Accurate Arterial Line Analysis

Michael Watson 2008

UHNT ICU

• How it works

• Correct tracing

• Waveform anatomy

• Stroke volume variance

How it works &

is it accurate?

How it works

How it works• An arterial vessel is cannulated under aseptic conditions with a 20 guage cannula

using seldinger (guideline) technique. There are 2 types of cannula in the UHH & UHNT trust – flow switch or vygon (vygon cannulas are used for more difficult cannulation). The arterial catheter is connected to a 1000ml flush bag of NaCl and pressurised to 300mmHg (also required for KVO running @ 3ml/hr). At the mid point in the pressurised connecting tube there is a transducer point and this is connected directly to the patients monitoring device. The pressure transducer converts the patients arterial blood pressure oscillations into an electrical waveform that is readable on the monitoring device.

• The resulting arterial pressure wave differs depending on the site of vascular cannulation i.e. radial, femoral, etc. this is due to several factors including

– Fluid status– Vessel pathology– Cannulation quality (including thrombus, phlebitis and/ or vasospasm)– Reflection waves throughout the arterial tree (more evident in the more distal catheters).

• The following slide shows the effects of wave reflection/deflection on the systolic and diastolic arterial blood pressure. Note that the mean arterial blood pressure is fairly constant. Wave reflection/deflection occurs as the blood passes through the arterial tree under pressure. If all of the vessels were straight, and had no branches, then the flow of blood would be direct and the pressure at each end would be the same. However, as arteries are under constant pressure adjustment (musclular wall adjustment) and as they have bends and many branches, the flow of blood becomes less laminar – it becomes turbulent – and it’s this turbulance that causes the systolic pressure to rise and the diastolic pressure to fall slightly in the more distal arteries.

Arterial trace at different sites

Before we analyse the arterial waveform,

Always make sure that the gain on the monitor is correctly set!!!!

Is it accurate?

• Now we know how the arterial pressure monitoring system works, we need to be able to decide whether or not the trace (and BP in numerical format) is accurate. Failure to notice this may lead to unnecessary, or missed treatments for our patients.

• There are 2 main abnormal tracing problems that can occur once the monitor gain is set correctly.

• Dampened trace

Dampened: wide, slurred, flattened tracing

• Dampening occurs due to:– air bubles– overly compliant, distensible tubing– catheter kinks– clots– injection ports– low flush bag pressure or no fluid in the flush bag

• This type of trace UNDERESTIMATES blood pressure.

• Resonant trace

Resonant: ‘spiked’ tracing

• Resonance occurs due to:– long tubing– overly stiff, non-compliant tubing– increased vascular resistance– reverberations in tubing causing harmonics that

distort the trace (i.e. high systolic and low diastolic)– non-fully opened stopcock valve

• This type of trace OVERESTIMATES blood pressure.

If in doubt: NIBP!

Waveform Anatomy

End diastole - systole

• Anacrotic limb reaches from point (a) to point (b)

point (y) is

known as the

anacrotic

notch

y

a

b

Anacrotic limb

• The anacrotic limb represents the first phase of the arterial pulse cycle

• It occurs as the ventricles eject the blood into the arterial tree and gives a visual record of the arterial pressure rising to that of the end systole.

• The steepness of the ascending phase can be affected by heart rate, increased systemic vascular resistance, and through the use of vasopressors such as noradrenaline (more steep incline) and vasodilators such as GTN (less steep incline).

• Myocardial contractility also effects the steepness of the anacrotic limb – during impaired contractility (post MI for example) the up-sweep, or the rate of pressure increase can be prolonged (see next slide).

• As the pressure reaches maximum, and the wave makes sharp turn to level off, this is called the anacrotic notch

• The black line represents the rate of pressure increase

Anacrotic notch abnormality

• In some patients, a second systolic notch can be seen (as point (a) in the next slide).

• It is detected when aortic insufficiency exists in association with aortic stenosis, and may be found in hypertrophic obstructive cardiomyopathy.

• It may OVERESTIMATE systolic blood pressure slightly.

Bisferiens tracing

a

End systole – diastole

• Dicrotic limb reaches from point (b) to point (c)

b

c

Dicrotic limb

• Descending limb of the arterial pressure trace as the pressure falls to that of the end diastolic pressure

• Dicrotic means ‘twice beating’ – meaning that this phase of the arterial pressure pulse should have a second, smaller wave, known as the dicrotic notch

• Dicrotic notch is point (x)

x

Dicrotic notch

• Can occur at any point that there is a fluctuation in pressure during the descending arterial limb.

• The most common time for this to occur is when the aortic and pulmonary valves snap shut causing pressure reverberations through the arterial system – this is displayed visually on the next 2 slides (the first slide is by far the easiest!)..

Dicrotic pressure changes

Dicrotic pressure changes

this part here is the dicrotic notch

Dicrotic notch – continued..

• Flat or non-existent notch can mean that the patient is dehydrated (line trace will also ‘swing’)

• Low notch can also mean high pulse pressure (due to the low diastole in septic shock for example)

• Flattened notch can be present in cardiopulmonary valve insufficiency.

Dicrotic limb: 2

• The rate of dicrotic ‘fall-off’, or the rate at which the arterial line trace falls from end-systole to early-diastole changes in relation to systemic vascular resistance.

• In patients with a severely reduced arteriolar resistance, fall-off time is rapid. This occurs as soon as end-systole finishes due to the greatly reduced pressure in the arterial tree (representing reduced afterload). The arterial waveform in this clinical state looks thin and pointed (don’t confuse this with resonance).

• In patients with increased vascular resistance, such as main vessel stenosis for example, the dicrotic fall-off time is greatly increased. This occurs due to the length of time it takes to return to end-diastolic pressure. The arterial waveform in this clinical state may be normal, or quite fat!

If in doubt: NIBP!

Stroke volume variance

Stroke volume variance • The systolic blood pressure reading can vary from time to time – this is

known as ‘arterial line swing’ and occurs more in dehydrated patients. This phenomenon occurs during respiration – both spontaneous and mechanically ventilated, although is more pronounced during volume controlled mechanical ventilation (SIMV).

• During spontaneous respiration, at the beginning of inspiration, the intrarthoracic pressure briefly drops before building up and becoming much higher than before due to the expansion of the lungs. This increased pressure causes a reduction in transmural blood flow back to the heart by compressing the intrarthoracic veins (reduced pre-load) causing stroke volume to drop. As expiration begins, after a brief period of further increased pressure as the muscles contract, the pressure drops significantly as the air leaves the lungs greatly increasing transmural blood flow (increased pre-load) causing stroke volume to rise. This can also be noted in the systolic blood pressure figure as it fluctuates with respiration.

• During positive pressure ventilation there is always a reduced transmural blood flow – this is why blood pressure is always lower during mechanical ventilation. Stroke volume variance is much more pronounced in relation to higher VTi and peak airways pressures.

• Of the next 2 slides, slide 1 represents changes during spontaneous respiration and slide 2 during mechanical ventilation.

Arterial ‘swing’

• sv = stroke volume

Ventilated arterial ‘swing’

• (A) = the expiratory phase – or the period where the only pressure in the chest is due to PEEP.

• (B) = the inspiratory phase, ASB + PEEP

Pulse contour analysis: swing

• The use of pulse contour analysis is available in the ICU and this can help in the selection of vasoactive drugs and/ or fluids.

• LiDCO also analyses arterial trace stroke volume variance and translates this into a visual percentage to determine if a patient will be pre-load responsive or not.

• LiDCO IS USELESS IF THE ARTERIAL LINE TRACE IS INACCURATE – please remember this.

If in doubt: NIBP!

The end.

08.04.2008 – UHNT ICU