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Objectives

  1. Discuss important heart-lung interactions during mechanical ventilation.

  2. List indications for hemodynamic monitoring.

  3. Describe the use of direct and derived hemodynamic measurements.

  4. Describe the effect of positive pressure ventilation on hemodynamic measurements.

  5. Describe how pulse pressure variation (PPV) can inform the response to fluid administration.

Introduction

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.

Heart-Lung Interactions

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. Thus, positive pressure ventilation can decrease preload and afterload through changes in pleural pressure. Pulmonary vascular resistance (PVR) is dependent on lung volume. Positive end-expiratory pressure (PEEP) may restore functional residual capacity (FRC) and decrease PVR. If applied in excess, however, PEEP can increase lung volume above FRC and increase 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 resulting from the positive pressure. However, as a result of ventricular interdependence, the rise in right ventricular volume that accompanies an acute increase in the PVR with the application of PEEP may decrease left ventricular compliance.

Hemodynamic Monitoring

Indications and complications for arterial and pulmonary artery catheters are listed in Table 30-1 and normal hemodynamic values are listed in Table 30-2.

Table 30-1Indications and Contraindications for Arterial and Venous Cannulation
Table 30-2Normal Values for Direct Measured and Derived Hemodynamic Values

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