Summary by Erik Manninen, MD
Breathing is exercise:
*Spontaneous breathing can have such high metabolic demand that gut ischemia can even be precipitated. Weaning from mechanical ventilation is a cardiac stress test. With more negative esophageal pressures seen with deep inspiration this can cause an increase in venous return and increase in pulmonary artery occlusion pressure. If demands are too high for the patient’s cardiovascular reserve liberation from the ventilator will not be possible. In this situation you can see an increase in oxygen extraction (i.e., increased arterial-venous oxygen saturation difference)
Heart and Lungs Interact (i.e., Ventilation affects circulation):
*Hemodynamic changes are due to changes in intrathoracic pressure not lung volume.
*Hypoxic vasoconstriction causes an increase in pulmonary vascular resistance, which occurs with PAO2<60mmHg.
*As lung volume deviates from FRC (Functional residual capacity) the pulmonary vascular resistance (PVR) increases. Therefore, all forms of lung disease whether increased lung volume as in COPD or decreased as in ARDS the PVR increases. This occurs either through transpulmonary pressure exceeding pulmonary artery pressure and collapsing vessels with hyperinflation or by O2 extraction from collapsed terminal airways with low lung volumes resulting in hypoxic vasoconstriction.
*Hyperinflation can induce acute right heart failure. This is why the right answer is to disconnect the patient with dynamic hyperinflation from the ventilator, as it will results in a decrease in esophageal pressure and pulmonary artery occlusion pressure while the cardiac index and blood pressure will increase.
*PEEP can increase PVR and over distend the lungs or it can open up collapsed alveoli and decrease PVR. Goal of PEEP is to improve oxygenation without overdistention.
*Cardiac output and venous return graphs have an equilibrium point.
Abolishing profoundly negative swings in intrathoracic pressure is good for the heart:
*Negative swings in intrathoracic pressure increase LV afterload.
*Using positive pressure breaths can decrease these swings in intrathoracic pressure and can decrease stress in patients such as those with acute cardiogenic pulmonary edema.
Recommended reading:
Pinsky. Intensive Care Med 26:1164-6, 2000.
Buda, Pinsky et al. NEJM 301:453-9, 1979.
