Respiratory failure is essentially failure to oxygenate, to eliminate carbon dioxide or a combination of both due to inadequate pulmonary gas exchange. It can be acute or chronic.
Inadequate pulmonary gas exchange can occur due to various reasons:
Obstruction of airflow to the alveolus can occur. This could be due to upper airway obstruction, or bronchoconstriction (asthma or chronic obstructive pulmonary disease).
Inadequate alveolar ventilation occurs if the alveoli are filled with fluid (pulmonary oedema) or inflammatory exudate (pneumonia) or blood (alveolar haemorrhage). The alveoli can also collapse down completely due to atelectasis or a blockage of bronchus more proximally (e.g. mucus plug or tumour). The whole lung can collapse if air, fluid or blood is able to enter the pleural space to cause a pneumothorax, pleural effusion or haemothorax respectively.
Inadequate ventilation can also occur in otherwise normal lungs when there is failure to ‘breathe enough’. This hypoventilation can occur due to various drugs that can suppress the respiratory drive such as opioids, benzodiazepines, alcohol and other sedatives. Hypoventilation can also occur due to a stroke that affects the brain stem or hypercapnoea.
A pulmonary embolism can cause inadequate perfusion of the lung where the pulmonary circulation is obstructed by an embolus. In this circumstance you have adequate ventilation of the alveoli but no perfusion therefore no gas exchange can occur.
Respiratory failure can be defined as a PaO2 (partial pressure of oxygen in arterial blood) less than 8 kPa. The PaO2 and PaCO2 levels are obtained from an arterial blood gas (ABG is explained later), which should be done as part of your assessment of B.
There are two types of respiratory failure depending on the PaCO2.
PaO2 < 8 kPa but normal or reduced PaCO2 (normal range 4.7 - 6.0 kPa).
Hypoxic respiratory failure is very common in acutely ill patients, and is normally due to a V/Q (Ventilation/Perfusion) mismatch.
PaO2 < 8 kPa with raised PaCO2 (usually > 6.7 kPa).
Hypoxia is accompanied by hypercapnoea. Hypercapnoea occurs due to inadequate ventilation (hypoventilation), specifically minute ventilation (the amount of air inspired/expired in one minute). Respiratory depressant drugs such as opioids can cause hypoventilation. Type 2 respiratory failure is also often seen in acutely ill patients with chronic obstructive pulmonary disease (COPD) who suffer with inelastic alveoli and large areas of pulmonary dead space, which they cannot effectively ventilate.
Importantly, patients with respiratory failure from any cause who become fatigued or exhausted will start to breathe less efficiently and retain CO2. This is a sign that ventilation may need to be assisted (e.g. non-invasive or invasive ventilation).