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Arterial Pressure Waveforms Section 3 Cardiovascular physiology Chapter 33 Arterial pressure waveforms The rate and character of the arterial pulse has been What is the arterial pressure wave? used for millennia for the diagnosis of a wide range of disorders. Perhaps more useful, however, is the direct Ejection of blood into the aorta generates both an cannulation of an artery, which allows quantitative arterial pressure wave and a blood flow wave. The information to be extracted. arterial pressure wave is caused by the distension of the elastic walls of the aorta during systole. The wave propagates down the arterial tree at a much faster rate What is the Windkessel effect? (around 4 m/s) than the mean aortic blood velocity During systole, the LV ejects around 70 mL of blood (20 cm/s). It is the arterial pressure wave that is felt as into the aorta (the SV). The elastic aortic walls expand the ‘radial pulse’, not the blood flow wave. to accommodate the SV, moderating the consequent increase in intra-aortic pressure from a DBP of Describethearterialpressurewaveform 80 mmHg to an SBP of 120 mmHg. The ejected blood possesses kinetic energy, whilst there is storage of for the aorta potential energy in the stretched aortic wall. In dia- Starting from end-diastole (Figure 33.1), the pressure stole, recoil of the aortic wall converts the stored generated by the LV ejects the SV into the aorta. The potential energy back into kinetic energy. This main- intra-aortic pressure rises to a peak value, the SBP, tains the onward flow of blood during diastole, and then falls to a trough, the DBP. The smooth thereby maintaining DBP; this is known as the ‘Wind- decent of the curve is interrupted at the dicrotic kessel effect’. This effect converts the sinusoidal pres- notch, when the aortic valve closes. sure wave generated in the heart into a positive and constant pressure at the tissues, much like converting How does the arterial pressure AC to DC electricity. With advancing age, there is degeneration of elastin in the wall of the aorta. The waveform differ at peripheral arteries? aortic wall becomes less compliant, and its ability to The morphology of the arterial pressure waveform accommodate SV without a large increase in pressure differs depending on where it is measured reduces. This accounts for the development of systolic (Figure 33.2). As the site of measurement moves more hypertension in the elderly. distally: SBP Systole Diastole Pressure increases as Figure 33.1 The arterial waveform. blood flows into the aorta 120 Aortic valve closes 100 Pressure falls as blood flows out of the aorta Pressure (mmHg) 80 Dicrotic notch DBP Aortic valve opens 60 0123 Time (s) Downloaded150 from https://www.cambridge.org/core. University College London (UCL), on 26 Dec 2017 at 06:18:44, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/CBO9781139226394.035 Chapter 33: Arterial pressure waveforms Figure 33.2 Arterial pressure waveform 120 Increasing SBP and steeper upstroke at different sites. 100 MAP 80 Pressure (mmHg) Decreasing DBP, dicrotic notch later and less sharp 60 Aorta Brachial Radial Femoral Dorsalis pedis Site of arterial waveform (a) Characteristic arterial pressure waveforms Reduced gradient Bifid waveform of upstroke 120 Steep downstroke 100 80 Pressure (mmHg) Reduced pulse Increased pulse Low dicrotic High dicrotic pressure pressure notch notch 60 ‘Normal’ Aortic stenosis Aortic regurgitation Low SVR High SVR (b) Respiratory swing with hypovolaemia and positive pressure ventilation Area 1 120 Area 2 100 80 Pressure (mmHg) Inspiratory phase Expiratory phase Inspiratory phase 60 051015 Time (s) Figure 33.3 (a) Characteristic changes of the arterial waveform; (b) variation through the respiratory cycle with positive pressure ventilation. The arterial upstroke is steeper and SBP is The morphology of the dicrotic notch changes: increased. – The dicrotic notch is positioned further down DBP is decreased. the pressure curve. Crucially, MAP is relatively constant wherever it is – Rather than being a sharp interruption in the measured; this is another reason why MAP is the pressure descent, the dicrotic notch becomes most important measure of blood pressure. more of a dicrotic wave. Downloaded from https://www.cambridge.org/core. University College London (UCL), on 26 Dec 2017 at 06:18:44, subject to the Cambridge Core terms of use, available151 at https://www.cambridge.org/core/terms. https://doi.org/10.1017/CBO9781139226394.035 Section 3: Cardiovascular physiology The change in shape and position of the dicrotic contractility but often has a pressure wave with a wave is due to it being caused by reflections of the bisferens appearance. arterial pressure wave rather than aortic valve SVR. The downstroke of the arterial pressure closure. waveform gives information about SVR: a steep downstroke with a low dicrotic notch indicates a low SVR – the arterial waveform looks thin and Can any other information be pointed (Figure 33.3a). Likewise, a high dicrotic gathered from the arterial pressure notch implies a high SVR. Hypovolaemia. In positive-pressure ventilated waveform? patients, a ‘respiratory swing’ in the arterial Although the arterial pressure waveform is often only pressure waveform is an indicator of used for measuring SBP, DBP, MAP and HR, it has hypovolaemia. There is beat-to-beat variation in many other clinical uses: the systolic pressure of the waveform, caused by Myocardial contractility. The slope of the the variation in preload throughout the waveform upstroke is a reflection of myocardial respiratory cycle (Figure 33.3b). contractility: an increased upstroke gradient Arterial pulse contour analysis.SVis suggests a greater pressure generated per unit proportional to the area under the systolic portion time. However, a reduced upstroke gradient is of the arterial pressure waveform; arterial pulse sometimes seen in aortic stenosis (Figure 33.3a). contour analysis allows calculation of the CO In contrast, aortic regurgitation is usually (see Chapter 28). SVV is calculated by dividing the associated with normal myocardial minimum SV (Area 2) by the maximum SV (Area 1). Further reading dysfunction. Circ Heart Fail 2010; Nephrol Dial Transplant 2010; 3(1): 149–56. 25(12): 3815–23. S. J. Denardo, R. Nandyala, G. L. Freeman, et al. Pulse wave analysis G. M. London, B. Pannier. Arterial B. Lamia, D. Chemla, C. Richard, et al. of the aortic pressure waveform in functions: how to interpret Interpretation of arterial pressure severe left ventricular systolic the complex physiology. wave in shock states. Crit Care 2005; 9(6): 601–6. Downloaded152 from https://www.cambridge.org/core. University College London (UCL), on 26 Dec 2017 at 06:18:44, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/CBO9781139226394.035.
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