Pulmonary Circulation

Respiratory-Cardiovascular Interactions During Mechanical Ventilation: Physiology and Clinical Implications

Positive-pressure inspiration and positive end-expiratory pressure (PEEP) increase pleural, alveolar, lung transmural, and intra-abdominal pressure, which decrease right and left ventricular (RV; LV) preload and LV afterload and increase RV afterload. The magnitude and clinical significance of the resulting changes in ventricular function are determined by the delivered tidal volume, the total level of PEEP, the compliance of the lungs and chest wall, intravascular volume, baseline RV and LV function, and intra-abdominal pressure. In mechanically ventilated patients, the most important, adverse consequences of respiratory-cardiovascular interactions are a PEEP-induced reduction in cardiac output, systemic oxygen delivery, and blood pressure; RV dysfunction in patients with ARDS; and acute hemodynamic collapse in patients with pulmonary hypertension. On the other hand, the hemodynamic changes produced by respiratory-cardiovascular interactions can be beneficial when used to assess volume responsiveness in hypotensive patients and by reducing dyspnea and improving hypoxemia in patients with cardiogenic pulmonary edema. Thus, a thorough understanding of the physiological principles underlying respiratory-cardiovascular interactions is essential if critical care practitioners are to anticipate, recognize, manage, and utilize their hemodynamic effects. © 2022 American Physiological Society. Compr Physiol 12:1-24, 2022.

Extracellular Vesicles as Unique Signaling Messengers: Role in Lung Diseases

Extracellular vesicles (EVs) are lipid bilayer-enclosed extracellular particles carrying rich cargo such as proteins, lipids, and microRNAs with distinct characteristics of their parental cells. EVs are emerging as an important form of cellular communication with the ability to selectively deliver a kit of directional instructions to nearby or distant cells to modulate their functions and phenotypes. According to their biogenesis, EVs can be divided into two groups: those of endocytic origin are called exosomes and those derived from outward budding of the plasma membrane are called microvesicles (also known as ectosomes or microparticles). Under physiological conditions, EVs are actively involved in maintenance of pulmonary hemostasis. However, EVs can contribute to the pathogenesis of diseases such as chronic obstructive pulmonary disease, asthma, acute lung injury/acute respiratory distress syndrome, interstitial lung disease, and pulmonary arterial hypertension. EVs, especially those derived from mesenchymal/stromal stem cells, can also be beneficial and can curb the development of lung diseases. Novel technologies are continuously being developed to minimize the undesirable effects of EVs and also to engineer EVs so that they may have beneficial effects and can be used as therapeutic agents in lung diseases. © 2021 American Physiological Society. Compr Physiol 11:1351-1369, 2021.

Clinical significance of pulmonary artery occlusion pressure

BACKGROUND

Ppao values are routinely used to assess pulmonary vascular status and LV performance. Regrettably, under many common clinically relevant conditions, even when Ppao values are measured accurately, Ppao values at baseline and in response to therapy often reflect an inaccurate measure of cardiovascular status.

RESULTS AND CONCLUSIONS

Thus, caution should be used when applying measures of Ppao in determining therapy if changes in RV volume, hyperinflation, or LV diastolic compliance are simultaneously occurring.

Structure, strength, failure, and remodeling of the pulmonary blood-gas barrier

The pulmonary blood-gas barrier needs to satisfy two conflicting requirements. It must be extremely thin for efficient gas exchange, but also immensely strong to withstand the extremely high stresses in the capillary wall when capillary pressure rises during exercise. The strength of the blood-gas barrier on the thin side is attributable to the type IV collagen in the basement membranes. However, when the wall stresses rise to very high levels, ultrastructural changes in the barrier occur, a condition known as stress failure. Physiological conditions that alter the properties of the barrier include intense exercise in elite human athletes. Some animals, such as Thoroughbred racehorses, consistently break their alveolar capillaries during galloping, causing hemorrhage. Pathophysiological conditions causing stress failure include neurogenic pulmonary edema, high-altitude pulmonary edema, left heart failure, and overinflation of the lung. Remodeling of the capillary wall occurs in response to increased wall stress, a good example being the thickening of the capillary basement membrane in diseases such as mitral stenosis. The blood-gas barrier is able to maintain its extreme thinness with sufficient strength only through continual regulation of its wall structure. Recent experimental work suggests that rapid changes in gene expression for extracellular matrix proteins and growth factors occur in response to increases in capillary wall stress. How the blood-gas barrier is regulated to be extremely thin but sufficiently strong is a central issue in lung biology.

The influence of respiratory acid-base status on adult pulmonary vascular resistance before and after cardiopulmonary bypass

Respiratory acid-base status has recently been shown to affect pulmonary vascular resistance (PVR) in adults following cardiac surgery. The purpose of this study was to examine what influence cardiopulmonary bypass has on the pulmonary vascular response to changes in respiratory acid-base status. Fifteen consecutive patients undergoing aortocoronary bypass were studied under general anesthesia both before and after cardiopulmonary bypass. Arterial PCO2 was manipulated by the addition of 5 percent carbon dioxide to the breathing circuit. Both before and after bypass, PVR increased significantly as PCO2 rose from 30 mm Hg to 50 mm Hg (p < 0.05). The PVR returned to baseline as PCO2 was returned to 30 mm Hg. These data suggest that increased PVR induced by hypercarbic acidemia is not simply a result of the effects of cardiopulmonary bypass on the pulmonary circulation. Instead, we conclude that respiratory acid-base status is an important determinant of adult PVR. We believe these data may be helpful in the treatment of mechanically ventilated patients, since patients are at particular risk of having abnormalities develop in respiratory acid-base status.

Primary pulmonary hypertension. Vascular structure, morphometry, and responsiveness to vasodilator agents

The use of pharmacologic agents in the treatment of pulmonary hypertension has not proved to be uniformly successful or predictable. One possible reason for the vagaries in response is that the pulmonary vascular lesions are not consistent. We examined the relation between the structure of the pulmonary resistance vessels in unexplained (primary) pulmonary hypertension and the response to pulmonary vasodilators. Our study involved 19 patients with clinically unexplained pulmonary hypertension (mean pressure, 59 +/- 14 mm Hg). After characterizing them clinically and performing control hemodynamic measurements, we determined the acute effects of a series of vasodilator agents that have different mechanisms of action. In 16 patients, lung biopsy material was related to the hemodynamic studies; in nine patients, including six who had undergone open lung biopsy, the hemodynamic studies were related to the pathologic changes found at autopsy. Histologic specimens from all 19 patients were evaluated qualitatively and sorted into three subsets of hypertensive pulmonary arteriopathy: medial hypertrophy (with minimal intimal proliferation), arteriopathy with plexiform lesions (associated predominantly with concentric laminar intimal proliferation and fibrosis), and arteriopathy with microthrombotic lesions (associated predominantly with eccentric intimal proliferation and fibrosis). The 16 lung biopsies were also quantitated by morphometric techniques. Using a decrease in calculated pulmonary vascular resistance of more than 30% accompanied by a decrease in mean pulmonary arterial pressure of at least 10% to define vasodilation, only four patients were responders. The patients varied considerably in their responses to different vasodilator agents. Patients with similar clinical and hemodynamic profiles differed considerably with respect to the nature of their pulmonary vascular obstructive lesions and their responses to vasodilator agents. Qualitative histologic examination of lung tissue did not provide a basis for predicting how individual patients would respond to vasodilator agents. However, quantitative morphologic analysis of the initial open lung biopsy specimens did prove helpful in predicting acute responsiveness to vasodilator agents and the subsequent clinical course of these patients with unexplained (primary) pulmonary hypertension. An intimal area of more than 18% of the vascular cross-sectional area had an 85% predictive value for identifying the patients who did poorly during the first 36 months of follow-up.