Traditionally, respiratory failure has been classified as hypoxemic respiratory failure with pO2 on room air less than 60 mm Hg or hypercapnic respiratory failure where pCO2 50 mm Hg or more with pH less than 7.35. This classification does not indicate the etiology of the respiratory failure and does not provide the physician with enough information that help in the management on mechanical ventilation.
In fact, the respiratory system maintains its functions by having an adequate neuromuscular competence (respiratory dive, transmission, and muscular strength) to provide the work of breathing (WOB) required to meet the resistive load, the elastic load (lung and chest wall), and the metabolic demand of the body.
With this understanding, we can look at the respiratory failure as an imbalance between the load (resistive, elastic, or metabolic) on one side and the competence of the neuromuscular system (respiratory dive, transmission, and muscular strength) on the other side. As long as the patient is able to provide the work of breathing needed for the required load, then the system remains in balance and the term impending respiratory failure is applied. Once the patient gets tired or the required load exceeds the patient capacity to provide the WOB then the neuromuscular competence will not be able to maintain the demand and the system will be imbalanced with development of hypoxic and/or hypercapnic respiratory failure.
The load can be resistive as it is the case in patients with asthma or upper airway obstruction, or elastic loaf as it is the case in ARDS, pulmonary edema, or pneumonia (it could also be from the chest wall or Intraabdominal such as pneumothorax or intraabdominal hypertension), or a high metabolic demand load as it is the case with sepsis or thyroid storm. Respiratory failure can also result from a reduced neuromuscular competence (with normal load) as in the case of toxic encephalopathy, CNS bleeding, myasthenia gravis, ALS, or myopathies.
Comments