Arterial Waveform Analysis

Information from the Arterial Waveform

From the measurements: - Heart rate - Systolic pressure - Diastolic pressure (coronary filling) - Mean arterial pressure (systemic perfusion) - Pulse pressure (high in AR, low in cardiac tamponade or cardiogenic shock) - Changes in amplitude associated with respiration (pulse pressure variation) - Slope of anacrotic limb associated with aortic stenosis

From the waveform shape: - Slope of anacrotic limb represents aortic valve and LVOT flow - Slurred wave in AS - Collapsing wave in AS - Rapid systolic decline in LVOTO - Bisferiens wave in HOCM - Low dicrotic notch in states with poor peripheral resistance - Position and quality of dicrotic notch as a reflection of the damping coefficient

Normal Arterial Waveform

The arterial waveform has three distinct components: 1. Systolic phase - rapid icnrease in pressure to a peak, then a rapid decline. Begins with AV opening and corresponds to LV ejection phase. 2. Dicrotic notch - widely believed to represent the closure of the aortic valve 3. Diastolic phase - run-off of blood into the peripheral circulation.

It is also separated into two limbs: anacrotic (upstroke) and dicrotic (downstroke).

The peak corresponds to the SBP. The trough corresponds to the DBP. The MAP is calculated from the AUC instead of directly from the SBP/DBP, and can vary even with identical SBP/DBP depending on the shape of the waveform:

Breaking down the pulse pressure waveform even more into its components:

The pulse pressure waveform has a delay from the beginning of systole on ECG (the R wave), probably about 1/4 of a second. This is due to the time it takes for depolarization, isovolumetric contraction, LV ejection, etc etc.

Systolic upstroke

This is ventricular ejection. Generated by the fast-moving LV ejection wave and corresponds to the peak aortic blood flow acceleration at the opening of the AV. The slope of this segment has some vague relationship with the rate of change in LV pressure and with the competence of the aortic valve. When the slope of this component is slurred, there may be aortic stenosis.

Peak systolic pressure

This is the maximum pressure in the central arteries, generated during the systolic ejection. The major contributions to this variable are the LV contraction, central arterial compliance and the reflected pressure wave.

The shape depends on the reflected waves coming back from terminal arterioles (high resistance vasculature) which augments the peak pressure and also can create an "anacrotic notch" where the reflected waves meets the initial forward wave. As one measures more distally, the effects of this are augmented (i.e. dorsalis pedis measures will have elevated peak pressures):

Systolic decline

This is the rapid decline in arterial pressure as the ventricular contraction comes to an end. The fall in pressure represents a period of time during which the efflux of blood from the central arterial compartment is faster than the influx of blood from the emptying left ventricle. 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).

Dicrotic notch

Widely believed to be the effect of the aortic valve closing (The valve closes and there is a sudden increase in pressure as the aortic blood volume suddenly discovers that it has nowhere else to go, apart from the peripheral circulation). It is generally believed that the peripheral dicrotic notch owes more of its shape to the vascular resistance of peripheral vessels than to the closing of the aortic valve.

Diastolic run-off

The diastolic run-off is the drop in pressure which occurs after the aortic valve has closed. There is no flow from the LV, but pressure does not drop suddenly - rather, it decreases gradually along an exponential curve. The reason for this is arterial "cushioning", or the reservoir effect of pumping blood into an elastic tube. This elastic recoil of large arteries contributes as much as 40% to the stroke volume.

Significant relationship to the age/vasuclar compliance of the individual. Waveform (a)  represents the radial waveform of a 25 year old person, (b) is 47 years old, and (c) is 80:

End-diastolic pressure

This is the pressure exerted by the vascular tree back upon the aortic valve. The diastolic pressure is what fills your coronary arteries, and should not be ignored.

Abnormal Arterial Waveforms

Hypertension and PVD

• steep systolic upstroke
• reflected wave with anacrotic notching

Aortic Stenosis

• decreased systolic upstroke slope
• possibly lowered pulse pressure + lowered systolic peak pressure
• incisura is los + dicrotic notch may attenuate

Aortic regurgitation

• typical belief is that there will be a sharp upstroke and a steep systolic decline
• pulse pressure will be wide
• "descendent" dicrotic notch

HOCM or dynamic LVOT

• this is supposed to produce a "bisferians pulse" characterized by a sudden midsystolic pressure drop

Arterial Line Damping

• The dynamic response of an arterial line system is tested using the "fast flush" test, where the transducer is briefly exposed to pressure straight from the counterpressure bag.
• When the fast flush abruptly ends, the transducer system oscillates at its natural frequency. This can be measured and assessed for adequacy. The time between oscillation "peaks" gives you the natural frequency of the system; i.e. a system with 50 msec between peaks has a natural frequency of 20Hz. The transducer system needs to have a natural frequency in excess of 24 Hz in order to resolve fine features of the arterial line trace (eg. dicrotic notch)

Square Wave Test

After a fast flush, the system returns to baseline and harmonically oscillates a few times. The bounces can help you determine the damping of the system. Regardless of damping however, the MAP should remain the same.

Note: the transducer needs to be levelled as well as zeroed to be properly calibrated: - Transducer height (levelling) – needs to be at level of right atrium (phlebostatic axis). For every 10cm below the phlebostatic axis the transducer will add 7.4mmHg of pressure and vice versa. - “Zero-ing” – important to ensure transducer zeroed. This ensures that the transducer references atmospheric pressure as zero.

Normal damping: - short time between oscillations (<20-30 ms) - 1-2 bounce oscillations - distint dicrotic notch (can be lost in some cardiovascular conditions)

Overdamping

• leads to underestimated systolic and overestimated diastolic pressure (tendency towards the mean)
• dicrotic notch is lost
• due to a blockage in the system (clot, air bubble, etc), loose connections, kinks in the tubing, arterial spasm, too narrow tubing

Underdamping

• leads to overestimated systolic and underestimated diastolic pressure (tendency away from the mean)
• die to catheter whip or artefact, stiff tubing, hypothermia, tachycardia or dysrhythmia

References

1. Normal arterial line waveforms | Deranged Physiology and Interpretation of abnormal arterial line waveforms | Deranged Physiology
2. Pressure Transducers And Arterial Line Waveforms - RK.md
3. Lamia B, Chemla D, Richard C, Teboul JL. Clinical review: Interpretation of arterial pressure wave in shock states. Crit Care. 2005;9(6):1-6. doi:10.1186/cc3891
4. Esper SA, Pinsky MR. Arterial waveform analysis. Best Practice & Research Clinical Anaesthesiology. 2014;28(4):363-380. doi:10.1016/j.bpa.2014.08.002
5. Arterial line dynamic response testing | Deranged Physiology
6. Arterial Lines – Intensive Care in a Flash