Discuss important heart-lung interactions during mechanical ventilation.
List indications for hemodynamic monitoring.
Describe the use of direct and derived hemodynamic measurements.
Describe the effect of positive pressure ventilation on hemodynamic measurements.
Describe how pulse pressure variation (PPV) can inform the response to fluid administration.
Invasive hemodynamic monitoring is commonly used with critically ill, mechanically ventilated patients. Because of the interactions between mechanical ventilation and hemodynamics, it is important that clinicians providing ventilatory support understand the basics of hemodynamic monitoring.
The heart and lungs share a common space in the thorax and, thus, are linked anatomically. With each breath, the lungs and thorax change both in volume and in pressure. These fluctuations affect cardiac function through changes in heart rate, preload, afterload, venous return, and contractility of the heart. Changes in intrapleural pressure can affect preload and afterload; positive pressure ventilation can decrease preload and afterload. Pulmonary vascular resistance (PVR) is dependent on lung volume; positive end-expiratory pressure (PEEP) may restore lung volume to the normal functional residual capacity (FRC) and decrease PVR. If applied in excess, however, PEEP can increase lung volume above FRC and increase the PVR. The descent of the diaphragm with respiration compresses the abdominal compartment and increases abdominal pressure, which increases the abdominal vascular pressures and increases the driving pressure for venous return. During positive pressure ventilation, the increase in abdominal pressure may partially compensate for the increase in right atrial pressure induced by the positive pressure. The application of PEEP can have complicated effects on the venous return depending on the change in abdominal pressure and the filling pressure of the ventricle. Ventricular interdependence occurs such that changes in the performance of one ventricle affects the other. The rise in right ventricular volume that accompanies an acute increase in the PVR will act to decrease LV compliance via effects on the ventricular septum.
Indications and complications for arterial and pulmonary artery catheters are listed in Table 29-1 and normal hemodynamic values are listed in Table 29-2.
Table 29-1Indications and Contraindications for Arterial and Venous Cannulation |Favorite Table|Download (.pdf) Table 29-1 Indications and Contraindications for Arterial and Venous Cannulation
• Indications: continuous blood pressure monitoring, frequent blood gases
• Complications: hemorrhage, infection, ischemia (embolus, thrombus, spasm)
Central venous catheter
• Indications: fluid administration, nutritional support, CVP measurements, central venous blood gases
• Complications: pneumothorax, embolus and thrombus formation, infection
Pulmonary artery cannulation
• Indications: PCWP measurements, cardiac output measurements, mixed venous blood gases
• Complications: pneumothorax, arrhythmias, embolus and thrombus formation, infection, cardiovascular injury
Table 29-2Normal Values for Direct Measured and Derived Hemodynamic Values