Normal Arterial Line Waveforms

Created on Mon, 06/29/2015 - 18:01
Last updated on Mon, 06/29/2015 - 18:03

The arterial pressure wave (which is what you see there) is a shockwave; it travels much faster than the actual blood which is ejected. It represents the impulse of left ventricular contraction, conducted though the aortic valve and vessels along a fluid column (of blood), then up a catheter, then up another fluid column (of hard tubing) and finally into your Wheatstone bridge transducer.

arterial pressure waveform

Systolic upstroke:

This is the ventricular ejection.
The slope of this segment has some vague relationship with the rate of flow through the aortic valve (probably more so when measured in the actual aorta). When its slope is slurred, there may be aortic stenosis.

Peak systolic pressure:

This is the maximum pressure generated during the systolic ejection.
Added to it is the reflected pressure from the rest of the vascular tree.
If the rest of the vascular tree is hardened and atheromatous, its poor compliance causes a powerful reflected wave, which when added to the systolic effort of the ventricle makes for a high peak systolic pressure.

The peak systolic pressure is what you bleed with. This is the pressure that blows the hemostatic thrombus plugs off the vessels you have so carefully cauterised, and stresses the wall of the fragile aneurysm.

Systolic decline

This is the rapid decline in arterial pressure as the ventricular contraction comes to an end. This decline is even more rapid when there is a left ventricular outflow tract obstruction (and systole comes to an abrupt halt before the left ventricle is finished with the ejection).

Mean Arterial Pressure (MAP)

MAP is roughly equivalent to the area under the arterial pressure curve, divided by the duration of the beat and averaged over several beats.
The MAP is what perfuses your organs, and what commands their blood flow autoregulation.

Dicrotic notch:

In perfect circumstances, when measured in the aorta, this notch is very sharp and it actually does represent the closing of the aortic valve. As you move further out

As mentioned below, the dicrotic notch position varies with the position of the arterial line.
A suspiciously low dicrotic notch could mean very poor vascular resistance, eg. in a situation like severe septic shock.

Diastolic runoff:

This is the rapid decline in arterial pressure as the ventricular contraction comes to an end.

End-diastolic pressure:

This is the pressure exerted by the vascular tree back upon the aortic valve.
Hardened non-compliant vessels will cause this pressure to be raised.
Soft vasoplegic vessels of a septic patient will offer little resistance, and the diastolic pressure will be lower.

A regurgitant aortic valve will cause this pressure to be lower than normal, because instead of meeting the aortic valve the pressure wave travels all the way though to the ventricle via the regurgitant jet.

The diastolic pressure is what fills your coronary arteries, and should not be ignored.

Pulse pressure:

A very widened pulse pressure suggests aortic regurgitation (as in diastole, the arterial pressure drops to fill the left ventricle though the regurgitating aortic valve)

A very narrow pulse pressure suggests cardiac tamponade, or any other sort of low output state (eg. severe cardiogenic shock, massive pulmonary embolism or tension pneumothorax)

Difference in waveforms according to site of insertion

Difference in arterial waveforms according to site of insertion

The further you get from the aorta,


                      • But, the MAP doesn't change very much.

                        This is because, from the aorta to the radial artery, there is little change in the resistance to flow.

                        MAP only really begins to change once you hit the arterioles.

                          • The taller the systolic peak (i.e. a higher systolic pressure)
                          • The further the dicrotic notch
                          • The lower the end-diastolic pressure (i.e. the wider the pulse pressure)
                          • The later the arrival of the pulse (its 60msec delayed in the radial artery)

This is called Distal systolic pulse amplification:

The systolic peak is steeper the further down the arterial tree you travel because of “reflected waves”.

That is to say, the narrowing and bifurcation of blood vessels reflects some of the pulse back at the aortic valve. As the resistance of the branching arterial tree increases, the pressure wave is reflected. The more resistant the tree (i.e. the more atheromatous the arteries) the more reflection there will be.

Thus the systolic blood pressure can increase by as much as 20mmHg by the time you get to the radial artery (as compared to the aorta).

In young people, this is a positive feature, as their relatively elastic vessels recoil slowly and the reflection wave is delayed, arriving to the aortic valve after it closes, and nourishing the coronary arteries

In the elderly, the reflection wave arrives early,during systole - adding to afterload and thus to the myocardial workload; while in diastole there may be no reflection wave, which means the coronary arteries miss out on its benefit.

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References

From Bersten and Soni's" Oh's Intensive Care Manual", 6th Edition; plus McGhee and Bridges Monitoring Arterial Blood Pressure: What You May Not Know (Crit Care Nurse April 1, 2002 vol. 22 no. 2 60-79 )

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