Chapter 17: Vascular access devices: insertion and management
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Anatomy and physiology
Arteries are elastic, muscular blood vessels that carry oxygenated blood from the left side of the heart (Jarvis [227]). The large pressure generated by the heart and resistance in the arteries ensure that organs and tissues receive highly oxygenated blood (Scales [400]). The safest and most common peripheral sites for arterial cannulation are the radial artery or dorsalis pedis. If peripheral arterial cannulation is not possible, femoral cannulation is the central site of choice.
Arterial pressure waveforms
The arterial waveform has three separate components that reflect the cardiac cycle (Figure 17.43):
- upstroke: the positive deflection occurs at the peak of systole
- dicrotic notch: occurs with aortic valve closure and reflects the beginning of diastole
- end‐diastole: the lowest point of the waveform.
Figure 17.43 Normal arterial trace.
Peripheral artery waveforms differ from central arterial waveforms; their traces have a higher peak systolic pressure, a wider pulse pressure and a more prominent dicrotic notch, described as ‘distal pulse amplification’ (Azim and Saigal [15]). This is because peripheral arteries are smaller and less compliant than central arteries. Despite ongoing debate about whether central arterial cannulation (e.g. femoral) is superior to peripheral arterial cannulation when monitoring mean arterial blood pressure (MAP), it appears that there is no statistically significant evidence to corroborate this (Mignini et al. [316]).
In addition to the continuous arterial pressure readings, the shape of the arterial waveform can be very informative when assessing the haemodynamic status of a patient. The slope of the arterial upstroke gives an indication of myocardial contractility. Additionally, an exaggerated swing in systolic pressure is seen on the monitor in pulsus paradoxus, a term used when the difference in peak systolic pressure between inspiration and expiration is greater than 10 mmHg. This is caused by:
- pericardial disease, for example cardiac tamponade causing impeded ventricular filling
- exaggerated inspiration by the patient
- changes in intrathoracic pressure caused by lung pathology, such as asthma
- hypovolaemia, particularly when the patient has concurrent high airway pressures.
