Arterial, Central Venous, and Pulmonary Artery Catheters
Arterial Catheters
What Do They Offer?
The placement of an arterial catheter permits (1) reliable and continuous monitoring of arterial pressure and (2) repeated blood sampling. Analysis of the arterial pulse pressure curve may also have other applications, including assessment of fluid responsiveness and estimation of cardiac output. The appearance of arterial pressure waves will vary according to the site at which the artery is sampled. As the arterial pressure wave is conducted away from the heart, three effects are observed: The wave appears narrower; the dicrotic notch becomes smaller; and the perceived systolic and pulse pressures rise and the perceived diastolic pressure falls.
Arterial Pressure Measurement
The optimal range of arterial pressure depends on individual patient characteristics, on underlying diseases, and also on treatment. Hence, it is impossible to give an optimal range of arterial pressure that is applicable in all patients. When arterial pressure needs to be evaluated accurately, oscillometric measurements become unreliable,1 and insertion of an arterial catheter is indicated.
1. Hypotensive states associated with (a risk of) altered tissue perfusion. Hypotension that is resistant to fluid administration requires the administration of vasopressor agents, and invasive measurement of arterial pressure is then necessary to titrate this form of therapy. Norepinephrine is the vasopressor agent most commonly used in this setting. A mean arterial pressure (MAP) of 65 to 70 mm Hg is usually targeted, but this level must be adapted to the individual patient and the clinical scenario; in particular, elderly patients with atherosclerotic disease may require higher levels than younger individuals with normal arteries.
2. Intravenous vasodilator therapy. Vasodilating therapy (e.g., nitrates and hydralazine) is a mainstay in the management of heart failure, because it can increase cardiac output. Close monitoring of arterial pressure is essential to avoid excessive hypotension.
3. Severely hypertensive states. Extreme hypertension may result in organ impairment, especially of the brain and the heart. Sodium nitroprusside or calcium entry blockers usually are used to lower arterial pressure, and careful, accurate monitoring is essential to titrate the antihypertensive therapy.
4. Induction of hypertension. Hypertension is sometimes induced in patients with neurologic diseases. Severe cerebral edema with intracranial hypertension, in particular, requires vasopressor support to maintain cerebral perfusion pressure (the gradient between the MAP and the intracranial pressure); likewise, hypertension may be used to treat or prevent the development of vasospasm secondary to subarachnoid hemorrhage, as part of the so-called triple-H therapy (hypertension, hypervolemia, hemodilution). Norepinephrine usually is used for this purpose.
Fluid Responsiveness
Variations in arterial pressure during positive-pressure ventilation have been used as a measure of fluid responsiveness. The transient increases in intrathoracic pressure influence venous return in patients who are likely to respond to fluid administration. This fluctuation in ventricular filling will translate into fluctuations in arterial pressure a few beats later. Accordingly, the greater the degree of systolic arterial pressure, or pulse pressure, variation during the respiratory cycle, the greater will be the increase in cardiac output in response to fluid administration (Fig. 4.1). However, this observation is valid primarily in patients without spontaneous respiratory movements and without significant arrhythmias, and only when a sufficient tidal volume is applied.2,3
Cardiac Output Assessment
The pulse contour analysis also can serve to estimate cardiac output less invasively than with the pulmonary artery catheter (PAC). Because the only determinants of arterial pressure are the stroke volume and the resistance and compliance factors of the blood and arteries, analysis of the pulse contour trace can help to monitor cardiac output over time. This can be done with regular calibrations whenever changes in vascular tone or blood volume occur, or even in the absence of calibration. These measurements are still approximations, so further technological developments can be expected to improve accuracy.
Blood Sampling
The presence of an arterial catheter can greatly facilitate blood sampling, especially in terms of enabling easy access to the circulation for regular monitoring of blood gases, such as may be required in severe respiratory failure or with acute metabolic alterations. Sensors can measure blood gases continuously, but widespread use of such sensors is limited by their cost.
Access
For placement of arterial catheters, usually the radial artery is used. The femoral artery can be easily cannulated and gives a better signal, but presence of a femoral catheter interferes more with patient mobility and warrants concern about infection.4 Use of other sites, such as the brachial or the axillary artery or even the dorsalis pedis artery,5 can be considered. An important point to keep in mind is that the pulse pressure increases from the core to the periphery. In other words, the systolic pressure is overestimated in smaller arteries (Fig. 4.2). Hence, it may be better to rely more on mean values than on systolic or diastolic pressures.
Complications
The most feared complication with use of arterial catheters is ischemia. With any suspicion of ischemia, the catheter must be removed immediately. Allen’s test, to determine occlusive arterial lesions distal to the wrist, is unreliable and is no longer widely used. The accidental disconnection of arterial lines can be associated with severe hemorrhage and even exsanguination. Infectious complications are rare.
Central Venous Catheters
What Do They Offer?
The central venous catheter can facilitate fluid administration. It also allows measurement of the central venous pressure (CVP) and enables access to central venous blood for sampling.
Fluid Administration
The large-bore central venous catheter allows fluids to be administered fast and reliably in the presence of acute hemorrhage. Placement of a central catheter is therefore essential in patients with hemorrhage due to polytrauma or with other forms of acute bleeding. It also allows irritant or hypertonic fluids to be administered, such as parenteral solutions, potassium-enriched solutions, and some therapeutic agents. Central venous lines also can be convenient in patients who need prolonged intravenous therapy when peripheral venous access becomes problematic.
Measurement of Central Venous Pressure
CVP is identical to right atrial pressure (RAP) (in the absence of vena cava obstruction) and to right ventricular (RV) end-diastolic pressure (in the absence of tricuspid regurgitation). It is thus equivalent to the right-sided filling pressure. CVP is determined by the interaction of cardiac function and venous return, which is itself determined by the blood volume and the compliance characteristics of the venous system. Hence, an elevated CVP can reflect an increase in blood volume as well as an impairment in cardiac function. Because the CVP evaluates the right-sided filling pressures, CVP can be increased in the presence of pulmonary hypertension, even if left ventricular (LV) function is normal. The normal value in healthy persons is very low, not exceeding 5 mm Hg. Thus, the CVP value may not be much lower than normal in the presence of hypovolemia. In general, a CVP below 10 mm Hg can be considered to indicate that the patient is more likely to respond to fluid resuscitation, but exceptions to this rule exist. A high CVP suggests a certain blood volume but does not guarantee sufficient LV filling.
Clinically, CVP can be assessed by evaluation of the degree of jugular distention6 or liver enlargement. A single CVP measurement is not very useful and is not a good indicator of a positive response to fluids; an increase in CVP without a concurrent increase in cardiac output is not only useless but also harmful, because it will lead to increased edema formation.
Access to Blood in Superior Vena Cava
Measuring the central venous oxygen saturation (ScvO2) is a surrogate for measurement of the true mixed venous oxygen saturation (
). Although absolute values of ScvO2 are not identical to single 
values, trends in ScvO2 over time follow the same pattern as trends in 
, making ScvO2 a useful measure in patients who do not require an arterial catheter.7 ScvO2 can be obtained either intermittently (by withdrawal of blood samples) or continuously (with the use of a catheter equipped with fiberoptic fibers).
Access
The central venous catheter generally is introduced via the internal jugular vein; the subclavian vein also can be used, although the risk of pneumothorax may be somewhat higher with this route. Peripherally inserted central catheters can also be placed, via the cephalic vein, basilic vein, or brachial vein. Introduction of a femoral catheter through the abdominal inferior vena cava to the right atrium can also yield reliable CVP measurements.8 The use of femoral catheters, however, is associated with a greater risk of infections and thrombophlebitis.9
Complications
Complications of central venous catheterization are related primarily to puncture of the central vein: Hemothorax can be life-threatening, especially in the presence of severe respiratory failure. In the presence of unilateral pathology, the catheter must be introduced on the affected side. Arterial puncture resulting in a local hematoma is not uncommon, but hematoma formation usually is without major consequences. Bedside ultrasonography can help guide the introduction of the catheter into the vein. Excessive advancement of a long catheter in a small patient can result in arrhythmias; such arrhythmias have been described with advancement of the catheter tip into the right ventricle, but this problem can be identified by the presence of an RV trace on the monitor display.
Catheter-related infections constitute the major long-term complication. Adherence to basic hygiene guidelines can decrease the incidence of catheter-related sepsis. Triple-lumen catheters may be associated with a higher incidence of catheter-related infection,10 primarily as a result of increased catheter manipulation. The use of antimicrobial-coated catheters may decrease the risk of infections,11 but fears remain about the risks of development of resistant organisms.12 Routine replacement of catheters after 3 to 7 days is not recommended.13
Pulmonary Artery Catheters
What Do They Offer?
PACs allow collection of data on right atrial, pulmonary artery, and pulmonary artery occlusive pressures (Fig. 4.3); flow (cardiac output); and oxygenation (
).
Pressures
Right Atrial Pressure
As indicated earlier, the RAP is identical to the CVP in the vast majority of cases.
Pulmonary Artery Occlusion Pressure
When the balloon on the catheter is inflated, it causes an obstruction (becomes wedged) in a small branch of the pulmonary artery, interrupting the flow of blood locally (but blood flow continues normally in the rest of the pulmonary circulation), so that (assuming the absence of an abnormal obstacle) a continuous column of blood is present between the tip of the PAC and the left atrium. This pulmonary artery occlusion pressure (PAOP), or pulmonary artery wedge pressure (PAWP), generally reflects the left atrial pressure well. Nevertheless, a number of steps must be taken to ensure the adequacy of the measurement.
A first question is whether the PAOP reflects the pressure in the pulmonary veins and not the alveolar pressure. The tip of the catheter should be in a West zone III position, where a continuous column of blood exists between the catheter tip and the left atrium (Fig. 4.4). These considerations are less important with fluid optimization and with today’s lower positive end-expiratory pressures (PEEPs).
To exclude a possible influence of airway pressure on PAOP readings, the changes in PAOP can simply be compared with the changes in pulmonary artery pressure (PAP) during the respiratory cycle. If PAOP reflects the pressure within the pulmonary veins, these changes should be identical, because the pulmonary artery and vein should be subjected to identical changes in intrathoracic pressures. If, on the other hand, the catheter tip is not in a West zone III, the changes in PAOP will be more significant than the changes in PAP. In these latter conditions, either fluid administration or some reduction in the PEEP level may abolish the differences.
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