The effects of passive leg raising on arterial wave reflection in healthy adults

Echocardiographic evidence of left ventricular untwisting-filling interplay

Background

Left ventricular untwisting generates an early diastolic intraventricular pressure gradient (DIVPG) than can be quantified by echocardiography. We sought to confirm the quantitative relationship between peak untwisting rate and peak DIVPG in a large adult population.

Methods

From our echocardiographic database, we retrieved all the echocardiograms with a normal left ventricular ejection fraction, for whom color Doppler M-Mode interrogation of mitral inflow was available, and left ventricular untwisting rate was measurable using speckle tracking. Standard indices of left ventricular early diastolic function were assessed by Doppler (peaks E, e’ and Vp) and speckle tracking (peak strain rate Esr). Load dependency of DIVPG and untwisting rate was evaluated using a passive leg raising maneuver.

Results

We included 154 subjects, aged between 18 to 77 years old, 63% were male. Test-retest reliability for color Doppler-derived DIVPG measurements was good, the intraclass correlation coefficients were 0.97 [0.91–0.99] and 0.97 [0.67–0.99] for intra- and inter-observer reproducibility, respectively. Peak DIVPG was positively correlated with peak untwisting rate (r = 0.73, P <  0.001). On multivariate analysis, peak DIVPG was the only diastolic parameter that was independently associated with untwisting rate. Age and gender were the clinical predictive factors for peak untwisting rate, whereas only age was independently associated with peak DIVPG. Untwisting rate and DIVPG were both load-dependent, without affecting their relationship.

Conclusions

Color Doppler-derived peak DIVPG was quantitatively and independently associated with peak untwisting rate. It thus provides a reliable flow-based index of early left ventricular diastolic function.

Features of the non-contact carotid pressure waveform: Cardiac and vascular dynamics during rebreathing

This report amplifies and extends prior descriptions of the use of laser Doppler vibrometry (LDV) as a method for assessing cardiovascular activity, on a non-contact basis. A rebreathing task (n = 35 healthy individuals) was used to elicit multiple effects associated with changes in autonomic drive as well as blood gases including hypercapnia. The LDV pulse was obtained from two sites overlying the carotid artery, separated by 40 mm. A robust pulse signal was obtained from both sites, in accord with the well-described changes in carotid diameter over the blood pressure cycle. Emphasis was placed on extracting timing measures from the LDV pulse, which could serve as surrogate measures of pulse wave velocity (PWV) and the associated arterial stiffness. For validation purposes, a standard measure of pulse transit time (PTT) to the radial artery was obtained using a tonometric sensor. Two key measures of timing were extracted from the LDV pulse. One involved the transit time along the 40 mm distance separating the two LDV measurement sites. A second measure involved the timing of a late feature of the LDV pulse contour, which was interpreted as reflection wave latency and thus a measure of round-trip travel time. Both LDV measures agreed with the conventional PTT measure, in disclosing increased PWV during periods of active rebreathing. These results thus provide additional evidence that measures based on the non-contact LDV technique might provide surrogate measures for those obtained using conventional, more obtrusive assessment methods that require attached sensors.

Effects of Ischemic Reperfusion Injury and Remote Conditioning on Passive Leg Raising-Induced Brachial-Artery Dilation

<b><i>Objectives:</i></b> Passive leg raising (PLR) has been proposed to assess arterial vasodilator reserve and possibly endothelial function. Since endothelial function is sensitive to ischemic-reperfusion (I-R) injury, we determined the effects of I-R injury and ischemic conditioning on PLR-induced brachial-artery dilation (BAD), i.e. PLR-BAD. <b><i>Methods:</i></b> We induced PLR-BAD before and after ipsilateral arm I-R injury (7.5 min of occlusion) in 20 healthy males aged 29 ± 6 years. The protocol was repeated in combination with remote conditioning stimuli (3 × 30 s of contralateral arm occlusions). <b><i>Results:</i></b> PLR resulted in significant BAD (3.85%, p < 0.001) before but not after prolonged ischemia (0.25%, p = 0.38). I-R injury, along with either preischemic or postischemic conditioning restored the PLR-BAD response (before: 3.11%, p < 0.001 and after: 3.74%, p < 0.001). <b><i>Conclusions:</i></b> I-R injury blunts the BAD induced by PLR. Remote pre- and postconditioning restore this response. These findings are similar to those previously reported using hyperemia and ultrasound to assess BAD.

Impact of body tilt on the central aortic pressure pulse

The present work was undertaken to investigate, in young healthy volunteers, the relationships between the forward propagation times of arterial pressure waves and the timing of reflected waves observable on the aortic pulse, in the course of rapid changes in body position. 20 young healthy subjects, 10 men, and 10 women, were examined on a tilt table at two different tilt angles, −10° (Head-down) and + 45° (Head-up). In each position, carotid-femoral (T_cf) and carotid-tibial forward propagation times (T_ct) were measured with the Complior device. In each position also, the central aortic pressure pulse was recorded with radial tonometry, using the SphygmoCor device and a generalized transfer function, so as to evaluate the timing of reflected waves reaching the aorta in systole (onset of systolic reflected wave, sT1r) and diastole (mean transit time of diastolic reflected wave, dMTT). The position shift from Head-up to Head-down caused a massive increase in both T_ct (women from 130 ± 10 to 185 ± 18 msec P < 0.001, men from 136 ± 9 to 204 ± 18 msec P < 0.001) and dMTT (women from 364 ± 35 to 499 ± 33 msec P < 0.001, men from 406 ± 22 to 553 ± 21 msec P < 0.001). Mixed model regression showed that the changes in T_ct and dMTT observed between Head-up and Head-down were tightly coupled (regression coefficient 2.1, 95% confidence interval 1.9–2.3, P < 0.001). These results strongly suggest that the diastolic waves observed on central aortic pulses reconstructed from radial tonometric correspond at least in part to reflections generated in the lower limbs.

The effect of lower body weight support on arterial wave reflection in healthy adults

Body weight support (WS) during treadmill exercise is used to rehabilitate orthopedic/neurological patients. WS lowers musculoskeletal strain and load. It compresses the lower body and increases intrathoracic volume. We studied short-term effects of WS on wave reflection indices using applanation tonometry during progressive WS of 25%, 50%, and 75% of body weight in 25 healthy men. WS decreased mean heart rate from 79 to 69 beats/min (P < .001). Peripheral and central mean arterial, systolic, and pulse pressures (PP) remained unchanged. There was a trend toward lower peripheral and central diastolic pressure. PP amplification ratio decreased significantly (P = .005). Reflected wave characteristics: Augmented pressure and index increased in a stepwise manner with WS (both P < .001). Both ejection duration and systolic duration of the reflected pressure wave (Ätr) increased progressively (both P < .001). The round-trip travel time (Δtp) was unchanged. Left ventricular workload and oxygen demand: Left ventricular wasted pressure energy increased (P < .001), and the subendocardial viability ratio decreased (P = .005), whereas the tension time index remained unchanged. In normal men, WS acutely decreases the PP amplification ratio, increases the amplitude and duration of the reflected aortic pressure wave, and increases measures of wasted left ventricular pressure energy and oxygen demand.

Should the augmentation index be normalized to heart rate?

Pulse wave analysis(PWA) is widely used to investigate systemic arterial stiffness. The augmentation index(AIx), the primary outcome derived from PWA, is influenced by the mean arterial pressure(MAP), age, gender and heart rate(HR). Gender- and age-specific reference values have been devised, and it is recommended that the MAP be used as a statistical covariate. The AIx is also commonly statistically adjusted to a HR of 75 b·min(-1); however, this approach may be physiologically and statistically inappropriate. First, there appears to be an important physiological chronic interaction between HR and arterial stiffness. Second, the method used to correct to HR assumes that the relationship with AIx is uniform across populations. A more appropriate practice may be to include HR as an independent predictor or covariate; this approach is particularly recommended for longitudinal studies, in which changes in HR may help to explain changes in arterial stiffness.

Increased wave reflection and ejection duration in women with chest pain and nonobstructive coronary artery disease: ancillary study from the Women's Ischemia Syndrome Evaluation

OBJECTIVE

Wave reflections augment central aortic SBP and increase systolic pressure time integral (SPTI) thereby increasing left ventricular (LV) afterload and myocardial oxygen (MVO2) demand. When increased, such changes may contribute to myocardial ischemia and angina pectoris, especially when aortic diastolic time is decreased and myocardial perfusion pressure jeopardized. Accordingly, we examined pulse wave reflection characteristics and diastolic timing in a subgroup of women with chest pain (Women's Ischemia Syndrome Evaluation, WISE) and no obstructive coronary artery disease (CAD).

METHODS

Radial artery BP waveforms were recorded by applanation tonometry, and aortic BP waveforms derived. Data from WISE participants were compared with data from asymptomatic women (reference group) without chest pain matched for age, height, BMI, mean arterial BP, and heart rate.

RESULTS

Compared with the reference group, WISE participants had higher aortic SBP and pulse BP and ejection duration. These differences were associated with increased augmentation index and reflected pressure wave systolic duration. These modifications in wave reflection characteristics were associated with increased SPTI and wasted LV energy (Ew) and a decrease in pulse pressure amplification, myocardial viability ratio, and diastolic pressure time fraction.

CONCLUSION

WISE participants with no obstructive CAD have changes in systolic wave reflections and diastolic timing that increase LV afterload, MVO2 demand, and Ew with the potential to reduce coronary artery perfusion. These alterations in cardiovascular function contribute to an undesirable mismatch in the MVO2 supply/demand that promotes ischemia and chest pain and may contribute to, or increase the severity of, future adverse cardiovascular events.

Comparison of passive leg raising and hyperemia on macrovascular and microvascular responses

Passive leg raising is a simple diagnostic maneuver that has been proposed as a measure of arterial vasodilator reserve and possibly endothelial function. While passive leg raising has previously been shown to lower blood pressure, increase flow velocity and cause brachial artery dilation, its effects on microvascular flow has not been well studied. Also, passive leg raising has been directly compared previously to upper arm but never to lower arm occlusion of blood flow induced hyperemia responses. We compared changes in macrovascular indices measured by brachial artery ultrasound and microvascular perfusion measured by Laser Doppler Flowmetry induced by passive leg raising to those provoked by upper arm and lower arm induced hyperemia in healthy subjects. Upper arm induced hyperemia increased mean flow velocity by 398%, induced brachial artery dilatation by 16.3%, and increased microvascular perfusion by 246% (p<.05 for all). Lower arm induced hyperemia increased flow velocity by 227%, induced brachial artery dilatation by 10.8%, and increased microvascular perfusion by 281%. Passive leg raising increased flow velocity by 29% and brachial artery dilatation by 5.6% (p<.05 for all), but did not change microvascular perfusion (-5%, p=ns). In conclusion, passive leg raising increases flow velocity orders of magnitude less than does upper arm or lower arm induced hyperemia. Passive leg raising-induced brachial artery dilatation is less robust than either of these hyperemic techniques. Finally, although upper arm and lower arm hyperemia elicits macrovascular and microvascular responses, passive leg raising elicits only macrovascular responses.

Passive leg raising induced brachial artery dilation: is an old technique a simpler method to measure endothelial function?

OBJECTIVES

Passive leg raising (PLR) is a diagnostic maneuver that has been shown to cause brachial artery dilation (BAD). The objectives of this study were to compare BAD induced by PLR with flow mediated dilation (FMD), and to investigate the mechanism of PLR-BAD. We studied a total of 75 subjects with and without cardiovascular risk factors/disease in order to provide a wide range of FMD responses.

METHODS

Using ultrasound, PLR-BAD and FMD induced by release of arterial cuff occlusion were measured.

RESULTS

BA diameter increased from 0.33+0.06 at baseline to 0.35+/-0.06 cm (p<.001) (4.8% increase) upon PLR and from 0.33+/-0.06 to 0.37+/-0.06 (11.8%) upon hyperemia. PLR induced BAD was significantly correlated with FMD (r=.82, p<.001). On receiver operating characteristic analysis of the two techniques, the area under the curve was 0.86 (95% CI 0.79-0.94, p<.001). Heart rate variability measures remained unchanged upon PLR indicating minimal contributions from changes in autonomic activity. The combination of FMD and PLR did not result in greater BAD than did FMD alone consistent with a common underlying mechanism. Mean blood flow velocity increased prior to BAD suggesting that shear stress increases prior to BAD.

CONCLUSIONS

BAD occurs in response to PLR and is proportional to FMD, although the magnitude of PLR-BAD is less than half that of FMD. It appears to occur by the same endothelial dependent mechanism as FMD. PLR-BAD may be used as a surrogate measure of FMD to evaluate vascular function, and has the advantage of being simpler to perform.

Effect of increased preload on the synthesized aortic blood pressure waveform

In the present study, we examined the influence of preload augmentation via passive leg elevation (PLE) on synthesized aortic blood pressure, aortic augmentation index (AIx), and aortic capacitance (a reflection of aortic reservoir function). Central and peripheral hemodynamics were measured via tonometry with a generalized transfer function in 14 young, healthy men (age = 24 yr). Aortic blood flow was calculated from the left ventricular outflow tract (LVOT) velocity-time integral (VTI) using standard two-dimensional echocardiographic-Doppler techniques. Measures were made in the supine position at rest (Pre), during PLE, and during recovery (Post). There was a significant increase in LVOT-VTI, synthesized aortic systolic blood pressure (BP) and AIx from Pre to PLE, with values returning to baseline Post (P < 0.05). There was a reduction in aortic capacitance from Pre to PLE, with values returning to baseline Post (P < 0.05). There was no change in heart rate, systemic arterial compliance, aortic elastance, aortic wave travel timing, or vascular resistance (P > 0.05). Change in AIx from Pre to PLE was associated with change in LVOT-VTI (r = 0.66, P < 0.05) and inversely associated with change in aortic capacitance (r = -0.73, P < 0.05). These data suggest that in a setting of isolated augmented preload with minimal changes in other potential confounders, the morphology of the synthesized aortic BP waveform and AIx may be related to changes in aortic reservoir function.