Orthopnea results from redistribution of fluid from the splanchnic circulation and lower extremities to the central circulation (Kasper 2015).
Heart failure
This extra volume of blood cannot be pumped on effectively with impaired left ventricular function. This results in an increase in pressure around the left ventricle, left atrium and pulmonary capillary occlusion. As a result:
- pulmonary edema develops
- the airway resistance increases
- lung compliance is reduced
- the pulmonary receptors are stimulated
- the vital capacity decreases
- occlusion of the small airways and so-called air trapping occurs
- Due to the altered distribution of ventilation and perfusion, there is:
- widening of the alveolar-arterial oxygen gradient
- Hypoxemia
- enlargement of the dead space
- The oedematous swelling of the bronchial wall causes an occlusion of the small airways and thus causes the typical wheezing of cardiac asthma.
- increasing limitation of expiratory airflow
- Increase in energy expenditure in the diaphragm to counteract the restrictive loads on the lungs (this must be overcome by the inspiratory muscles)
(Dennis 2019)
Mechanical impairment of the diaphragm.
When the diaphragm is mechanically impaired, lung dynamics are found to be favored by the upright posture (Kasper 2015).
Bilateral diaphragmatic paresis.
In bilateral diaphragmatic paresis, there is a cranial shift of the abdominal contents in the supine position. This causes the residual volume to decrease. Since the diaphragm cannot contract, the abdominal contents cannot shift caudally either (Dennis 2019).
Chronic obstructive pulmonary disease
In COPD, increased inspiratory effort occurs in the supine position due to increased airway resistance and intrinsic PEEP.
Premature airway closure, destruction of lung architecture, airway collapse at rest, and increased airway resistance limit expiratory airflow. Dynamic hyperinflation of the lungs develops as a result.
Intrinsic positive end-expiratory pressure thus occurs, requiring increased inspiratory work of breathing. The supine position increases flow restriction and airway resistance increases (Dennis 2019).