Increased renal and forearm vasoconstriction in response to exercise after heart transplantation

Physiological confounders of renal blood flow measurement

OBJECTIVES

Renal blood flow (RBF) is controlled by a number of physiological factors that can contribute to the variability of its measurement. The purpose of this review is to assess the changes in RBF in response to a wide range of physiological confounders and derive practical recommendations on patient preparation and interpretation of RBF measurements with MRI.

METHODS

A comprehensive search was conducted to include articles reporting on physiological variations of renal perfusion, blood and/or plasma flow in healthy humans.

RESULTS

A total of 24 potential confounders were identified from the literature search and categorized into non-modifiable and modifiable factors. The non-modifiable factors include variables related to the demographics of a population (e.g. age, sex, and race) which cannot be manipulated but should be considered when interpreting RBF values between subjects. The modifiable factors include different activities (e.g. food/fluid intake, exercise training and medication use) that can be standardized in the study design. For each of the modifiable factors, evidence-based recommendations are provided to control for them in an RBF-measurement.

CONCLUSION

Future studies aiming to measure RBF are encouraged to follow a rigorous study design, that takes into account these recommendations for controlling the factors that can influence RBF results.

Influence of different types of stressors on the waveform of the peripheral arterial pulse in humans

BACKGROUND

To characterize further the different reflected waves visible at the peripheral arterial pressure waveform, we recorded changes in the pulse wave contour during different stressful interventions.

METHODS

Ten young healthy volunteers underwent two protocols. At the first occasion, the digital arterial pressure waveform was continuously recorded with the Portapres device during 5 min of rest, a mental arithmetic stress test (MAT) and during the cold pressor test (COP). At a second occasion, the pulse waveform was recorded at rest and during bicycle exercise.

RESULTS

The reflected pressure peak seen during systole arrived earlier and was increased in amplitude during both MAT and COP compared to rest (p < 0.05). The first reflected pressure peak during diastole arrived earlier and was reduced in amplitude compared to the systolic peak during MAT (p < 0.01), while COP induced smaller effects. Bicycle exercise at 100 and 140 heartbeats/min caused the reflected peak in systole to be undetectable in most subjects and induced a marked reduction in the amplitude of the first diastolic reflected peak (p < 0.01), and time to its appearance was markedly reduced at 140 beats/min.

CONCLUSION

Several stressors accounted in daily life, such as mental stress, pain and exercise-induced rapid changes in the arterial pulse waveform. In terms of timing and amplitude, the reflected systolic pressure peak and the first diastolic pressure peak did not always change in the same direction during the interventions, implying different origins of these two reflected waves. Combining the stressors used with pulse wave analysis is a simple method for studying vascular reactivity non-invasively.

Acute effects of a single dose of phosphodiesterase type 5 inhibitor (sildenafil) on systemic arterial blood pressure during exercise and 24-hour ambulatory blood pressure monitoring in heart transplant recipients

BACKGROUND

Arterial systemic hypertension (SH) can be associated with a decrease in endothelium-dependent nitric oxide (NO). Sildenafil increases cyclic guanosine monophosphate (cGMP), a mediator of NO. However, little is known about the effects of PDE5 inhibition on 24-hour ambulatory pressure (ABP) and exercise blood pressure, noreprinephrine (Nor), and exercise capacity, especially after orthotopic heart transplantation (OHT).

METHODS

We studied 22 OHT patients who on the 1st day underwent a cardiopulmonary (CP) self-controlled treadmill 6' walk test (6') and, then, an ECG monitored CP treadmill maximal exercise test (Ex) within 60 and 90 minutes after oral Sildenafil (Sil; 50 mg) or placebo (Pl) given at random, and ABP. We determined at basal position (b), in the last minute of the 6' and at the peak Ex, the HR (bpm), Systolic blood pressure (SBP), and diastolic blood pressure (DBP), (mm Hg), VO2 (mL/kg/min), Slope VE/VCO2, exercise time (ET, min), distance (D; miles), and Nor (pg/mL). Also, after CP tests, 24-h SBP and DBP, the measurements were repeated on the 2nd day when the cross-over was done.

RESULTS

Sil significantly reduced blood pressure in the basal position and during exercise. It also promoted a significant reduction in SBP and DBP during 24 hours, daytime and nighttime. Sil did not change exercise capacity.

CONCLUSION

The NO-cGMP pathway seems to play a role in blood pressure control in OHT. In addition to antihypertensive therapy, PDE5 inhibition may have potential beneficial effects on hypertensive OHT.

Exercise responses following heart transplantation: 5 year follow-up

Heart transplantation is an established treatment for end stage heart failure. In addition to increased life expectancy, heart transplant recipients report a remarkable improvement in symptoms and functional capacity. Exercise performance following heart transplantation, however, remains impaired even in the absence of exertional symptoms. We have assessed the response to exercise in 47 patients with cardiac failure prior to and then at yearly intervals to five years post transplantation. All patients performed incremental symptom limited exercise tests during which minute ventilation (V'E), oxygen consumption (V'O2) and carbon dioxide production (V'CO2) and heart rate (HR) were measured. Ventilatory response (V'E/V'CO2), anaerobic threshold (V'O2 AT %predicted) and heart rate response (HR/VO2) were calculated. The dead space to tidal volume ratio (VD/VT) and alveolar-arterial oxygen gradient (A-aO2) were computed from transcutaneous monitoring. Despite substantial improvement in subjective functional capacity, heart transplant recipients continue to have limited exercise performance [Maximal V'O2% predicted pre-transplant 41.3 (2.2); 1 year 48.6 (1.7), p <0.001: V'O2 AT% 31.5 (1.1); 1 year 35.6 (1.0); respectively p<0.05]. The maximal oxygen uptake continued to improve at two years post-transplant but, thereafter, there was no further significant change at up to 5 years post transplant [50.9 (1.5)]. At one year post-transplantation peak HR [65.2 (0.9) vs 79.1(1.4)] and the HR/VO2 response [24.0(1.8) vs 79.6(4.2)] were significantly reduced compared to pre-transplant values. The heart rate response remained lower compared to predicted at 5 years post-transplant although there was a significant increase compared to one year post-transplant (32.9 vs 24.0mls/bt). There was a weak but significant relationship between maximal VO2 and peak HR (0.39, p<0.05) and HR/VO2 (r= 0.37, p<0.05) at one year post-transplant. Prior to transplantation the ventilatory response to exercise was elevated [V'E/V'CO2 45.6 (2.5)] and decreased significantly following transplantation [1 yr 34.1 (1.3), respectively p<0.001]. In addition, despite significant improvement in VD/VT after transplantation, it remained higher than normal [Pre VD/VT at maximum exercise 0.35 (0.02); 1 yr 0.31 (0.02); p<0.05]. There was a further fall in the VE/VCO2 and VD/VT at two years post-transplantation with no further change at up to 5 years post transplantation [VE/VCO2 32.0 (1.0); VD/VT 0.29 (0.01)]. Although cardiac output is markedly improved after transplantation, due to chronotropic incompetence associated with denervation, its response remains subnormal and this may explain the residual abnormalities of ventilatory and gas exchange responses to exercise following transplantation.

Serial measurements of peripheral vascular reactivity and exercise capacity in congestive heart failure and after heart transplantation

BACKGROUND

The regulation of nutritive blood flow to skeletal muscles during exercise seems to make an important contribution to exercise capacity. In congestive heart failure (CHF) this regulation seems to be impaired, with attenuated peripheral vasodilatory capacity. The results regarding improvement of peripheral vasoreactivity after heart transplantation (HTx) are conflicting, and the contribution of impaired peripheral vasoreactivity to the observed reduced exercise capacity among heart transplant recipients (HTR) has not been well elucidated. We therefore assessed the reversibility of impaired vasoreactivity in forearm and calf after HTx with relationship to exercise capacity.

METHODS AND RESULTS

The vasoreactivity of both forearm and calf was studied with venous occlusion plethysmography and related to exercise capacity in 64 patients with CHF and in 22 controls. Of these patients, 29 patients underwent HTx, and the same measurements were performed 10 days, 6 months and 1 year after HTx, and in a group of 15 HTR who had undergone HTx several years ago. Our main findings were (1) impaired resting blood flow in patients with CHF improved after HTx and even surpassed levels of controls; (2) peak forearm blood flow remained attenuated early after HTx, but normalized during the first year postoperatively; (3) both forearm and calf minimal resistance remained elevated after HTx; (4) vascular reactivity displays regional variations in forearm and calf both during CHF and after HTx; and (5) peripheral vascular reactivity relate to exercise performance in both patients with CHF and HTR, but the relationship seemed more pronounced in CHF.

CONCLUSION

With impaired vasoreactivity related to limited exercise capacity in CHF, improvement is evident after HTx, but both forearm and calf minimal resistance remains elevated. These findings suggest increased vasoconstrictor drive to both exercising and non-exercising muscles, possibly contributing to persistent physical limitation after HTx.

Influence of progressive renal dysfunction in chronic heart failure

Chronic heart failure (CHF) is often associated with impaired renal function due to hypoperfusion. Such patients are very sensitive to changes in renal perfusion pressure, and may develop acute tubular necrosis if the pressure falls too far. The situation is complicated by the use of diuretics, ACE inhibitors and spironolactone, all of which may affect renal function and potassium balance. Chronic renal failure (CRF) may also be associated with fluid overload. Anaemia and hypertension in CRF contribute to the development of left ventricular hypertrophy (LVH), which carries a poor prognosis, so correction of these factors is important.

Vascular mechanisms of sudden death in hypertrophic cardiomyopathy, including blood pressure responses to exercise

Approximately a third of patients with hypertrophic cardiomyopathy fail to increase blood pressure appropriately during exercise, a consequence of an inappropriate vasodilator response in nonexercising beds, leading to an exaggerated fall in systemic vascular resistance. The precise mechanism responsible for this abnormal vascular control in hypertrophic cardiomyopathy is still unclear, but is thought to be secondary to enhanced cardiac baroreceptor activity. However, alternate or synergistic mechanisms, including enhanced release of brain natriuretic peptide, may be involved. Normal exercise blood pressure responses have been shown to have a high (97%) negative predictive accuracy for sudden death during an average follow-up of approximately 3 years, providing considerable reassurance. Patients with abnormal blood pressure responses on exercise were at markedly increased risk of sudden cardiac death, although the positive predictive accuracy during this brief follow-up period was low (15%). It is likely that vascular instability may act as a trigger for sudden cardiac death in patients with an underlying electrophysiologic substrate. Recent evidence suggests that this vascular instability may also result in hypotension during ordinary daily activity, or even at rest, and may be an important cause of syncope in hypertrophic cardiomyopathy. Further studies are required to identify mechanisms of attenuating or reversing this vascular instability. Such measures might have the potential to improve symptoms of recurrent syncope and perhaps reduce the risk of sudden cardiac death.

Exercise intolerance following heart transplantation: the role of pulmonary diffusing capacity impairment

STUDY OBJECTIVES

Although impairment of the diffusing capacity of the lung for carbon monoxide (DLCO) in heart transplant recipients is well-documented, there are limited data on its impact on exercise capacity in these patients. The aim of this study was to determine the effect of DLCO reduction on exercise capacity in heart transplant recipients.

DESIGN

Descriptive cohort study.

SETTING

A regional cardiopulmonary transplant center.

PARTICIPANTS

Twenty-six heart transplant recipients who were studied before and after transplantation compared with 26 healthy volunteers.

MEASUREMENTS

Spirometry and static lung volumes were measured using body plethysmography, DLCO was measured using the single-breath technique, and progressive cardiopulmonary exercise was performed using a bicycle ergometer, continuous transcutaneous blood gas monitoring, and on-line analysis of minute ventilation, oxygen uptake (VO(2)), and carbon dioxide production.

RESULTS

Before transplantation, the mean percent predicted for hemoglobin-corrected DLCO was reduced in patients (73.2%) compared to healthy control subjects (98.8%; p < 0.001) and declined significantly after transplantation (60.1%; p < 0.05). Although the mean maximal symptom-limited VO(2) (VO(2)max) increased after transplantation (increase, 41.3 to 48.6% of predicted; p < 0.05), it remained substantially lower than normal (92.9%; p < 0.001). There was a significant correlation between DLCO and VO(2)max after transplantation (r = 0.61; p = 0.001), but not before transplantation (r = 0.09; p = 0.66). DLCO was also inversely correlated with other respiratory responses to exercise, including the following: the ventilatory response to exercise (r = -0.44; p < 0.05); dead space to tidal volume ratio (r = -43; p < 0.05); and the alveolar-arterial oxygen gradient (r = -0. 45; p < 0.05), but there was no correlation between any of these variables and DLCO before transplantation.

CONCLUSION

DLCO reduction after heart transplantation appears to represent persistent gas exchange impairment and contributes to exercise limitation in heart transplant recipients.

Forearm vasoconstriction during dynamic leg exercise in patients with chronic heart failure

Previous studies assessing vascular responses in nonexercising beds during exercise in patients with chronic heart failure (CHF) have yielded varying results. We proposed that the clinical and hemodynamic severity of heart failure may explain some of the variation. We reasoned that diastolic ventricular interaction (DVI), by limiting the ability of such patients to increase left ventricular (LV) volume and stroke volume during exercise, would attenuate baroreflex activation, resulting in increased sympathetic activation and hence exaggerated vasoconstriction. We hypothesized therefore that vasoconstriction in nonexercising beds would be exaggerated in patients with symptomatic and hemodynamically severe heart failure, particularly if associated with DVI. We measured forearm vascular resistance (FVR) during semierect cycle exercise in 22 CHF patients and 23 control subjects. DVI was assessed by measuring changes in ventricular volumes (radionuclide ventriculography) during volume unloading (-30 mm Hg lower-body negative pressure) in the heart failure patients and was inferred when LV end-diastolic volume paradoxically increased. Patients with symptoms of heart failure developed larger increases in FVR during exercise than did asymptomatic patients. There were significant correlations between the change in FVR during peak exercise and the resting mean pulmonary arterial pressure and pulmonary vascular resistance. CHF patients with DVI developed exaggerated increases in FVR (median [25th to 75th percentile]) compared with the remaining patients during low-workload exercise (138 [66 to 171] vs 6.4 [-4.3 to 28] units, P = 0.002) and during peak exercise (160 [90 to 384] vs 61 [-7.4 to 75] units, P < 0.02). Vasoconstriction in nonexercising beds is exaggerated in CHF patients with clinically and hemodynamically severe heart failure, particularly if associated with DVI. This may explain some of the reported variation in the degree of sympathetic activation that occurs during exercise in CHF patients.

Forearm vascular responses during semierect dynamic leg exercise in patients following myocardial infarction

We assessed forearm vascular and blood pressure responses to dynamic leg exercise in patients 7 and 28 days postmyocardial infarction. To determine a possible association between abnormal exercise vascular responses and baroreflex dysfunction, integrated and carotid baroreflex sensitivity and forearm vascular responses (during application of subhypotensive lower body negative pressure) were assessed. On day 7, 42 patients were compared with 21 age- and sex-matched controls. All subjects were assessed for (1) forearm vascular resistance during semierect exercise, (2) blood pressure measurements during erect treadmill exercise, and (3) integrated, cardiopulmonary, and carotid baroreceptor sensitivity. These studies were repeated in 13 patients on day 28. Forearm vascular resistance increased during exercise by 36% +/- 63% in patients versus 121% +/- 105% in controls (P = 0.0001), and fell in 15 patients, a response seen in none of the controls. Exercise hypotension was demonstrated in 5 patients, all of whom had abnormal vasodilator vascular responses. Those patients with vasodilator responses had a lower left ventricular ejection fraction (52% +/- 12% vs 62% +/- 9%; P = 0.007), and lower cardiopulmonary mechanoreceptor sensitivity (-6.6 +/- 3.9 units vs +6.4 +/- 10.4 units, P = 0.02) than those with constrictor responses, respectively. In the 13 patients studied on day 28, the change in forearm vascular resistance was similar to that observed on day 7 (36% +/- 63% vs 46% +/- 73%). Paradoxical vasodilation of forearm vessels during leg exercise is common in patients studied 7 and 28 days postmyocardial infarction, and is associated with lower left ventricular ejection fraction and abnormal vascular responses during subhypotensive lower body negative pressure.

Exercise vascular responses in health and disease

In normal subjects during exercise, there is vasoconstriction of non-exercise resistance vessels and an increase in blood pressure. We have investigated patients with vasovagal syncope with structurally normal hearts and patients with hypertrophic cardiomyopathy compared with normal controls and found a failure of vasoconstriction in both patients with vasovagal syncope and hypertrophic cardiomyopathy compared with normals and this has been associated with exercise hypotension. An association between exercise hypotension and sudden death has previously been reported in hypertrophic cardiomyopathy. We speculate that in patients with vasovagal syncope and structurally and electrically normal hearts, exercise hypotension is well tolerated, but in patients with structurally abnormal hearts, exercise hypotension can have catastrophic consequences.

Abnormal forearm vascular responses during dynamic leg exercise in patients with vasovagal syncope

BACKGROUND

We have reported previously that in some patients with normal hearts who present with exercise syncope, abnormal forearm vasodilation is seen during leg exercise and tilt table tests are positive. This suggests that exercise syncope may be a variant of vasovagal syncope. In this study we tested the hypothesis that there is loss of the normal forearm vasoconstrictor response during dynamic leg exercise in an unselected population of patients with classic vasovagal syncope.

METHODS AND RESULTS

We evaluated forearm vascular responses during maximal semierect cycle exercise in 28 consecutive patients with vasovagal syncope and compared them with 30 age-matched control subjects. We also evaluated blood pressure responses during erect treadmill exercise (Bruce protocol). While forearm vascular resistance at rest was similar in the patients with vasovagal syncope and the control group, forearm vascular resistance was markedly lower in the patients than in control subjects at peak exercise (85 +/- 54 versus 149 +/- 94 units, P = .002). Forearm vascular resistance fell by 3 +/- 48% during exercise in patients versus an increase of 135 +/- 103% in control subjects (P < .0001). Systolic blood pressure during erect exercise was lower in patients versus control subjects (155 +/- 32 versus 188 +/- 17 mm Hg, P < .0001). Six of the vasovagal patients complained of exercise syncope or presyncope on specific inquiry, and 4 of these 6 exhibited exercise hypotension during erect treadmill exercise testing.

CONCLUSIONS

Patients with vasovagal syncope exhibit a failure of the normal vasoconstrictor response in the forearm during dynamic leg exercise. Exercise syncope and presyncope are not uncommon in unselected patients with classic vasovagal syncope, as is exercise hypotension.