Increased left ventricular stiffness impairs filling in dogs with pulmonary emphysema in respiratory failure

A change of heart: mechanisms of cardiac adaptation to acute and chronic hypoxia

Over the last 100 years, high‐altitude researchers have amassed a comprehensive understanding of the global cardiac responses to acute, prolonged and lifelong hypoxia. When lowlanders are exposed to hypoxia, the drop in arterial oxygen content demands an increase in cardiac output, which is facilitated by an elevated heart rate at the same time as ventricular volumes are maintained. As exposure is prolonged, haemoconcentration restores arterial oxygen content, whereas left ventricular filling and stroke volume are lowered as a result of a combination of reduced blood volume and hypoxic pulmonary vasoconstriction. Populations native to high‐altitude, such as the Sherpa in Asia, exhibit unique lifelong or generational adaptations to hypoxia. For example, they have smaller left ventricular volumes compared to lowlanders despite having larger total blood volume. More recent investigations have begun to explore the mechanisms underlying such adaptive responses by combining novel imaging techniques with interventions that manipulate cardiac preload, afterload, and/or contractility. This work has revealed the contributions and interactions of (i) plasma volume constriction; (ii) sympathoexcitation; and (iii) hypoxic pulmonary vasoconstriction with respect to altering cardiac loading, or otherwise preserving or enhancing biventricular systolic and diastolic function even amongst high altitude natives with excessive erythrocytosis. Despite these advances, various areas of investigation remain understudied, including potential sex‐related differences in response to high altitude. Collectively, the available evidence supports the conclusion that the human heart successfully adapts to hypoxia over the short‐ and long‐term, without signs of myocardial dysfunction in healthy humans, except in very rare cases of maladaptation.

The altitude at which a calf is born and raised influences the rate at which mean pulmonary arterial pressure increases with age

Right heart failure secondary to pulmonary hypertension is a leading cause of mortality among suckling beef calves in the Rocky Mountain region. The objective of this study was to track changes in pulmonary arterial pressures (PAP) in healthy calves born and raised at altitudes ranging from 1,470 to 2,730 m. It was hypothesized that calves located at higher altitudes would show a greater increase in mean PAP (mPAP) with age than would be experienced by calves located at lower altitudes. The rationale is that high altitude hypobaric hypoxia causes a greater rate of vascular remodeling and, consequently, greater resistance to blood flow than calves located at lower altitudes. A prospective study was conducted on 5 cohorts of suckling calves from 4 herds located at altitudes of 1,470, 2,010, 2,170, and 2,730 m. In total, 470 PAP measurements were obtained from 258 calves. As hypothesized, calves located at altitudes ≥2,170 m showed a significant increase in mPAP with age ( ≤ 0.002) whereas calves at 1,470 m did not ( = 0.16). Except for calves at 2,170 m ( < 0.001), systolic PAP did not increase with age ( ≥ 0.16). Diastolic PAP increased with age at altitudes ≥ 2,170 m ( ≤ 0.09) but did not change in calves at 1,470 m ( = 0.20). In summary, mPAP and the rate at which mPAP increases with age are positively associated with the altitude at which calves are born and raised.

Impaired myocardial function does not explain reduced left ventricular filling and stroke volume at rest or during exercise at high altitude

Impaired myocardial systolic contraction and diastolic relaxation have been suggested as possible mechanisms contributing to the decreased stroke volume (SV) observed at high altitude (HA). To determine whether intrinsic myocardial performance is a limiting factor in the generation of SV at HA, we assessed left ventricular (LV) systolic and diastolic mechanics and volumes in 10 healthy participants (aged 32 ± 7; mean ± SD) at rest and during exercise at sea level (SL; 344 m) and after 10 days at 5,050 m. In contrast to SL, LV end-diastolic volume was ∼19% lower at rest (P = 0.004) and did not increase during exercise despite a greater untwisting velocity. Furthermore, resting SV was lower at HA (∼17%; 60 ± 10 vs. 70 ± 8 ml) despite higher LV twist (43%), apical rotation (115%), and circumferential strain (17%). With exercise at HA, the increase in SV was limited (12 vs. 22 ml at SL), and LV apical rotation failed to augment. For the first time, we have demonstrated that EDV does not increase upon exercise at high altitude despite enhanced in vivo diastolic relaxation. The increase in LV mechanics at rest may represent a mechanism by which SV is defended in the presence of a reduced EDV. However, likely because of the higher LV mechanics at rest, no further increase was observed up to 50% peak power. Consequently, although hypoxia does not suppress systolic function per se, the capacity to increase SV through greater deformation during submaximal exercise at HA is restricted.

Short-term adaptation and chronic cardiac remodelling to high altitude in lowlander natives and Himalayan Sherpa

NEW FINDINGS

What is the topic of this review? At high altitude, the cardiovascular system must adapt in order to meet the metabolic demand for oxygen. This review summarizes recent findings relating to short-term and life-long cardiac adaptation to high altitude in the context of exercise capacity. What advances does it highlight? Both Sherpa and lowlanders exhibit smaller left ventricular volumes at high altitude; however, myocardial relaxation, as evidenced by diastolic untwist, is reduced only in Sherpa, indicating that short-term hypoxia does not impair diastolic relaxation. Potential remodelling of systolic function, as evidenced by lower left ventricular systolic twist in Sherpa, may facilitate the requisite sea-level mechanical reserve required during exercise, although this remains to be confirmed. Both short-term and life-long high-altitude exposure challenge the cardiovascular system to meet the metabolic demand for O2 in a hypoxic environment. As the demand for O2 delivery increases during exercise, the circulatory component of oxygen transport is placed under additional stress. Acute adaptation and chronic remodelling of cardiac structure and function may occur to facilitate O2 delivery in lowlanders during sojourn to high altitude and in permanent highland residents. However, our understanding of cardiac structural and functional adaption in Sherpa remains confined to a higher maximal heart rate, lower pulmonary vascular resistance and no differences in resting cardiac output. Ventricular form and function are intrinsically linked through the left ventricular (LV) mechanics that facilitate efficient ejection, minimize myofibre stress during contraction and aid diastolic recoil. Recent examination of LV mechanics has allowed detailed insight into fundamental cardiac adaptation in high-altitude Sherpa. In this symposium report, we review recent advances in our understanding of LV function in both lowlanders and Sherpa at rest and discuss the potential consequences for exercise capacity. Collectively, data indicate chronic structural ventricular adaptation, with adult Sherpa having smaller absolute and relative LV size. Consistent with structural remodelling, cardiac mechanics also differ in Sherpa when compared with lowlanders at high altitude. These differences are characterized by a reduction in resting systolic deformation and slower diastolic untwisting, a surrogate of relaxation. These changes may reflect a functional cardiac adaptation that affords Sherpa the same mechanical reserve seen in lowlanders at sea level, which is absent when they ascend to high altitude.

Biventricular function at high altitude: implications for regulation of stroke volume in chronic hypoxia

The myocardium is well protected against chronic hypoxia. In chronic hypoxia stroke volume falls both at rest and on exercise. The fall in stroke volume is associated with reduction in left ventricular dimensions and filling pressure. An obvious explanation for this is the reduction in plasma volume observed at high altitude, but this does not appear to be the whole story. Neither is left ventricular systolic function abnormal even at the summit of Mount Everest. Hypoxia itself may have a direct effect on impairing myocardial relaxation. Increased pulmonary vascular resistance leads to right ventricular pressure overload. This may impair right ventricular function, and reduce stroke volume and venous return to the left atrium. Interaction between the right and left ventricles, which share a common septum and are potentially constrained in volume by the pericardium, may impair diastolic left ventricular filling as a consequence of right ventricular pressure overload, and hence reduce stroke volume. It is questionable how clinically significant is this left ventricular diastolic dysfunction. The relative importance of different mechanisms which reduce stroke volume probably depends whether hemodynamics are measured at rest or on exercise. Intervention with sildenafil to ameliorate hypoxic pulmonary vasoconstriction is associated with both an increase in exercise capacity and stroke volume in hypoxia. Whether these have a causal association remains to be demonstrated.

Alveolar hypoxia induces left ventricular diastolic dysfunction and reduces phosphorylation of phospholamban in mice

Chronic obstructive pulmonary disease (COPD) may lead to pulmonary hypertension (PH) and reduced function of the right ventricle (RV). However, COPD patients may also develop left ventricular (LV) diastolic dysfunction. We hypothesized that alveolar hypoxia induces LV diastolic dysfunction and changes in proteins governing Ca(2+) removal from cytosol during diastole. Mice exposed to 10% oxygen for 1, 2, or 4 wk were compared with controls. Cardiac hemodynamics were assessed with Doppler echocardiography and a microtransducer catheter under general anesthesia. The pulmonary artery blood flow acceleration time was shorter and RV pressure was higher after 4 wk of hypoxia compared with controls (both P < 0.05). In the RV and LV, 4 wk of hypoxia induced a prolongation of the time constant of isovolumic pressure decay (51% RV, 43% LV) and a reduction in the maximum rate of decline in pressure compared with control (42% RV, 42% LV, all P < 0.05), indicating impaired relaxation and diastolic dysfunction. Alveolar hypoxia induced a 38%, 47%, and 27% reduction in Ser16-phosphorylated phospholamban (PLB) in the RV after 1, 2, and 4 wk of hypoxia, respectively, and at the same time points, Ser16-phosphorylated PLB in the LV was downregulated by 32%, 34%, and 25% (all P < 0.05). The amounts of PLB and sarco(endo)plasmic reticulum Ca(2+) ATPase (SERCA2a) were not changed. In conclusion, chronic alveolar hypoxia induces hypophosphorylation of PLB at Ser16, which might be a mechanism for impaired relaxation and diastolic dysfunction in both the RV and LV.

Left atrial and ventricular filling in chronic obstructive pulmonary disease. An echocardiographic and Doppler study

Abnormal left ventricular (LV) diastolic function has frequently been reported in patients with chronic obstructive pulmonary disease (COPD). In the present work, diastolic function was studied by a combined analysis of pulmonary venous and mitral blood flow velocities in 34 patients with COPD clinically stable and without history of heart disease, and 20 control subjects. We confirmed the increased contribution of the atrial contraction to the LV filling in COPD patients in comparison with control subjects; furthermore, a decreased left atrial (LA) filling during the ventricular systole was observed. Changes in LV filling were not the consequence of a systolic dysfunction, because LV systolic function was normal. Doppler indices indicated that LA pressure was below 15 cm H(2)O in all the patients with COPD and control subjects. Several factors can be put forward to explain these changes; the first one is tachycardia. In addition to hypoxemia and medications, echocardiography suggested that a decreased LV preload participated in increased heart rate. Analysis of Doppler transmitral and pulmonary venous flows demonstrated the role of the ventricular interdependence because a correlation existed between LA and LV filling pattern and right ventricle pressure and diameter.

Altered left ventricular chamber stiffness and isovolumic relaxation in dogs with chronic pulmonary hypertension caused by emphysema

BACKGROUND

In chronic obstructive lung disease, a right to left ventricular septal shift that occurs as a consequence of right ventricular pressure overload is the usual mechanism given to explain a decrease in left ventricular (LV) diastolic performance. The purpose of the present study was to examine the extent to which this mechanism could account for a decrease in LV diastolic function in a canine model in which pulmonary artery pressure was elevated to a level found in human disease.

METHODS AND RESULTS

Severe emphysema was produced in dogs by repeated instillations of the enzyme papain into the lung. To assess LV diastolic function, we used sonomicrometry, in which three pairs of subendocardial crystal transducers were implanted along the three orthogonal axes of the LV. LV end-diastolic dimensions and pressure-strain relations along the three axes, as well as the time constant of LV isovolumic relaxation (T), were measured before (baseline) and after 1 year of emphysema (post-1-year study). The results showed that after 1 year of pulmonary hypertension, LV pressure-strain relations were decreased along the septal-lateral and anterior-posterior axes, but a right to left ventricular septal shift was not detected. The relation of average midwall circumferential stress to midwall circumferential strain was used to describe the intrinsic compliance of the LV. The results showed that myocardial stiffness increased in emphysema but that chamber volume was not reduced. At the post-1-year study, T was abnormally increased in the emphysema group in response to augmented preload and afterload compared with preemphysema measurements.

CONCLUSIONS

We conclude that mechanisms other than ventricular interdependence may be operative in leading to altered LV diastolic filling in chronic emphysema.