The routine management of mechanical ventilation in the ICU includes monitoring of peak airway pressures, plateau pressures and determining airway resistance.
When volume or pressure is pushed through an airway, a peak pressure is generated. This peak pressure is the sum of the amount of pressure necessary to get through the airways, inflate the alveoli and displace the chest wall and diaphragm. An inspiratory hold is performed on the ventilator to measure how much this pressure (plateau pressure) is actually being sensed in the alveoli once the lungs are inflated. By subtracting the plateau pressure from the peak pressure, we can calculate the resistance from the airways.
In managing mechanical ventilation, we routinely look at the plateau pressure to determine the limits to which we can increase our ventilating volumes. For the majority of patients, the chest wall and diaphragm are relatively compliant so are not a major factor in ability to ventilate patients. In cases of stiff chest wall or distended abdomens, the plateau pressure may be misleading as the pressure sensed within the alveoli is in part due to the pressures from the stiff chest wall or diaphragm.
Recently, esophageal catheters have been used to help optimize ventilation of patients with concerns re. stiff chest walls or diaphragms (distended abdomens). A catheter inserted in the esophagus is in close proximity to the pleural space. Esophageal pressures can be used as a surrogate to pleural pressures. Use of esophageal pressure monitoring can then help to differentiate between:
o pressure in the pleural space, attributable to chest wall and diaphragm and
o pressure distending the lungs (transpulmonary pressure) which might result in barotraumaPtpt (transpulmonary) = Paw (plateau) – Pes (esophageal)
