Pressure Measurement 3
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Transcript of 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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