 30 +/- 11%, respectively. Details of the wave shape were reproduced slightly better with ITF but not significantly, thus individualization of pressure transfer is not effective in resting patients.

Variable day/night bias in 24-h non-invasive finger pressure against intrabrachial artery pressure is removed by waveform filtering and level correction

Background Twenty-four-hour finger arterial pressure (FAP) recordings show a negative bias against intrabrachial artery pressure (BAP) and the bias is greater during the night, thereby overestimating the nocturnal blood pressure dip. We have available a methodology with which to reconstruct BAP from FAP by waveform filtering (transfer function) and generalized level (bias) correction that reduces the bias for short-term blood pressure records. Objective To investigate if this methodology also decreases the extra bias during the night, thereby yielding a better estimate of the nocturnal dip. Methods Twenty-four-hour FAP and BAP blood pressure recordings were simultaneously obtained in eight healthy normotensive volunteers and 14 patients with hypertension (ages 19–60 years), during standardized scheduled activities. The data were analysed off-line, applying the brachial reconstruction technique (reBAP) consisting of a waveform filter and level correction. Simultaneous beats yielded systolic, diastolic and mean pressures that were averaged per 30 min, per day, per night, per activity, over the 24-h period, and for volunteers and patients separately. Results Over the full 24 h, FAP systolic, diastolic and mean values for the total group differed from BAP by +1 ± 10, −8 ± 7 and −10 ± 8 mmHg (mean ± SD), respectively. Similarly, reBAPs differed by +1 ± 11, −2 ± 7 and −2 ± 7 mmHg. BAPs dipped by 20 ± 8, 13 ± 6 and 15 ± 6 mmHg, respectively, during the night. These dips were overestimated by +8, +4 and +4 mmHg by FAP, but not by reBAP: −1, +1 and +1 mmHg. The volunteer and the patient groups showed slight differences in results, but these were not statistically significant. Conclusions The generalized reconstruction technique to obtain near-brachial pressure from non-invasive FAP almost completely removed bias over the full 24-h day–night period and improved tracking of diurnal changes for all three blood pressure values.

Limited accuracy of the hyperbaric index, ambulatory blood pressure and sphygmomanometry measurements in predicting gestational hypertension and preeclampsia.

Objective The aim of this study was to validate the hyperbaric index (HBI) for first trimester prediction of preeclampsia and gestational hypertension. Methods Participants were low-risk and high-risk nulliparous women and high-risk multiparous women, and were recruited between April 2004 and June 2006. At a gestational age of 9 weeks (range 8–11 weeks), blood pressure (BP) was measured first by sphygmomanometry and thereafter by ambulatory BP measurement (ABPM) for 48 h. The first 90 low-risk women who had an uneventful pregnancy formed the reference group for calculation of a time-specified tolerance interval with 90% confidence limits. In the validation group, consisting of the remaining women, the HBI was calculated as the time-specified BP excess over this tolerance limit for SBP, DBP and mean arterial pressure. Results The validation group contained 101 women. Fifteen women developed preeclampsia and 13 developed gestational hypertension. For preeclampsia, the maximum HBI had the best predictive capacity with a sensitivity of 73% and a specificity of 86%. However, the difference with standard ABPM measurement or sphygmomanometry was small with a sensitivity between 75 and 73% and a specificity between 86 and 95%. The predictive efficacy for gestational hypertension was poor with all methods (sensitivity between 54 and 77%, specificity between 41 and 78%). Conclusion Standardized sphygmomanometry, ABPM measurement and the HBI calculated from 48-h ABPM had a comparable, restricted predictive efficacy. The high predictive value of HBI as observed in earlier studies could not be reproduced.

Aortic stiffness and the balance between cardiac oxygen supply and demand: the Rotterdam Study.

Objectives Aortic stiffness is an independent predictor of cardiovascular morbidity and mortality. We investigated whether aortic stiffness, estimated as aortic pulse wave velocity, is associated with decreased perfusion pressure estimated as the cardiac oxygen supply potential. Methods Aortic stiffness and aortic pressure waves, reconstructed from finger blood pressure waves, were obtained in 2490 older adults within the framework of the Rotterdam Study, a large population-based study. Cardiac oxygen supply and demand were estimated using pulse wave analysis techniques, and related to aortic stiffness by linear regression analyses after adjustment for age, sex, mean arterial pressure and heart rate. Results Cardiac oxygen demand, estimated as the Systolic Pressure Time Index and the Rate Pressure Product, increased with increasing aortic stiffness [0.27 mmHg s (95% confidence interval: 0.21; 0.34)] and [42.2 mmHg/min (95% confidence interval: 34.1; 50.3)], respectively. Cardiac oxygen supply potential estimated as the Diastolic Pressure Time Index decreased [−0.70 mmHg s (95% confidence interval: −0.86; −0.54)] with aortic stiffening. Accordingly, the supply/demand ratio Diastolic Pressure Time Index/Systolic Pressure Time Index −1.11 (95% confidence interval: −0.14; −0.009) decreased with increasing aortic stiffness. Conclusion Aortic stiffness is associated with estimates of increased cardiac oxygen demand and a decreased cardiac oxygen supply potential. These results may offer additional explanation for the relation between aortic stiffness and cardiovascular morbidity and mortality.