Constructive and destructive addition of forward and reflected arterial pulse waves

Although the physics of arterial pulse wave propagation and reflection is well understood, there is considerable debate as to the effect of reflection on vascular input impedance (Z(in)), pulsatile pressure, and stroke work (SW). This may be related to how reflection is studied. Conventionally, reflection is experimentally abolished (thus radically changing unrelated parameters), or a specific model is assumed from which reflection can be removed (yielding model-dependent results). The present work proposes a simple, model-independent method to evaluate the effect of reflection directly from measured pulsatile pressure (P) and flow (Q). Because characteristic impedance (Z(0)) is Z(in) in the absence of reflection, the P with reflection theoretically removed can be calculated from Q x Z(0). Applying this insight to an illustrative case indicates that reflection has the least effect on P and SW at normal pressure but a greater effect with vasodilation and vasoconstriction. Z(in), P, and SW are increased or decreased depending on the relative amount of constructive and destructive addition of forward and reflected arterial pulse waves.

True arterial system compliance estimated from apparent arterial compliance

A new method has been developed to estimate total arterial compliance from measured input pressure and flow. In contrast to other methods, this method does not rely on fitting the elements of a lumped model to measured data. Instead, it relies on measured input impedance and peripheral resistance to calculate the relationship of arterial blood volume to input pressure. Generally, this transfer function is a complex function of frequency and is called the apparent arterial compliance. At very low frequencies, the confounding effect of pulse wave reflection disappears, and apparent compliance becomes total arterial compliance. This study reveals that frequency components of pressure and flow below heart rate are generally necessary to obtain a valid estimate of compliance. Thus, the ubiquitous practice of estimating total arterial compliance from a single cardiac cycle is suspect under most circumstances, since a single cardiac cycle does not contain these frequencies.

Disparate effects of three types of extracellular acidosis on left ventricular function

Effects of acidosis on muscle contractile function have been studied extensively. However, the relative effects of different types of extracellular acidosis on left ventricular (LV) contractile function, especially the temporal features of contraction, have not been investigated in a single model. We constituted perfusion buffers of identical ionic composition, including Ca2+ concentration ([Ca2+]), to mimic physiological control condition (pH 7.40) and three types of acidosis with pH of 7.03: inorganic (IA), respiratory (RA), and lactic (LA). Isolated rabbit hearts (n = 9) were perfused with acidotic buffers chosen at random, each preceded by the control buffer. Under steady-state conditions, instantaneous LV pressure (Pv) and volume (Vv) were recorded for a range of Vv. The results were as follows. 1) LV passive (end-diastolic) elastance increased with IA and RA. However, this increase may not be a direct effect of acidosis; it can be explained on the basis of myocardial turgor. 2) Although LV inotropic state (peak active Pv and elastance) was depressed by all three acidotic buffers, the magnitude of inotropic depression was significantly less for LA. 3) Temporal features of Pv were altered differently. Whereas IA and RA reduced time to peak Pv (tmax) and hastened isovolumic relaxation at a common level of LV wall stress, LA significantly increased tmax and retarded relaxation. These results and a model-based interpretation suggest that cooperative feedback (i.e., force-activation interaction) plays an important role in acidosis-induced changes in LV contractile function. Furthermore, it is proposed that LA-induced responses comprise two components, one due to intracellular acidosis and the other due to pH-independent effects of lactate ions.

Apparent arterial compliance

Recently, there has been renewed interest in estimating total arterial compliance. Because it cannot be measured directly, a lumped model is usually applied to derive compliance from aortic pressure and flow. The archetypical model, the classical two-element windkessel, assumes 1) system linearity and 2) infinite pulse wave velocity. To generalize this model, investigators have added more elements and have incorporated nonlinearities. A different approach is taken here. It is assumed that the arterial system 1) is linear and 2) has finite pulse wave velocity. In doing so, the windkessel is generalized by describing compliance as a complex function of frequency that relates input pressure to volume stored. By applying transmission theory, this relationship is shown to be a function of heart rate, peripheral resistance, and pulse wave reflection. Because this pressure-volume relationship is generally not equal to total arterial compliance, it is termed "apparent compliance." This new concept forms the natural counterpart to the established concept of apparent pulse wave velocity.

Ejection has both positive and negative effects on left ventricular isovolumic relaxation

In isovolumically beating hearts, the speed of left ventricular (LV) relaxation is uniquely determined by peak active stress (sigma max). In contrast, such a succinct description of relaxation is lacking for the ejection beats, although ejection is generally thought to hasten relaxation. We set out to determine how ejection modifies the relaxation-sigma max relationship obtained in the isovolumically beating hearts. Experiments were performed on five isolated rabbit hearts subjected to various loading conditions. Instantaneous LV pressure and volume were recorded and converted to active stress, from which isovolumic relaxation time (Tr) was defined as the time for stress to fall from 75 to 25% of sigma max (isovolumic beats) or its end-ejection value (ejection beats). Steady-state and transient isovolumic beat and steady-state ejection beat data were used to develop a multiple regression model. This model identified stress, current beat ejection, and previous beat ejection history as independent predictor variables of Tr and fit the data well in all hearts (r2 > 0.98). Furthermore, this model could predict relaxation in transient ejection beats (r2 = 0.30 for all hearts). Whereas the coefficient for the current beat ejection was negative (i.e., negative effect or hastening relaxation), the ejection history coefficient was positive (i.e., positive effect or slowing relaxation). The sum of these two coefficients was negative, corresponding to the commonly observed net negative effect of ejection on relaxation. The expected positive inotropic effect of ejection was also observed. The dissipations of both positive inotropic and relaxation effects were slow, suggesting a nonmechanical underlying mechanism(s). We postulate that these two effects are linked and caused by ejection-mediated changes in myofilament Ca2+ sensitivity.

Serial assessment of the cardiovascular system in normal pregnancy. Role of arterial compliance and pulsatile arterial load

BACKGROUND

Temporal changes in systemic arterial compliance and wave propagation properties (pulsatile arterial load) and their role in ventricular-systemic arterial coupling during gestation have not been explored. Noninvasive methods combined with recently developed mathematical modeling techniques were used to characterize vascular and left ventricular (LV) mechanical adaptations during normal gestation.

METHODS AND RESULTS

Fourteen healthy women were studied at each trimester of pregnancy and again postpartum. Experimental measurements included instantaneous aortic pressure (subclavian pulse tracings) and flow (aortic Doppler velocities) and echocardiographic imaging of the LV. A small increase in LV muscle mass and end-diastolic chamber dimension occurred by late gestation, with no significant alterations in myocardial contractility. Cardiac output increased and the steady component of arterial load (total vascular resistance) decreased during pregnancy. Several changes in pulsatile arterial load were noted: Global arterial compliance increased (approximately 30%) during the first trimester and remained elevated thereafter. The magnitude of peripheral wave reflections at the aorta was reduced. The mathematical model-based analysis revealed that peripheral wave reflections at the aorta were delayed and that both conduit and peripheral vessels contributed to the increased arterial compliance. Finally, coordinated changes in the pulsatile arterial load and LV properties were responsible for maintaining the efficiency of LV-to-arterial system energy transfer.

CONCLUSIONS

The rapid time course of compliance changes and the involvement of both conduit and peripheral vessels are consistent with reduced vascular tone as being the main underlying mechanism. The pulsatile arterial load alterations during normal pregnancy are adaptive in that they help to accommodate the increased intravascular volume while maintaining the efficiency of ventricular-arterial coupling and diastolic perfusion pressure.

Acute primary human immunodeficiency virus type 1 infection in a patient with concomitant cytomegalovirus encephalitis

We report what we believe is the first case of primary human immunodeficiency virus type 1 (HIV-1) infection and simultaneous cytomegalovirus (CMV) encephalitis, which was confirmed by detection of CMV DNA in the patient's cerebrospinal fluid with use of the polymerase chain reaction. This coinfection had an unusual course, and the patient's clinical status deteriorated despite administration of combination antiretroviral therapy. The patient responded clinically only after therapy for CMV infection was added to his combination antiretroviral regimen. An atypical course and duration of symptomatic primary HIV-1 infection should suggest a possible coincident infection with other opportunistic agents that are normally expected to cause disease later in the course of HIV-1 infection. Current recommendations from the Centers for Disease Control and Prevention list CMV encephalitis as an AIDS-defining event.

Wave propagation in coupled left ventricle-arterial system. Implications for aortic pressure

The objective of this study was to examine the effects of wave propagation properties (global reflection coefficient gamma IG; pulse wave velocity, c(ph); and characteristic impedance zeta(o) on the mechanical performance of the coupled left ventricle-arterial system. Specifically, we sought to quantify effects on aortic pressure (P(ao)) and flow Q(ao) while keeping constant other determinants of P(ao) and Q(ao) (left ventricular end-diastolic volume, V(ed), and contractility, heart rate, and peripheral resistance, R(s)). Isolated rabbit hearts were subjected to real-time, computer-controlled physiological loading. The arterial circulation was modeled with a lossless tube terminating in a complex load. The loading system allowed for precise and independent control of all arterial properties as evidenced by accurate reproduction of desired input impedances and computed left ventricular volume changes. While propagation phenomena affected P(ao) and Q(ao) morphologies as expected, their effects on absolute P(ao) values were often contrary to the current understanding. Diastolic (Pd) and mean (Pm) P(ao) and stroke volume decrease monotonically with increases in gamma G, c(ph), or zeta(o) over wide ranges. In contrast, these increase had variable effects on peak systolic P(ao) (Ps): decreasing with gamma G, biphasic with c(ph), and increasing with zeta(o). There was an interaction between gamma G and c(ph) such that gamma G effects on P(m) and P(d) were augmented a higher C(ph) and vice versa. Despite large changes in system parameters, effects on Pm and Ps were modest ( < 10% and < 5%, respectively); effects on Pd were always two to four times greater. Similar results were obtained when the single-tube model of the arterial system was replaced by an asymmetrical T-tube configuration. Our data do not support the prevailing hypothesis that P(s) (and therefore ventricular load) can be selectively and significantly altered by manipulating gamma G, c(ph), and/or zeta o.

Combating malnutrition in HIV infection

Physicians should use nutrition as adjunctive therapy for all patients affected by HIV disease. However, when wasting is already present, a thorough evaluation (examination and labs) to elucidate and treat the underlying causes of malnutrition should be initiated, dietary counseling made available, and nutritional and vitamin/mineral supplementation encouraged. Appetite stimulants may be helpful when appropriate. If wasting syndrome develops despite intervention, enteral or parenteral nutrition may be considered. Clinical trials are currently underway to assess the utility of anabolic therapies and cytokine inhibitors as additional options. Malnutrition is a leading cause of morbidity in HIV disease. With continued research and early nutritional interventions (i.e., prevention as well as treatment strategies), the task of preventing wasting may eventually become "a piece of cake."

Arterial wave propagation phenomena, ventricular work, and power dissipation

The effects of wave propagation phenomena, namely global reflection coefficient (gamma G[omega]) and pulse wave velocity (Cph), are studied in a model of the coupled left ventricle/arterial system. The left ventricle consists of a time-varying elastance, while the arterial system is modeled as a single, uniform, elastic tube terminating in a complex load. Manipulation of model parameters allowed for the precise control of gamma G(omega) and Cph independent of each other, peripheral resistance, and characteristic impedance. Reduction of gamma G(omega) and Cph were achieved through increases in load compliance and tube compliance, respectively. The equations describing the system were solved for left ventricular and aortic pressures and aortic flow. From these, stroke volume (SV), left ventricular stroke work (SW), and steady (Ws), oscillatory (Wo), and total power dissipation (Wt) in the arterial system were calculated. An index of arterial system efficiency was the ratio Wo/Wt (%Wo), with lower values indicating higher efficiency. Reduction of gamma G(omega) yielded initial increases in Ws, while Wo increased for the entire range of gamma G(omega), resulting in increased %Wo. This reduced efficiency is imposed on the ventricle, resulting in increased SW without increased SV. On the other hand, decreased Cph yielded in a steady increase in Ws and a biphasic response in Wo, resulting in reduced %Wo for most of the range of reduced Cph. These results suggest that differential effects on arterial system efficiency can result from reductions of gamma G(omega) and Cph. In terms of compliance, changes in arterial compliance can have different effects on efficiency, depending on where the compliance change takes place. Reasons for these results are suggested, and the role of distributed compliances is raised as a new problem.

Physiological relevance of T-tube model parameters with emphasis on arterial compliances

The T-tube model of systemic arterial circulation was examined with respect to the physiological relevance of model parameters. root aortic pressure [Pao(t)] and flow [Qao(t)] and descending aortic flow [Qb(t)] were measured in anesthetized, open-chest dogs under control conditions, during inflation of a balloon positioned in the left external iliac artery (n = 5), and during infusion of vasoactive drugs nitroprusside (NTP, n = 4) and phenylephrine (PHL, n = 5). With Pao(t) as the input, the model accurately predicted both Qao(t) and Qb(t) under all conditions (r2 > 0.96). The balloon inflation data established the ability of the model to discriminate between proximal and distal arterial mechanical properties. Furthermore, proximal properties (i.e., tube characteristic impedances and transit times) were independent of distal properties such as terminal compliances and resistances (or equivalently, wave reflections). The effects of NTP and PHL were pharmacologically consistent and served to further validate this model. NTP primarily affected distal (load) properties, whereas PHL altered both load and tube parameters. Physiological interpretation of model parameters, particularly compliance, is also discussed. The ability of the model to correctly discriminate between proximal and distal arterial properties is relevant because these properties may affect cardiovascular function differently.

Alterations in systemic arterial mechanical properties during septic shock: role of fluid resuscitation

The effects of septic shock (endotoxin; EDTX) on arterial mechanical properties were studied in anesthetized rabbits, both in the absence (EDTX alone) and presence (EDTX + fluids) of fluid resuscitation. Aortic pressure-flow (n = 20) and pressure-diameter (n = 10) measurements were used to calculate systemic arterial and regional aortic mechanical properties. At 3 h of EDTX shock, EDTX-alone rabbits had elevated total peripheral resistance (TPR, + 30%, P < 0.05), reduced cardiac output (CO, -40%, P < 0.05), and increased aortic characteristic impedance (Zc, +78%, P < 0.05). In contrast, the EDTX + fluids group responded with decreased TPR (-30%, P < 0.05), a tendency to increase CO (+23%), and elevated Zc (+46%, P < 0.05). A reduction in aortic diameter (-20%, P < 0.05) and an increase in elastic modulus (+50%, P < 0.05) and water content (+23%, P < 0.02) of the aortic wall were observed following endotoxemia. Thus following EDTX 1) "hyperdynamic" septic shock profile (i.e., low TPR, high CO) was observed only when concomitant fluid replacement was provided, 2) aortic wall stiffening was present due to both increased smooth muscle tone and vessel wall edema, and 3) fluid resuscitation resulted in discordant changes in TPR and Zc, suggesting differential flow-induced vasodilation between arteriolar and aortic smooth muscle.

Differential effects of chronic oral antihypertensive therapies on systemic arterial circulation and ventricular energetics in African-American patients

BACKGROUND

A comprehensive evaluation of arterial load characteristics and left ventricular energetics in systemic hypertension has been limited by the need for invasive techniques to access instantaneous aortic pressure and flow. As a consequence of this methodological limitation, no data exist on the effects of long-term antihypertensive therapy on global arterial impedance properties and indexes of myocardial oxygen consumption (MVO2). Using recently validated noninvasive techniques, we compared in hypertensive patients the effects of chronic oral treatment with ramipril, nifedipine, and atenolol on arterial impedance and mechanical power dissipation as well as indexes of MVO2.

METHODS AND RESULTS

Sixteen African-American subjects with systemic hypertension were studied with a randomized, double-blind, crossover protocol. Instantaneous central aortic pressure and flow, from which arterial load characteristics can be derived, were estimated from calibrated subclavian pulse tracings (SPTs) and continuous-wave aortic Doppler velocity in conjunction with two-dimensional (2D) echocardiographic measurements of the aortic annulus, respectively. To derive ventricular wall stress and indexes of MVO2, left ventricular short- (M-mode) and long-axis (2D echo) images were acquired simultaneously with SPTs. Data were collected at the end of a 2-week washout period (predrug control) and after 6 weeks of treatment with each agent. Although all three agents reduced diastolic blood pressure to the same extent, different effects on mean and systolic pressures and vascular impedance properties were noted. Nifedipine reduced total peripheral resistance (TPR; 1744 +/- 398 versus 1290 +/- 215 dyne-s/cm5) and increased arterial compliance (ACL; 1.234 +/- 0.253 versus 1.776 +/- 0.415 mL/mm Hg). This improvement in arterial compliance was not entirely accounted for by the reduction in distending pressure. Ramipril also decreased TPR (1740 +/- 292 versus 1437 +/- 290 dyne-s/cm5) and increased ACL (1.214 +/- 0.190 versus 1.569 +/- 0.424 mL/mm Hg), but with this agent, the change in arterial compliance was explained solely on the basis of a reduction in distending pressure. Atenolol, in contrast, did not affect either TPR or ACL. In agreement with the compliance results, nifedipine and ramipril significantly lowered the first two harmonics of the impedance spectrum, but atenolol did not. None of these agents resulted in a significant change in characteristic impedance or in the relative amplitude of the reflected pressure wave. Total vascular mechanical power and percent of oscillatory power remained unaltered with all antihypertensive treatments. Only ramipril and nifedipine reduced the integral of both meridional and circumferential systolic wall stresses, indicating that MVO2 per beat was reduced with these agents. Stress-time index, a measure of MVO2 per unit time, decreased significantly with ramipril but not with nifedipine because of an increase in heart rate noted in 10 of 16 patients (mean increase, 10 beats per minute). Thus, a reduction in MVO2 coupled with unchanged total vascular mechanical power suggests improved efficiency of ventriculoarterial coupling with ramipril and with nifedipine in the subset of patients in whom heart rate remained unchanged. In contrast, there was no evidence of a reduction in wall stress, stress integral, or stress-time index with atenolol.

CONCLUSIONS

The noninvasive methodology used in this study constitutes a new tool for serial and simultaneous evaluation of arterial hemodynamics and left ventricular energetics in systemic hypertension. In this study, we demonstrate the differential effects of chronic antihypertensive therapies on systemic arterial circulation and indexes of MVO2 in African-American subjects. Consideration of drug-induced differential responses of arterial load and indexes of MVO2 with each drug may provide a more physiological approach to the treatment of systemic hypertension in indivi

Differential effects of wave reflections and peripheral resistance on aortic blood pressure: a model-based study

It has been generally accepted that arterial system wave reflections act to increase aortic blood pressure and the load placed on the left ventricle. Using a mathematical model of the coupled left ventricle-arterial system, we predict that this is not the case. With the model, two aspects of wave reflection, the global reflection coefficient [TG(omega)] and the pulse wave velocity (cph), were adjusted independently. In addition, TG(omega) and cph could be altered independently of the direct-current properties of the arterial system model. Reduction of TG(omega) yielded increases in stroke volume (SV) as well as in peak systolic (Ps), diastolic (Pd), and mean aortic (Pao) pressures and, hence, increased the load on the left ventricle. SV and Pao increased only in the range where strong reflection occurs. Reduced cph also yielded higher pressures, whereas increased cph resulted in reduced Pao and Pd but increased Ps. The changes in pressures and SV in response to altered TG(omega) and cph were relatively small compared with absolute levels. Simulated vasoconstriction and vasodilation further demonstrated the much greater importance of peripheral resistance on pressure and SV levels and lead to the prediction that pressure reduction in vasodilation occurs not because of, but in spite of, reduced wave reflections. We conclude that these results have not yet been observed experimentally, because reflection cannot yet be separated from the direct-current properties of the arterial system; therefore wave reflections themselves have not yet been adequately studied in the intact animal.

Repeated reflection of waves in the systemic arterial system

Traditional analysis of pulse-wave propagation and reflection in the arterial system treats measured pressure and flow waves as the sum of a single forward wave (traveling away from the heart) and a single backward wave (traveling toward the heart). The purpose of this study was to develop a more general wave reflection theory that allows repeated reflection of these waves. The arterial system was modeled as a uniform viscoelastic tube terminating in a complex load with reflections occurring at the tube load interface and the heart tube interface. The resulting framework considers the forward wave to be the sum of an initial wave plus a series of antegrade waves. Similarly, the backward wave is the sum of a series of retrograde waves. This repeated reflection theory contains within it the traditional forward/backward wave reflection analysis as a special case. In addition, the individual antegrade and retrograde waves, at the tube entrance, are shown to be independent of the tube length. Aortic pressure and flow data, from dog experiments, were used to illustrate the phenomenon of repeated reflections. Alteration of the arterial system loading conditions, brought about through pharmacological intervention, affected the number and morphology of repeated waves. These results are compared with those found in traditional forward/backward reflection analysis.

Cardiovascular applications of the ALOPEX optimization technique

ALOPEX is a general optimization process incorporating a cost function containing a large number of parameters which may be simultaneously adjusted until the cost function reaches an optimum (maximum or minimum); local extremes are avoided by introducing random noise into the procedure. In this paper, ALOPEX is incorporated into a simple haemodynamic study in which an electric analogue model of the left ventricle is used to develop equations of myocardial stroke work. Pilot experiments were undertaken in rabbits (n = 5) to gauge the effectiveness of this optimizing technique. In the control state, calculated stroke work for the rabbit was determined to be 50 +/- 7 mmHg ml, while ALOPEX predicted a stroke work of 51 +/- 7 mmHg ml. ALOPEX is capable of following changing cardiovascular states when pharmacological agents are introduced. For example, after nitroprusside treatment, stroke work was reduced by 38 +/- 6% (P < 0.05) while ALOPEX predicted a 42 +/- 4% reduction from baseline (P < 0.05). Methoxamine treatment increased stroke work by 74 +/- 34%, while ALOPEX predicted a 73 +/- 43% increase above control values. There were no statistical differences between calculated and ALOPEX predicted values. Individual model parameters such as maximum left ventricular elastance (Emax) and left ventricular end diastolic volume (EDV) were also predicted correctly by ALOPEX. We have found that the ALOPEX optimization technique is useful in predicting components of multi-parametric functions. In particular, we have shown it to be adaptable to a simple haemodynamic model.

Temporal relationship between left ventricular and arterial system elastances

Arterial compliance is an important component of ventricular afterload. Although its pressure dependence has been recognized, its temporal relationship to ventricular elastance (Elv(t)) has not been established. We investigated this in five open chest anesthetized dogs where simultaneous aortic pressure and flow and left ventricular pressure were measured. Elv(t) was derived using an elastance-resistance model of the left ventricle assuming an ejection fraction of 0.50 and a dead volume (Vd) of 3.0 mL. The nonlinear pressure-dependent compliance (C(P)) of the arterial system was incorporated in a three-element Windkessel model and determined by accurate prediction of aortic pressure from aortic flow. The resulting arterial elastance (Eas(t)) was computed as Eas(t) = 1/C(P). Results show that Eas(t) reaches a minimum value at or near the start of ventricular ejection and attains its peak value at or near the same time maximum LV elastance (Emax) is reached, at end-systole. Finally, numerical simulation of the model demonstrates its ability to adequately reproduce measured pressure and flow. Thus, the arterial system, in terms of elastance, is dynamically and temporally coupled to the left-ventricle during ejection.

Concurrent compliance reduction and increased peripheral resistance in the manifestation of isolated systolic hypertension

The hemodynamic mechanisms responsible for producing isolated systolic hypertension (ISH) in the elderly are generally attributed to a decrease in arterial compliance. However, no consistent theoretical or experimental model has been proposed for the production of ISH. This problem was investigated with the use of computer simulation of the modified Windkessel model, an often-used tool in the study of arterioventricular function. Aortic pressure (Pa(t] and aortic flow (Qa(t] data were used to obtain the model parameters: peripheral resistance (Rs), arterial compliance (C) and characteristic impedance of the proximal aorta (Zo). Using Qa(t) as the input to the model, the effects of altered vascular properties on Pa(t) were studied by changing these model parameters. Graded reductions of C (25, 50 and 75%) alone increased systolic pressure (Ps), but also decreased diastolic pressure (Pd) to values below those found in ISH. On the other hand, an increase in Rs of 25% along with a 50 to 75% increase in C resulted in percent changes in Ps and Pd that would result in ISH from a normal pressure level. These results were consistent for a wide range of pressures. Decreased arterial compliance alone is not always responsible for the production of ISH. Rather, isolated systolic hypertension is usually the result of greatly reduced arterial compliance along with a smaller but significant increase in peripheral resistance.