Abstract
When the rapid detection of haemodynamic change is imperative, the ‘gold standard’ is direct arterial pressure monitoring. Other indications for invasive arterial pressure monitoring include severe underlying cardiovascular disease, the inability to obtain indirect measurements and the need for frequent blood sampling. While the radial artery is the most frequently used site, other commonly used arterial cannulation sites include the femoral, brachial, axillary and dorsalis pedis arteries. Complications of arterial cannulation include haemorrhage, thrombosis, vasospasm, distal ischaemia, dissection, infection, unintentional arterial drug administration, pseudoaneurysm and arteriovenous fistula formation.
Arterial Blood Pressure Monitoring
When the rapid detection of haemodynamic change is imperative, the ‘gold standard’ is direct arterial pressure monitoring. Other indications for invasive arterial pressure monitoring include severe underlying cardiovascular disease, the inability to obtain indirect measurements and the need for frequent blood sampling. While the radial artery is the most frequently used site, other commonly used arterial cannulation sites include the femoral, brachial, axillary and dorsalis pedis arteries. Complications of arterial cannulation include haemorrhage, thrombosis, vasospasm, distal ischaemia, dissection, infection, unintentional arterial drug administration, pseudoaneurysm and arteriovenous fistula formation.
The MAP is nearly constant throughout the arterial tree and provides the most accurate single measure of the pressure driving the blood flow to the organs. In contrast, the values for the systolic and diastolic BP vary throughout the arterial tree (Figure 30.1). As the monitoring site moves more distally, the arterial pressure waveform changes – the sharper systolic upstroke, the higher systolic peak, the delayed and less distinct dicrotic notch, more prominent diastolic wave and lower end-diastolic pressure (Figure 30.2). As a result, peripherally recorded arterial pressure waveforms have a wider pulse pressure than central aortic pressure. However, in contrast to the normal distal pulse pressure amplification seen in peripheral arterial pressure traces, pressure waveforms recorded during hypothermic CPB often underestimate both the systolic and the mean central aortic pressure.
Figure 30.1 The effect of monitoring site on arterial pressure waveform – aortic arch versus femoral artery. Although the mean pressure remains unchanged, the systolic and pulse pressure are both amplified as the monitoring site moves peripherally in the arterial tree.
Figure 30.2 Normal arterial BP waveform components include (1) systolic upstroke, (2) systolic peak pressure, (3) systolic decline, (4) dicrotic notch, (5) diastolic runoff and (6) end-diastolic pressure.
For accurate pressure measurements, the monitoring transducer should be referenced to the level of the heart, which is typically chosen to be the mid-axillary level. Patient or bed movement without adjustment of the transducer height yields inaccurate pressure values.
An underdamped system (Figure 30.3A) can lead to falsely high systolic, falsely low diastolic or an exaggerated pulse pressure. Conversely, an overdamped system (Figure 30.3B) will cause a slurred arterial systolic upstroke and narrowed pulse pressure, which result in an underestimation of the systolic pressure and an overestimation of the diastolic pressure. The most common causes are blood clots, air bubbles, compliant or kinked tubing, loose connections or low flush bag pressure.
(A) Underdamped arterial BP waveform. Note that the dicrotic notch follows the second systolic peak.
(B) Overdamped arterial BP waveform.
Central Venous Pressure Monitoring
The CVP, an estimate of the RA pressure, is dependent upon intravascular volume, RV function and venous tone. Common sites for central venous cannulation include the internal jugular veins (IJVs), the subclavian veins (SCVs) and the femoral veins. Complications include bleeding, haematoma, arterial puncture, nerve injury, arrhythmias, infection, pneumothorax, thrombosis, airway compromise, air embolism and cardiac tamponade.
Arrhythmias encountered while advancing the guidewire can be avoided by limiting its insertion depth to a point above the superior cavo-atrial junction (CAJ). The right IJV–CAJ distance is the shortest (16 cm) and the left SCV–CAJ is the longest (21 cm). Right SCV–CAJ and left IJV–CAJ distances are intermediate (18 and 19 cm, respectively). The use of the SCV is associated with the lowest infection rate and is preferred for long-term use. However, the SCV is not easily compressible and its use is relatively contraindicated in coagulopathic patients. Although the femoral veins are more compressible and associated with fewer mechanical complications, this site is associated with a greater infection risk. Multiple clinical studies have shown that the use of ultrasound decreases both the rate of failed cannulation and the incidence of complications.
Although the CVP is often used as a surrogate for intravascular volume, its measurement is influenced by patient positioning, positive-pressure ventilation, TV integrity, RV dysfunction, and pulmonary and pericardial disease. Consequently, trends in the CVP provide a better guide to fluid management compared with isolated absolute values.
Analysis of the RA pressure or CVP waveform reveals three systolic components (c-wave, x–descent, v-wave) and two diastolic components (y-descent, a-wave). The a-wave represents atrial contraction and the c-wave, closure of the TV at the beginning of systole. During the ensuing x-descent, rapid atrial filling begins as the ventricle contracts and ejects, resulting in the v-wave at the end of systole. As the TV opens during diastole, the CVP falls as blood rapidly flows into the RV during the y-descent. The blood flow from the vena cavae into the RA is maximal during the x- and y-pressure descents (Figure 30.4).
Several diagnostic clues to cardiac rhythm abnormalities can be gleaned from careful observation of the CVP waveform. In AF, the absence of synchronized atrial contraction eliminates the a-wave. Atrial flutter may produce saw-tooth waves on the pressure tracing, known as f-waves. Junctional rhythm, complete heart block and VT are all forms of atrioventricular dissociation in which cannon a-waves may be evident. These result from atrial contraction against a closed TV during ventricular systole.
The normal CVP (RA pressure) ranges between 0 mmHg and 8 mmHg, and it is typically lower than the PAWP. However, in cardiac tamponade, the CVP and PAWP are both markedly increased and of similar value, with diastolic right and left heart pressure equalization being a characteristic haemodynamic finding.
Pulmonary Artery Catheterization
The PAFC has been used to manage cardiac and critically ill patients since the 1970s. To date, however, no randomized trial or meta-analysis has been able to definitively demonstrate an improved patient outcome attributable to PAFC monitoring. Some studies even suggest that PAFC use increases complications, length of stay and hospital costs. The PAFC allows direct measurement of the RA, PA and PA wedge pressures. Additionally, the SvO2 and the RV CO can be measured (Figure 30.5).
