Central Aortic Pressure Waveform Research Articles

Central Aortic Reservoir-Wave Analysis Improves Prediction of Cardiovascular Events in Elderly Hypertensives

Several morphological parameters based on the central aortic pressure waveform are proposed as cardiovascular risk markers, yet no study has definitively demonstrated the incremental value of any waveform parameter in addition to currently accepted biomarkers in elderly, hypertensive patients. The reservoir-wave concept combines elements of wave transmission and Windkessel models of arterial pressure generation, defining an excess pressure superimposed on a background reservoir pressure. The utility of pressure rate constants derived from reservoir-wave analysis in prediction of cardiovascular events is unknown. Carotid blood pressure waveforms were measured prerandomization in a subset of 838 patients in the Second Australian National Blood Pressure Study. Reservoir-wave analysis was performed and indices of arterial function, including the systolic and diastolic rate constants, were derived. Survival analysis was performed to determine the association between reservoir-wave parameters and cardiovascular events. The incremental utility of reservoir-wave parameters in addition to the Framingham Risk Score was assessed. Baseline values of the systolic rate constant were independently predictive of clinical outcome (hazard ratio, 0.33; 95% confidence interval, 0.13-0.82; P=0.016 for fatal and nonfatal stroke and myocardial infarction and hazard ratio, 0.38; 95% confidence interval, 0.20-0.74; P=0.004 for the composite end point, including all cardiovascular events). Addition of this parameter to the Framingham Risk Score was associated with an improvement in predictive accuracy for cardiovascular events as assessed by the integrated discrimination improvement and net reclassification improvement indices. This analysis demonstrates that baseline values of the systolic rate constant predict clinical outcomes in elderly patients with hypertension and incrementally improve prognostication of cardiovascular events.

Abstract 132: Compliance Changes in the Aorta Following Chest Compressions in a Standard and High Impulse Model

Introduction: In cardiopulmonary resuscitation (CPR), chest compressions eject blood from the aorta and into the circulation. Following return of spontaneous circulation (ROSC), the circulation generated by the heart may be hemodynamically compromised, notably in its aortic compliance, or the aorta’s ability to resist blood pressure changes in response to blood volume changes. We investigated the relationship between CPR force delivery and relative change in aortic compliance in standard and high impulse chest compression models. Hypothesis: We hypothesized that average compliance would increase following chest compressions and that high impulse CPR would yield larger compliance changes than compliance changes in standard CPR. Methods: Central aortic pressure (CAP) waveforms were retrospectively analyzed via Millar catheters in female swine separated into standard CPR (n=4) and high impulse CPR (n=5). Ventricular fibrillation was induced and left untreated for 8 minutes, followed by 5 minutes of CPR and drugs. Rescue shocks were then delivered. Compliance was defined as stroke volume divided by the pressure difference between the dicrotic notch and end diastole. Stroke volume was estimated by using a previously validated model, which defined stroke volume as the area beneath the systolic portion of the CAP waveform multiplied by both a calibration constant and a ratio of time in diastole over time in systole. Compliances were calculated from CAP waveforms in ten consecutive beats before and after CPR with custom software built using MATLAB. A 2-way ANOVA was performed on average compliance across ten beats before and after chest compressions in both standard and high impulse CPR. Results: The average compliance (mL/mmHg) at baseline or pre-CPR (203), was significantly different from the average compliance post-CPR (335) (p=.03). The compliance change in standard CPR (151) did not significantly differ from compliance change in high impulse CPR (116) (p=.27). Conclusion: We observed a significant compliance increase in beats post-CPR compared to beats pre-CPR in both standard and high impulse groups. A significant difference in compliance change was not found when comparing standard vs. high impulse CPR.

Validation of Peripheral Pulse Wave Analysis Technique to Detect Aortic Valve State in Continuous-Flow LVADs1

Validation of Peripheral Pulse Wave Analysis Technique to Detect Aortic Valve State in Continuous-Flow LVADs1

Lower potassium intake is associated with increased wave reflection in young healthy adults.

BackgroundIncreased potassium intake has been shown to lower blood pressure (BP) even in the presence of high sodium consumption however the role of dietary potassium on vascular function has received less attention. The aim of this study was to evaluate the relationship between habitual intake of sodium (Na) and potassium (K) and measures of arterial stiffness and wave reflection.MethodsThirty-six young healthy adults (21 M, 15 F; 24 ± 0.6 yrs; systolic BP 117 ± 2; diastolic BP 63 ± 1 mmHg) recorded their dietary intake for 3 days and collected their urine for 24 hours on the 3rd day. Carotid-femoral pulse wave velocity (PWV) and the synthesis of a central aortic pressure waveform (by radial artery applanation tonometry and generalized transfer function) were performed. Aortic augmentation index (AI), an index of wave reflection, was calculated from the aortic pressure waveform.ResultsSubjects consumed an average of 2244 kcals, 3763 mg Na, and 2876 mg of K. Average urinary K excretion was 67 ± 5.3 mmol/24 hr, Na excretion was 157 ± 11 mmol/24 hr and the average Na/K excretion ratio was 2.7 ± 0.2. An inverse relationship between AI and K excretion was found (r = -0.323; p < 0.05). A positive relationship between AI and the Na/K excretion ratio was seen (r = 0.318; p < 0.05) while no relationship was noted with Na excretion alone (r = 0.071; p > 0.05). Reflection magnitude, the ratio of reflected and forward waves, was significantly associated with the Na/K excretion ratio (r = 0.365; p <0.05) but not Na or K alone. PWV did not correlate with Na or the Na/K excretion ratio (p > 0.05) but showed an inverse relationship with K excretion (r = -0.308; p < 0.05).ConclusionsThese data suggest that lower potassium intakes are associated with greater wave reflection and stiffer arteries in young healthy adults.

Application of the N-Point Moving Average Method for Brachial Pressure Waveform–Derived Estimation of Central Aortic Systolic Pressure

The N-point moving average (NPMA) is a mathematical low-pass filter that can smooth peaked noninvasively acquired radial pressure waveforms to estimate central aortic systolic pressure using a common denominator of N/4 (where N=the acquisition sampling frequency). The present study investigated whether the NPMA method can be applied to brachial pressure waveforms. In the derivation group, simultaneously recorded invasive high-fidelity brachial and central aortic pressure waveforms from 40 subjects were analyzed to identify the best common denominator. In the validation group, the NPMA method with the obtained common denominator was applied on noninvasive brachial pressure waveforms of 100 subjects. Validity was tested by comparing the noninvasive with the simultaneously recorded invasive central aortic systolic pressure. Noninvasive brachial pressure waveforms were calibrated to the cuff systolic and diastolic blood pressures. In the derivation study, an optimal denominator of N/6 was identified for NPMA to derive central aortic systolic pressure. The mean difference between the invasively/noninvasively estimated (N/6) and invasively measured central aortic systolic pressure was 0.1±3.5 and -0.6±7.6 mm Hg in the derivation and validation study, respectively. It satisfied the Association for the Advancement of Medical Instrumentation standard of 5±8 mm Hg. In conclusion, this method for estimating central aortic systolic pressure using either invasive or noninvasive brachial pressure waves requires a common denominator of N/6. By integrating the NPMA method into the ordinary oscillometric blood pressure determining process, convenient noninvasive central aortic systolic pressure values could be obtained with acceptable accuracy.

Arterial wave reflections and ventricular-vascular interaction in patients with left ventricular systolic dysfunction.

Central aortic pressure waveform (AoPW) is the summation of a forward-traveling wave generated by the left ventricle and a backward-traveling wave caused by the reflection of the forward wave. The aim of this study was to evaluate the effect of ventricular-vascular coupling on the morphology of AoPW in chronic heart failure patients with different degrees of left ventricular systolic dysfunction (LVSD) using pulse wave analysis (PWA). PWA of AoPW and left ventricular (LV) function were evaluated by applanation tonometry in 26 control subjects, in 12 patients with left ventricular ejection fraction (LVEF) ≤ 30%, and in 14 patients with LVEF > 30%. Augmentation pressure, augmentation index, wasted energy, and ejection duration were lower in patients with LVEF ≤ 30% than in those with LVEF > 30% and in control subjects. Furthermore, augmentation index showed an inverse correlation with Doppler mitral E-wave amplitude (r = -0.40; P = 0.04) and E/A ratio (r = -0.42; P = 0.03) and a direct correlation with deceleration time of mitral E-waves (r = 0.39; P = 0.04). In patients with severe LVSD (LVEF ≤ 30%), aortic wave reflections negatively interfere with LV function and induce a shortening of ejection duration. In contrast, AoPW is similar in patients with moderate LVSD (LVEF > 30%) and in control subjects.

Individualized Estimation of the Central Aortic Blood Pressure Waveform: A Comparative Study

This paper presents a comparative study on the relative performance of alternative strategies for estimating an individualized central aortic blood pressure (BP) waveform. Based on the transmission line representation of the central aortic-radial arterial line, a fully individualized (ITF 1), two partially individualized (ITF 2 and ITF 3 ), and a fully nonindividualized (NITF) transfer functions (i.e., frequency-dependent central aortic-radial BP relationships) were constructed using experimental data collected from nine swine subjects. The central aortic BP waveforms estimated by these transfer functions were compared against their measured gold standards, with the root-mean-squared waveform error and the absolute errors associated with systolic and pulse pressures as performance measures. Overall, the advantage of the individualized over the nonindividualized approach was modest but significant. The superiority of the individualized approach to its nonindividualized counterpart was increasingly pronounced under nonnominal or extreme physiologic conditions, as the subject's pulse transit time deviated from the averaged nominal value. The results suggest that the use of a fully individualized transfer function (ITF 1) is strongly recommended for nonnominal physiologic conditions, whereas a partially (ITF 2 and ITF 3) or even fully nonindividualized transfer function (NITF) may also yield acceptable performance under nominal physiologic conditions.

Non-invasive method for the aortic blood pressure waveform estimation using the measured radial EBI

The paper presents a method for the Central Aortic Pressure (CAP) waveform estimation from the measured radial Electrical Bio-Impedance (EBI). The method proposed here is a non-invasive and health-safe approach to estimate the cardiovascular system parameters, such as the Augmentation Index (AI). Reconstruction of the CAP curve from the EBI data is provided by spectral domain transfer functions (TF), found on the bases of data analysis. Clinical experiments were carried out on 30 patients in the Center of Cardiology of East-Tallinn Central Hospital during coronary angiography on patients in age of 43 to 80 years. The quality and reliability of the method was tested by comparing the evaluated augmentation indices obtained from the invasively measured CAP data and from the reconstructed curve. The correlation coefficient r = 0.89 was calculated in the range of AICAP values from 5 to 28. Comparing to the traditional tonometry based method, the developed one is more convenient to use and it allows long-term monitoring of the AI, what is not possible with tonometry probes.

Guidelines on guidelines

Current guidelines on isolated systolic hypertension (ISH) suggest the same treatment to patients of all ages. Application of these guidelines in youth with ISH may not be appropriate, as presently no data show adverse outcome or benefit of drug therapy in this group. Simple noninvasive tonometric techniques now enable physicians to measure the central aortic pressure waveform and amplification of the pressure pulse. ISH in youth is usually caused by high amplification of the central pressure wave, whereas ISH in the elderly (>age 60) is attributable to aortic stiffening. This is the only group with ISH shown to have an adverse prognosis and to warrant drug therapy.

Lower potassium intake is associated with increased wave reflection in young healthy males

Increased potassium intake has been shown to lower blood pressure (BP) even in the presence of high sodium consumption however the role of dietary potassium on vascular function has received less attention. The objective of this study was to evaluate the relationship between habitual intake of sodium (Na) and potassium (K) and measures of arterial stiffness and wave reflection. 20 healthy males (24±0.7 yrs; systolic BP 121±3; diastolic BP 64±2 mmHg) recorded their dietary intake for 3 days and collected their 24‐hr urine on the 3rd day. Carotid‐femoral pulse wave velocity (PWV) and the synthesis of a central aortic pressure waveform (by radial artery applanation tonometry and generalized transfer function) were performed. Aortic augmentation index (AI), an index of wave reflection, was calculated from the aortic pressure waveform. Subjects consumed an average of 2,492 kcals, 4,212 mg Na, and 2,967 mg of K. Average urinary Na/K excretion ratio was 2.69±0.3. An inverse relationship between AI and K excretion was found (r= −0.3122; p&lt;0.001) while no relationship was noted with Na excretion (r= − 0.005; p&gt;;0.05). AI positively correlated with the Na/K excretion ratio (r= 0.153; p&lt;0.05). PWV did not correlate with either nutrient or the Na/K excretion ratio (p&gt;;0.05). These data suggest that lower potassium intakes are associated with stiffer arteries in young healthy males. Supported by R01 HL 104106.

Comparative Study on Tube-Load Modeling of Arterial Hemodynamics in Humans

In this paper, we assess the validity of two alternative tube-load models for describing the relationship between central aortic and peripheral arterial blood pressure (BP) waveforms in humans. In particular, a single-tube (1-TL) model and a serially connected two-tube (2-TL) model, both terminated with a Windkessel load, are considered as candidate representations of central aortic-peripheral arterial path. Using the central aortic, radial and femoral BP waveform data collected from eight human subjects undergoing coronary artery bypass graft with cardiopulmonary bypass procedure, the fidelity of the tube-load models was quantified and compared with each other. Both models could fit the central aortic-radial and central aortic-femoral BP waveform pairs effectively. Specifically, the models could estimate pulse travel time (PTT) accurately, and the model-derived frequency response was also close to the empirical transfer function estimate obtained directly from the central aortic and peripheral BP waveform data. However, 2-TL model was consistently superior to 1-TL model with statistical significance as far as the accuracy of the central aortic BP waveform was concerned. Indeed, the average waveform RMSE was 2.52 mmHg versus 3.24 mmHg for 2-TL and 1-TL models, respectively (p < 0.05); the r² value between measured and estimated central aortic BP waveforms was 0.96 and 0.93 for 2-TL and 1-TL models, respectively (p < 0.05). We concluded that the tube-load models considered in this paper are valid representations that can accurately reproduce central aortic-radial/femoral BP waveform relationships in humans, although the 2-TL model is preferred if an accurate central aortic BP waveform is highly desired.

A “backward” calculation method for the estimation of central aortic pressure wave in a 1D arterial model network

The central aortic pressure waveform plays an important role in medicine. Several clinical outcomes can be determined using this information. However, direct measurement is difficult and risky, so that estimation methods are preferred. In the current paper a novel method is introduced which is a new basis for this estimation process. A one-dimensional arterial model is created using basic equations of fluid dynamics and a viscoelastic material model. In case of “forward” calculation the volumetric flow rate curve of the heart is set as a boundary condition. Pressure, velocity, wave propagation speed and deformation are calculated in the whole network. In case of “backward” calculation the pressure waveform is given at an arbitrary point of the network. With modification of the equations the calculation is turned backwards: knowing a pressure waveform at the downstream end of a vessel segment, the pressure waveform at the upstream end is calculated. The central aortic pressure waveform can be estimated within the 1D arterial model using this method. Details of the computation method and results of testing calculations are presented. The developed method is a powerful tool that might be used for central aortic pressure wave estimation in the future.

76 COMPARISON OF THE USE OF CENTRAL AND PERIPHERAL PRESSURE WAVEFORMS FOR OPTIMIZATION OF CARDIAC RESYNCHONIZATION THERAPY

Background: In patients with cardiac dyssynchrony, optimization of cardiac resynchronization therapy (CRT) is achieved by ascertaining the atrial-ventricular (AV) and inter-ventricular (VV) delays that give maximum cardiac output (CO). As continuous CO measurements are not readily available, this study assesses the use of the peripheral (radial) and central aortic pressure waveform for optimal AV and VV delay using a haemodynamic model. Methods: The model consisted of left and right heart chambers simulated by time varying elastance (variable capacitors), systemic and pulmonary sections simulated by windkessel segments (resistance, capacitance) and a delay line for the arm. Model parameters were altered to obtain maximal values (optimization criterion) for central and peripheral systolic, diastolic and pulse pressure (cSBP, cDBP, cPP, pSBP, pDBP, pPP) and CO from baseline settings of AV (120 ms) and VV (0 ms) delays. Results: The optimal AV delay was not different for all hemodynamic measures. However, optimal VV delay was significantly different for blood pressure parameters compared to CO (*p<0.001).Conclusions: The study indicates that both peripheral and central pressure parameters give similar results for CRT optimisation for AV and VV delay. However optimization for pressure parameters gives significant differences for VV delay compared to CO. This suggests that the ventricular -vascular coupling is an important parameter to be considered for CRT optimisation.

Comparison of two generalized transfer functions for measuring central systolic blood pressure by an oscillometric blood pressure monitor

Central aortic systolic blood pressure (SBP-C) can be estimated from a cuff oscillometric waveform derived during the pulse volume plethysmography (PVP) by applying a device-specific aortic pressure-to-PVP waveform-generalized transfer function (A2P(GTF)). The present study compared the performance of an aortic-to-brachial pressure waveforms generalized transfer function (A2B(GTF)), which is independent of any PVP devices, with an A2P(GTF). Generalized transfer function of aortic-to-brachial (A2B(GTF)) and aortic-to-PVP (A2P(GTF)) were generated from the simultaneously obtained central aortic and brachial pressure waveforms recorded by a high-fidelity dual pressure sensor catheter, and the PVP waveform recorded by a customized noninvasive blood pressure monitor during cardiac catheterization in 40 patients, and were then applied in another 100 patients with simultaneously recorded invasive aortic pressure and noninvasively calibrated (using cuff SBP and diastolic blood pressures) PVP waveforms. The mean difference±s.d. between the noninvasively estimated and invasively recorded SBP-C was -2.1±7.7 mm Hg for A2B(GTF), which was not greater than that of -3.0±7.7 mm Hg for A2P(GTF) (P<0.01). In conclusion, SBP-C can be measured reliably using a noninvasive blood pressure monitor by applying either an A2P(GTF) or A2B(GTF) to a noninvasively calibrated PVP waveform. The performance of an A2B(GTF) is not inferior to that of an A2P(GTF).

Estimation of central aortic systolic pressure from the second systolic peak of the peripheral upper limb pulse depends on central aortic pressure waveform morphology

Direct identification of second systolic peaks of peripheral upper limb pulses (pSBP2) has been used to represent central systolic blood pressure (cSBP), but its accuracy at low SBP was questioned. We investigated the relationship of pSBP2 with characteristics of central pressure waveforms. High-fidelity central aortic and right brachial pressure waveforms were simultaneously recorded using a custom-made dual pressure sensor catheter in 78 patients (65.9 ± 12.9 years) during catheterization for 285 measurements. Overall agreement between cSBP and pSBP2 was good (mean difference -0.9 ± 4.8, r = 0.98), with a systematic bias at low SBP. We examined agreements of different waveform types according to the relationship of the second systolic peak of aortic pressure waveforms (cSBP2) to cSBP. Of type A (positive late systolic augmentation) and type B (zero augmentation) aortic pressure waveforms, in which cSBP = cSBP2, agreement between pSBP2 and cSBP was excellent (mean difference -0.4 ± 4.1, r = 0.99). There were 40 type C aortic pressure waveforms (negative augmentation; cSBP > cSBP2) with cSBP 107.2 ± 13.9 mmHg. Their cSBP2, compared with cSBP, showed closer agreement (mean difference -0.6 ± 3.2 vs. -4.0 ± 7.2 mmHg) and better correlation (r = 0.97 vs. 0.85, P = 0.03) with pSBP2. pSBP2 can be used with type A and B aortic pressure waveforms for estimation of cSBP. However, it should not be used with type C aortic pressure waveforms, typically at low SBP, because pSBP2 is closer to cSBP2 than cSBP. This explains why pSBP2 underestimates cSBP at low SBP.

Asynchronous arterial systolic expansion as a marker of vascular aging

Arterial elastic properties change with aging. Measurements of pulse wave velocity and augmentation index are useful for the evaluation of arterial stiffness. However, they likely represent only global characteristics of the arterial tree rather than local vascular alterations. The aim of this study was to evaluate whether local vascular properties assessed by velocity vector imaging differed with aging. Vascular properties of carotid arteries with ages were assessed in 100 healthy volunteers (52 men) ranging from 20 to 68 years using velocity vector imaging. The peak circumferential strain and strain rate of the six segments in left common carotid arteries were analyzed and the standard deviation of the time to peak circumferential strain and strain rate of the six segments, representing the synchronicity of the arterial expansion, were calculated. Central blood pressure, augmentation index and pulse wave velocity were assessed by commercially available radial artery tonometry, the SphygmoCor system (AtCor Medical, West Ryde, Australia). A validated generalized transfer function was used to acquire the central aortic pressures and pressure waveforms. Pulse wave velocity, augmentation index and velocity vector imaging parameters showed significant changes with age. However, the age-related changes in pulse wave velocity, augmentation index and velocity vector imaging parameters were different. The increase in pulse wave velocity was more prominent in older individuals, whereas the changes in augmentation index and carotid strain and strain rate were evident earlier, at the age of 30 years. Unlike augmentation index, which showed little change in older individuals, the standard deviation of time to peak strain and strain rate showed a steady increase from younger to older individuals. Asynchronous arterial expansion could be a useful discriminative marker of vascular aging independent of individual's age.

Subject-Specific Estimation of Central Aortic Blood Pressure Using an Individualized Transfer Function: A Preliminary Feasibility Study

This paper presents a new approach to the estimation of unknown central aortic blood pressure waveform from a directly measured peripheral blood pressure waveform, in which a physics-based model is employed to solve for a subject- and state-specific individualized transfer function (ITF). The ITF provides the means to estimate the unknown central aortic blood pressure from the peripheral blood pressure. Initial proof-of-principle for the ITF is demonstrated experimentally through an in vivo protocol. In swine subjects taken through wide range of physiologic conditions, the ITF was on average able to provide central aortic blood pressure waveforms more accurately than a nonindividualized transfer function. Its usefulness was most evident when the subject's pulse transit time deviated from normative values. In these circumstances, the ITF yielded statistically significant reductions over a nonindividualized transfer function in the following three parameters: 1) 30% reduction in the root-mean-squared error between estimated versus actual central aortic blood pressure waveform (p < 10 (-4)), 2) >50% reduction in the error between estimated versus actual systolic and pulse pressures ( p < 10 (-4)), and 3) a reduction in the overall breakdown rate (i.e., the frequency of estimation errors >3 mmHg, p < 10 (-4)). In conclusion, the ITF may offer an attractive alternative to existing methods that estimates the central aortic blood pressure waveform, and may be particularly useful in nonnormative physiologic conditions.

Quantification of the Calibration Error in the Transfer Function-Derived Central Aortic Blood Pressures

The accuracy of the central aortic systolic (SBP-C) and pulse (PP-C) blood pressures estimated noninvasively by a generalized transfer function technique has been questioned. The purpose of the present study was to quantify precisely the impact of the input errors (differences between the oscillometric (SBP-O, DBP-O, PP-O) and invasive (SBP-B, DBP-B, PP-B) brachial systolic, diastolic, and pulse blood pressures) on the output errors (differences between the estimated and invasively measured SBP-C and PP-C). Invasive high-fidelity right brachial and central aortic pressure waveforms, and SBP-O, DBP-O, and PP-O (=SBP-O - DBP-O) were obtained simultaneously in 40 patients during cardiac catheterization. A generalized transfer function was applied on the individual brachial pressure waveform to derive predicted SBP-C and PP-C. Observed input errors were -2.3 ± 5.8 mm Hg from SBP-O, 8.1 ± 5.3 mm Hg from DBP-O, and -10.4 ± 7.1 mm Hg from PP-O, respectively. The output errors were -2.2 ± 6.4 mm Hg and -10.3 ± 8.0 mm Hg for SBP-C and PP-C, respectively, when the brachial pressure waveforms were recalibrated using SBP-O and DBP-O. The outputs were determined by the inputs according to the Equation (1): SBP-C error = 0.97 × SBP-O error + 0.03 (r = 0.88, P < 0.01); and the Equation (2): PP-C error = 0.96 × PP-O error - 0.30 (r = 0.86, P < 0.01). Noninvasive application of the generalized transfer function techniques produces estimates of SBP-C and PP-C with errors equivalent to those of the oscillometric blood pressure monitor in the estimation of SBP-B and PP-B. The output errors can be predicted from input errors of SBP-O and DBP-O.

Influence of Aortic Pressure Wave Components Determined Noninvasively on Myocardial Oxygen Demand in Men and Women

Myocardial oxygen consumption is increased by arterial stiffening. It is not known precisely how. This study aimed to evaluate the role of the incident and reflected pressure wave in raising myocardial oxygen demand. Central (aortic) pressure waveforms were generated from radial waveforms using a generalized transfer function in 1628 cardiology outpatients (1038 males and 590 females). Aortic waveforms were used to derive measures of incident and reflected waves, as well as to measure mean central systolic pressure (an indicator of systolic ventricular load), left ventricular ejection duration, and tension time index (a surrogate of myocardial oxygen demand) using validated techniques. Incident and reflected waves were measured using the conventional and an alternative method (aortic flow triangulation). Relationships were tested before and after correction for age, height, weight, heart rate, and mean arterial pressure using simple and multivariate linear regression models. Analyses were conducted separately by gender. In both genders (according to conventional or alternative methods of wave measurement), both the incident and reflected wave were strong predictors of tension time index (P<0.001). Both pressure waves raised the mean central systolic pressure (P<0.001). The reflected wave (P<0.001), unlike the incident wave (P>0.05), was also associated with a longer cardiac ejection duration. Tension time index (P<0.0001), mean central systolic pressure (P<0.001), and ejection duration (P<0.0001) were greater in women. Changes in arterial properties alter the nature of pressure wave propagation and predispose to cardiac ischemia (especially in women).

Calculation of Forward and Backward Arterial Waves by Analysis of Two Pressure Waveforms

We developed a technique to calculate forward and backward arterial waves from proximal and distal pressure waveforms. First, the relationship between the waveforms is represented with an arterial tube model. Then, the model parameters are estimated via least-squares fitting. Finally, the forward and backward waves are calculated using the parameter estimates. Thus, unlike most techniques, the arterial waves are determined without a more difficult flow measurement or an experimental perturbation. We applied the technique to central aortic and femoral artery pressure waveforms from anesthetized dogs during drug infusions, volume changes, and cardiac pacing. The calculated waves predicted an abdominal aortic pressure waveform measurement more accurately (2.4 mmHg error) than the analyzed waveforms (5.3mmHg average error); reliably predicted relative changes in a femoral artery flow measurement (14.7% error); and changed as expected with selective vasoactive drugs. The ratio of the backward- to forward-wave magnitudes was 0.37 ± 0.05 during baseline. This index increased by ∼50% with phenylephrine and norepinephrine, decreased by ∼60% with dobutamine and nitroglycerin, and changed little otherwise. The time delay between the waves in the central aorta was 175 ± 14 ms during baseline. This delay varied by ±∼25% and was inversely related to mean pressure.