ablating seizures and treating hyperdynamic states such as sepsis, thyrotoxicosis
and helpful in modulating oxygen consumption.
II. Cardiorespiratory Interactions
When manipulating parameters of oxygen delivery, one should pay close
attention to the respiratory oxygen. The heart and lungs interact so closely
together that to only consider one system may prove detrimental to the patient as a
whole.
Let us consider each ventricle separately. The right atrium (RA) fills from the
superior vena cava and the inferior vena cava. Typically, the RA is passively filled
by blood and the filling is augmented by negative intrathoracic pressure. Therefore,
positive pressure ventilation decreases RV preload by decreasing the gradient
between the SVC/IVC and the RA. Increases in the mean airway pressure (MAP)
further decrease the pressure between systemic venous system and RA. In
addition, increases in MAP, may translate to increase in pulmonary venous
resistance (PVR) depending on the lung volume.
The left ventricle (LV) is also affected by positive pressure ventilation. (PPV)
The effects of PPV on the LV preload are directly derived on the preload delivered
to the right heart from the systemic vascular system. Therefore, PPV and higher
MAP can decrease LV preload. Interestingly, however, PPV can decrease LV
afterload. The positive intrathoracic pressure already present adds to total
pressure that is needed to be generated by the LV to generate a certain systemic