Pulmonary Vascular Diseases

Abstract

Diseases of the pulmonary vasculature are a cause of increased pulmonary vascular resistance (PVR) in pulmonary embolism, chronic thromboembolic pulmonary hypertension (CTEPH), and pulmonary arterial hypertension or decreased PVR in pulmonary arteriovenous malformations on hereditary hemorrhagic telangiectasia, portal hypertension, or cavopulmonary anastomosis. All these conditions are associated with a decrease in both arterial PO2 and PCO2. Gas exchange in pulmonary vascular diseases with increased PVR is characterized by a shift of ventilation and perfusion to high ventilation-perfusion ratios, a mild to moderate increase in perfusion to low ventilation-perfusion ratios, and an increased physiologic dead space. Hypoxemia in these patients is essentially explained by altered ventilation-perfusion matching amplified by a decreased mixed venous PO2 caused by a low cardiac output. Hypocapnia is accounted for by hyperventilation, which is essentially related to an increased chemosensitivity. A cardiac shunt on a patent foramen ovale may be a cause of severe hypoxemia in a proportion of patients with pulmonary hypertension and an increase in right atrial pressure. Gas exchange in pulmonary arteriovenous malformations is characterized by variable degree of pulmonary shunting and/or diffusion-perfusion imbalance. Hypocapnia is caused by an increased ventilation in relation to an increased pulmonary blood flow with direct peripheral chemoreceptor stimulation by shunted mixed venous blood flow.

Vascular and right ventricular remodelling in chronic thromboembolic pulmonary hypertension

“…Measurements of ventilation/perfusion (V9/Q9) distributions using the multiple inert gas elimination technique in both conditions generally show: preserved matching of V9/Q9modes; a mild to moderately increased perfusion to lung units with lower than normal V9/Q9; no or minimal pulmonary shunting; and no diffusion limitation [68][69][70][71][72]. The mean V9/Q9in both CTEPH and PAH is shifted to higher V9/Q9in relation to hyperventilation, which decreases the efficiency of gas exchange and increases physiologic dead space [73]. Anatomic dead space, or inert gas dead space defined by a V9/Q9 .100, remains normal or near-normal, and V9/Q9distributions do not usually exhibit higher than normal V9/Q9modes.…”

Hypoxaemia during pregnancy: pulmonary arteriovenous dilatation as a likely cause

“…However, haemorrhagic hereditary telangiectasia causes a true shunt, and the first hyperoxia test performed in our patient excluded a true shunt. Although hepatopulmonary syndrome was excluded by definition in the absence of portal hypertension, a perfusion-diffusion defect, as found in hepatopulmonary syndrome, was the most likely cause of hypoxaemia in our patient [2,7]. The association of a marked increase in PA-aO 2 when breathing room air, together with the measurement of a small shunt effect when breathing 100% oxygen is the functional hallmark of this anomaly.…”

“…Numerous conditions are associated with a decrease of both PaO 2 and PaCO 2 [2]. Pulmonary embolism and pneumonia are frequent during pregnancy but were excluded here [3].…”

“…The fact that blood is perfused in fewer over-dilated alveolar-capillary units than normal also explains the low TLCO [5]. In most cases, an increase in alveolar oxygen pressure provides enough driving pressure to overcome this relative diffusion defect [2,8]. However, severe forms of pulmonary arteriovenous dilatation are poorly responsive to administration of 100% oxygen [9] .…”

The Role of Exercise Testing in the Modern Management of Pulmonary Arterial Hypertension

“…Severe exercise intolerance due to impairment of oxygen transport is a hallmark of pulmonary arterial hypertension (PAH) and the reasons for that are evident from inspection of the Fick equation. In subjects with PAH, maximum SV is severely reduced [1], peak predicted HR is often not reached [2], relative anaemia is a common finding [3] and desaturation on exercise is present in the majority of cases due to a combination of precipitate fall in mixed venous oxygen saturation (S v O 2 ) and ventilation-perfusion mismatch [4]. There are other factors which can in general limit exercise such as ventilatory limitation and peripheral muscle dysfunction but their contribution to impairment of exercise capacity in PAH is less relevant.…”