Pulmonary Artery Catheter Placement

Arterial Pressure Monitoring

Noninvasive measurement of blood pressure is one of the most widely undertaken procedures in clinical medicine. Invasive techniques are more commonly employed in intensive care patients for several reasons. Most importantly, the accuracy provided by intra-arterial lines is vital to assess the mean arterial pressure in critically ill patients when they are hemodynamically unstable. In addition, continuous surveillance of arterial pressure is of paramount importance when vasoactive agents are used. Furthermore, frequent noninvasive arterial pressure monitoring adds to the discomfort of the patient. Finally, an arterial line also permits frequent arterial blood gas estimations. Historically it has been relatively easy to measure pressure in the major peripheral arteries. Reliance has therefore been put on the maintenance of systemic pressure under the assumption that adequate pressure will also provide adequate flow and thus adequate tissue perfusion.

Studies in intensive care patients where the focus has been the maintenance of blood pressure have not been particularly fruitful. Hypotension is defined as a systolic pressure less than 90 mm Hg or a mean pressure less than 65 mm Hg. Most intensivists accept that pressure needs to be kept at a level that allows adequate tissue perfusion, particularly of the major organs, but that maintenance of blood flow through these organs is paramount.

Interpretation of the changes seen in the arterial waveform in relation to changes in intrathoracic pressure can now also give information about whether the patient is likely to respond to a fluid challenge ( Box 67-1-1 ). A greater than 10% or 12% variability of systolic pressure, pulse pressure, and/or stroke volume caused by the regular and consistent positive pressure associated with positive-pressure inspiration indicates that the patient is probably hypovolemic and is likely to respond to fluid resuscitation. It should be stressed, however, that this technique can only be used in sedated and ventilated patients in whom there is no spontaneous breathing. This is an important technological development because occult hypovolemia is probably not uncommon in critically ill patients and if unrecognized is likely to contribute to an increase in both morbidity and mortality.

Central Venous Pressure

Central venous pressure (CVP) is the intravascular pressure in the great thoracic veins, measured relative to atmospheric pressure. It is conventionally measured at the junction of the superior vena cava and the right atrium and provides an estimate of the right atrial pressure. The CVP is often used as a marker of volemic status or preload, although the ability of this measurement to provide this information is limited.

The CVP is influenced by the volume of blood in the central venous compartment and also the compliance of that compartment ( Box 67-1-2 ). Starling demonstrated the relationships between CVP and ventricular contraction, and Guyton the relationship between venous return and CVP. By plotting the two relationships on the same set of axes, it can be seen that the “ventricular function curve” and the “venous return curve” intersect at only one point, demonstrating that if all other factors remain constant in an individual patient, a given CVP can, at equilibrium, be associated with only one possible cardiac output ( Figure 67-1-1 ). Both curves can of course be affected by a number of factors: total blood volume and distribution of that blood volume between the different vascular compartments (determined by vascular tone). The inotropic state of the right ventricle will affect the shape of the ventricular function curve. When any one of these factors is altered, there will be an imbalance between cardiac output and venous return that will persist for a short time until a new equilibrium is reached at a new central venous blood volume and/or an altered central venous vascular tone.