Lung

The lungs are a pair of spongy, air-filled organs of respiration situated in the chest cavity on either side of the heart, enclosed by a thin membrane called the pleura. The right lung has three lobes and the left lung has two (the left being slightly smaller to accommodate the heart). Their primary function is gas exchange: oxygen from inhaled air passes into the bloodstream and carbon dioxide from the blood passes into the airways to be exhaled.

Gas exchange occurs in approximately 300 to 500 million tiny air sacs called alveoli, which are surrounded by a dense network of pulmonary capillaries. The barrier between the alveolar air and the blood in these capillaries is extremely thin, allowing efficient diffusion of gases. Oxygenated blood returns to the left atrium via the pulmonary veins and is then pumped by the left ventricle to the body, while deoxygenated blood arrives from the right ventricle via the pulmonary artery. This circuit is the pulmonary circulation.

In the context of cardiac arrest and critical illness, the lungs are centrally important to management. Patients who are unconscious or in cardiac arrest require their airway to be opened and breathing to be supported, either by rescue breaths, a bag-valve-mask, or mechanical ventilation via an endotracheal tube. During targeted temperature management (TTM) in the intensive care unit, ventilator settings are adjusted to maintain normal oxygen and carbon dioxide levels, as both hypoxia and over-ventilation (causing low carbon dioxide) are harmful to the injured brain.

Complications affecting the lungs are common after prolonged critical illness. Aspiration pneumonia (infection from stomach contents entering the airways), acute respiratory distress syndrome (ARDS, a form of severe inflammatory lung injury), pneumothorax (collapsed lung, a risk of central line insertion and high ventilator pressures) and ventilator-associated pneumonia are all recognised post-arrest complications. Monitoring of oxygen levels, regular chest X-rays and careful ventilator management are central to intensive care after cardiac arrest.