pressure measurement 3

8/3/2019 Pressure Measurement 3

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DR.N.VISWANATHAN


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` Recognizing the appearance of normal pressure

waveforms is a prerequisite to identifying

abnormalities that characterize certain

cardiovascular disorders` Forward pressure and flow waves, as seen in the

central aorta, are intrinsically identical in shape

and timing.


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` Forward pressure and flow waves, as seen in thecentral aorta, are intrinsically identical in shape andtiming.

` The pressure wave is modified by summation with a

reflected pressure wave (Pbackward), and the resultantmeasured central aortic pressure wave shows asteady increase throughout ejection .

` The flow wave is also modified by summation with areflected flow wave (Fbackward), but because flow isdirectional, Fbackward reduces the magnitude of flow in

late ejection, giving the characteristic Fmeasured as isseen with aortic flowmeters or Doppler signals


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` The reflections for pressure occur from many sites

within the arterial tree, but the major effective

reflection site in humans appears to be the region

of the terminal abdominal aorta.` Ascending aortic pressure is increased

substantially within one beat after bilateral

occlusion of the femoral arteries by external

manual compression


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` Ascending aortic (ASC Ao) pressure waveform in

a patient before and after bilateral occlusion of the

femoral arteries by external manual compression

(left arrow).` On the right, high-speed recordings show that the

major portion of the increase in pressure results

from augmentation of the late (reflected) wave. .


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` Factors that augment pressure wave reflections

Vasoconstriction

Heart failure

HypertensionAortic or iliofemoral obstruction

Valsalva maneuver after release


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` Vasodilation

Physiologic (e.g., fever)

Pharmacologic (e.g., nitroglycerin,

nitroprusside)Hypovolemia

Hypotension

Valsalva maneuvere strain phase


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` Pressure reflections are diminished during the

strain phase of the Valsalva maneuver with the

result that pressure and flow waveforms become

similar in appearance .

` After release of the Valsalva strain, reflected

waves return and are exaggerated.


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` The commonly noted late-peaking appearance ofcentral aortic and left ventricular pressure tracings inhumans referred to as the type A waveform pattern isa result of strong pressure reflections in late systole.

` In addition to the Valsalva maneuver, pressurereflections are diminished during hypovolemia,hypotension, and in response to a variety ofvasodilator agents .

` In these circumstances, the left ventricular and

central aortic pressure waves exhibit a type C pattern


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` A wedge pressure is obtained when an end-hole

catheter is positioned in a designated blood vessel

with its open end-hole facing a capillary bed, with

no connecting vessels conducting flow into oraway from the designated blood vessel between

the catheter tip and the capillary bed.

` A true wedge pressure can be measured only in

the absence of flow


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` In the absence of flow, pressure equilibrates

across the capillary bed so that the catheter tip

pressure is equal to that on the other side of the

capillary bed.


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` If minimal damping occurs between the cathetertip and the opposite side of the capillary bed thatis,

` 1.if there is a large, relatively dilated capillary bed,

` 2.if the precapillary arterioles and postcapillaryvenules are not constricted, and

` 3.if there is no other source of obstruction, such asthe presence of microthrombi

phasic as well as mean pressure may betransmitted to the wedged catheter.


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` Thus, an end-hole catheter wedged in a hepatic

vein may be used to measure portal venous

pressure.

` A catheter wedged in a distal pulmonary arterymeasures pulmonary venous pressure;


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In the absence of cor triatriatum or

obstruction to pulmonary venous outflow, the

pulmonary venous and left atrial pressures are

equal, so that pulmonary artery wedgepressure may be used as a substitute for left

atrial pressure


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` Some common sources of error and artifact in

clinical pressure measurement include

1.Deterioration in frequency response,

2.Catheter whip artifact,3. End pressure artifact,

4.Catheter impact artifact,

5.Systolic pressure amplification in the periphery,

and6.Errors in zero level, balancing, and calibration.


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` Although frequency response may be high and

damping optimal during setup of the transducers,

substantial deterioration in the characteristics can

develop during the course of a catheterization study.

` Air bubbles may be introduced into the catheters,

stopcocks, or tubing, or dissolved air may come out of

the saline solution used to fill the transducer (just as

dissolved air may come out of solution in a glass of

water allowed to stand unperturbed for a few hours).


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` Even the smallest air bubbles have a drastic effect

on pressure measurement because they cause

excessive damping and lower the natural

frequency (by serving as an added compliance).


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Flushing out the catheter, manifold, and

transducer dispels these small air bubbles and

restores the frequency response of the pressuremeasurement system.


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Motion of the tip of the catheter within the heart

and great vessels accelerates the fluid contained

within the catheter.

Such catheter whip artifacts may producesuperimposed waves of 10 mm Hg.

Catheter whip artifacts are particularly common in

tracings from the pulmonary arteries and are

difficult to avoid.


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Catheter impact artifact is similar but not

identical to catheter whip artifact. When a

fluid-filled catheter is hit (e.g., by valves in

the act of opening or closing or by the wallsof the ventricular chambers), a pressure

transient is created.


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Catheter impact artifacts are common with pigtail

catheters in the left ventricular chamber, where the

terminal pigtail may be hit by the mitral valve

leaflets as they open in early diastole.


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` When radial, brachial, or femoral arterial

pressures are measured and used to represent

aortic pressure, it must be remembered that peak

systolic pressure in these arteries may beconsiderably higher (e.g., by 20 to 50 mm Hg)

than peak systolic pressure in the central aorta ,

although mean arterial pressure will be the same

or slightly lower.


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` Pressure waveforms in a patient undergoing

cardiac catheterization, Is a function of distance

from the aortic valve (Ao V).

`

First vertical line marks onset of primary (forward)pressure wave, which occurs progressively later

after the QRS complex with increasing distance

from the aortic valve.

` Second vertical line marks onset of secondarypressure rise associated with the backward or

reflected pressure wave


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` The change in waveform of arterial pressure as it

travels away from the heart is largely a

consequence of reflected waves.

`

These waves, presumably reflected from the aorticbifurcation, arterial branch points, and small

peripheral vessels, reinforce the peak and trough

of the ante grade pressure waveform, causing

amplification of the peak systolic and pulsepressures .


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` The peripheral arterial systolic pressure may

commonly appear to be 20 mm Hg higher than the

left ventricular systolic pressure as a result of this

phenomenon.


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` Error in the quantitation of pressures because of

improper zero reference is common.

` As mentioned earlier, in many laboratories thezero reference point is taken at the midchest with

the patient supine, although some laboratories use

a point 10 cm vertically up from the back or 5 cm

vertically down from the sternal angle.


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` All manometers must be zeroed at the same point

, and the zero reference point should be changed

if the patient's position is changed during the

course of the study (e.g., if pillows are placed toprop up the patient).


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` All manometers must be zeroed at the same

point , and the zero reference point should

Be changed if the patient's position is

changed during the course of the study (e.g., ifpillows are placed to prop up the patient


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pressure measurement 3

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