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Choice of Venous Access Sites
Commonly, the right internal jugular vein is used for
insertion of a venous sheath through which the pulmonary
artery catheter is passed. Other sites can be advantageous,
depend-ing on the situation (Table 1). For a patient’s
initial catheteri-zation, use of the femoral veins for
catheterization may be preferred, because it allows the
greatest flexibility with which the clinician can perform
the most thorough evaluation. This is especially important
for excluding left heart pathology when direct measuremebnt
of left ventricular end diastolic pressure is necessary.
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| Table—Common
Venous Access Sites |
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| Site |
|
Advantages |
|
Disadvantages |
|
Complications |
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| Right internal jugular vein |
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Facilitates pulmonary artery access;
proximity to heart; may not need fluoroscopy |
|
Cutaneous access can be difficult |
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Hematoma, pneumothorax,
tracheal obstruction |
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| Left subclavian vein |
|
Facilitates pulmonary artery access;
proximity to heart |
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Vascular control of bleeding difficult |
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Pneumothorax, hemothorax |
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| Femoral veins |
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Easiest to cannulate; easiest for
vascular control of bleeding |
|
Most problematic for pulmonary artery
access; small
risk of infection; limits patient
mobility; fluoroscopy required |
|
Hematoma |
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Measurements to Record
Standard right-heart catheterization measurements (Figure
3) include:
- right atrial pressure (RAP)
- right ventricular pressure (RVP)
- pulmonary arterial pressure (PAP)
- pulmonary capillary wedge pressure (PCWP)
- systemic arterial pressure (BP) and heart rate
- cardiac output (CO)
- pulmonary arterial vasoreactivity
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- pulmonary arterial (PA) (“mixed venous”)
saturation
- superior vena cava (SVC) saturation*
- inferior vena cava (IVC) saturation* •
right atrial (RA) saturation*
- right ventricular (RV) saturation*
*When indicated.
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Normal pressure waveforms are shown in Figure 3. PCW
pressure measurements are made when the balloon of the
catheter is inflated after the catheter has been properly
advanced into the pulmonary artery. The inflated balloon
prevents the  measurement
of any pressure proximal to the balloon, and thus measurements
recorded from the tip of the catheter reflect only left
atrial pressure, which is commonly used as a surrogate
for left ventricular end diastolic pressure.
The PCW pressure tracing should display three waveforms:
the awave represents contraction of the left atrium. The
cwave is due to a rapid rise in the left ventricular pressure
in early systole, causing the mitral valve to bulge backward
into the left atrium, so that the atrial pressure increases
momen-tarily. The vwave is produced when blood enters
the left atrium during late systole, the time at which
most filling of the left atrium occurs.
Hemodynamic calculations– The following formulas
are used to calculate standard hemodynamic parameters
derived from the above measurements:
Mean* systemic arterial pressure (mBP) =
diastolic BP + (systolic-diastolic BP)/3
Mean* pulmonary arterial pressure (mPAP) =
diastolic PAP + (systolic-diastolic PAP)/3
Pulmonary vascular resistance (PVR) =
(mPAP-PCW pressure)/Cardiac output (CO)
Pulmonary vascular resistance index (PVRI) =
PVR/Body surface area (BSA)
Systemic vascular resistance (SVR) = (mBP-RAP)/CO
Systemic vascular resistance index (SVRI) = SVR/BSA
*Mean values may be more readily obtained
by taking readings from bedside electronic monitoring
equipment, which obviates the need for adjusting arithmetic
means for extreme heart rates.
Cardiac output measurements– There are two standard
methods for determining cardiac output. Both methods meas-ure
pulmonary blood flow, which in the absence of an intra-cardiac
shunt is equal to systemic blood flow.
The thermodilution method for determining cardiac output
uses the indicator dilution principle, where the indicator
is cold saline infused as a bolus injection into the proximal
port of the right-heart catheter. The thermistor at the
distal end of the catheter then measures the appearance
and disappear-ance of indicator over time, and a cardiac
output is then calculated. This method can be inaccurate
at very high or very low cardiac outputs, and can underestimate
cardiac output when significant valve regurgitation is
present.
When using this technique, the clinician must ensure
that the proximal right atrial port for injection is actually
in the right atrium, since the port can be in the right
ventricle when the catheter is wedged.
The Fick method for determining cardiac output is based
on the principle that consumption of a substance (oxygen
in this case) must equal blood flow to the organ multiplied
by the difference between the arterial and venous concentrations
of the substance. For this method, the formula for cardiac
output is as follows:
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CO =
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oxygen consumption per minute
(VO 2 )
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|
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(arterial oxygen content –
venous oxygen content)
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where oxygen content is calculated as: 1.34 x [Hb] x
oxygen saturation/100.
In this case, the oxygen consumption can either be esti-mated
or directly measured using standard techniques.6
Arterial oxygen saturation is usually determined by arterial
blood gas analysis, while venous oxygen saturation is
determined by mixed venous (pulmonary arterial) blood
gas analysis.
Note: In order to measure systemic arterial oxygen
saturation for determining cardiac output using the Fick
method, caution should be exercised when relying on pulse
oximetry, since both overestimation and underestimation
can lead to significant errors in cardiac output calculations.7,8,9
Additionally, pulse oximetry may not be reliable in
patients with Raynaud’s phenomenon, a common finding in
patients with PAH.
Shunt measurements– An abnormally high pulmonary
arterial saturation suggests a right-to-left shunt due
to congen-ital heart disease and requires further evaluation
and testing to identify and quantitate the shunt. Quantitation
of left-to-right and/or right-to-left shunting is an integral
part of right-heart catheterization.10 However,
these calculations are beyond the scope of this article.
Left heart catheterization – Left-heart catheterization
is not required in all patients with suspected PAH and
should be reserved for patients for the following diagnostic
purposes:
- validation of abnormal PCW pressure/evaluation of
left ventricular diastolic dysfunction
- suspected left-sided valvular lesion (mitral, aortic)
- suspected coronary artery disease
Some PAH specialists prefer to perform left-heart catheter-ization
in all patients with suspected PAH as part of their initial
(diagnostic) evaluation, to assure that the all-exclusive
workup of PAH is complete.
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