Types of Respiratory Failure.
Type I or “Hypoxemic” Respiratory Failure.
Type I ARF is defined by PaO2 < 60 mm Hg, with normal or decreased PaCO2.
Causes:
1) inadequately oxygenated alveoli (due to low FiO2 and/or alveolar collapse and/or the presence of alveoli filled with fluid, cells, debris, or blood);
2) compromised transition of oxygen from the alveoli to the blood (due to interstitial processes or pulmonary vascular disease);
3) compromised ability of the blood to become oxygenated (due to obstructed blood flow, shunting, low Hgb concentration, or the presence of dysfunctional Hgb). The analysis of ABG values and calculation of the alveolar-arterial (A-a) gradient are important for the assessment of type I ARF.
Type II or “Hypercapnic” Respiratory Failure.
Type II ARF (PaCO2 > 45 mm Hg) represents the failure of the lungs to remove a sufficient amount of CO2 and is characterized by decreased alveolar minute ventilation. An increase in PaCO2 leads to hypoxemia because CO2 displaces O2 and effectively reduces the alveolar partial pressure of oxygen (PAO2). In contrast to some cases of type I ARF, hypoxemia in type II ARF is easily corrected with supplemental oxygen. This type of respiratory failure is frequently due to acute or chronic neuromuscular dysfunction or the inability of the airways or lungs to ensure adequate ventilation and CO2 exchange.
Type III or “Perioperative” Respiratory Failure.
Type III respiratory failure is synonymous with perioperative respiratory failure and is related to atelectasis of the lung. It is often a consequence of abnormal abdominal and chest wall mechanics in the setting of surgery or trauma, especially with intrapleural or subdiaphragmatic pathologies. The patient usually splints the chest to limit involuntary movement of the injured region, leading to inadequate expansion of the dependent parts of the lungs, with resultant regional atelectasis and hypoventilation. As a result, type III ARF shares features with both type I (hypoxemic) and type II (hypercapnic) ARF. This type of ARF can be prevented or ameliorated by certain anesthetic strategies as well as perioperative measures such as elevating the head of the bed, early ambulation, incentive spirometry, avoiding excessive sedation, and lowering intraabdominal pressure.
Type IV or “High-demand” Respiratory Failure.
Type IV respiratory failure is related to an inability of (normal or relatively normal lungs) to keep up with increased ventilatory demands associated with systemic hypermetabolism (e.g., secondary to sepsis). Under these conditions, respiratory muscle fatigue can lead to a requirement for mechanical ventilation (MV) to support adequate minute ventilation.