Left Atrial Pressure, Pulmonary Artery Wedge Pressure,
and Pulmonary Capillary Pressure
Direct left atrial pressure monitoring
is usually restricted to patients undergoing cardiac surgery. The most common technique
for monitoring left atrial pressure involves catheterization of the left atrium with
a thin catheter introduced through the right superior pulmonary vein and secured
with a purse-string suture. The catheter is brought out through the skin beneath
the xiphoid process and attached to a closed pressure monitoring tubing-transducer
system. Because left atrial monitoring is discontinued in the postoperative period
simply by withdrawing the left atrial catheter through the skin, trans-septal methods
for monitoring left atrial pressure during surgery have been proposed to reduce the
risk of bleeding after catheter removal.[432]
[433]
Left atrial pressure measurement has also been performed for diagnostic cardiac
catheterization by a retrograde arterial approach, trans-septal venous catheterization,
and even percutaneous direct left atrial puncture from a right paravertebral insertion
site.[434]
Today, direct left atrial pressure monitoring has largely been
supplanted by PAC monitoring. This has occurred, no doubt, because the PAC simultaneously
provides an estimation of left ventricular filling pressure as well as additional
useful monitoring information, including CVP and cardiac output. Although some authors
have shown modest discrepancies between left atrial pressure and PAWP in the period
immediately after cardiopulmonary bypass, wedge pressure generally provides an excellent
estimate of left atrial pressure in postoperative cardiac surgical patients.[426]
[435]
[436]
However,
a host of pathophysiologic conditions can alter the relationships
between pulmonary artery diastolic, wedge, and left atrial pressure, and these conditions
must be recognized to avoid misinterpretation of the data provided by these monitors
(see later).
Direct left atrial pressure monitoring continues to be particularly
useful in pediatric patients undergoing complex congenital heart surgery because
of the difficulty of using some of the standard percutaneous techniques in this patient
population. Gold and coauthors reported the use of more than 6000 transthoracic
left atrial and right atrial pressure monitoring catheters, with complications from
left-sided monitoring occurring in only 0.68% of 2393 catheters.[437]
However, direct access to the left heart chambers always carries the risk of air
and particulate embolization to the systemic circulation and thus requires close
attention to proper management by the entire team of caregivers to ensure that line
patency is maintained and flushing is performed with care.
The most common complication of direct left atrial pressure monitoring
actually results from catheter removal. Because of the risk of bleeding from the
cardiac site of catheter entry, the left atrial catheter must be removed before the
mediastinal portion of the chest drains, and if the catheter cannot be withdrawn
through the skin, surgical re-exploration is needed. Other rare complications of
left atrial pressure monitoring include entrapment of the catheter in mechanical
aortic or mitral valve prostheses,[437]
[438]
fistula formation between the right superior pulmonary vein and the right main stem
bronchus,[439]
and unrecognized retained catheter
fragments that serve as a source of systemic embolization.[440]
Normal left atrial pressure waveforms resemble CVP or right atrial
pressure waveforms, although there are a few subtle morphologic distinctions. Because
atrial depolarization originates in the sinoatrial node located at the junction of
the superior vena cava and the right atrium, the right-sided a wave appears slightly
earlier than the left-sided a wave ( Fig.
32-35
). Although the a wave is the most prominent pressure peak in a normal
CVP trace, the v wave is often taller than the a wave in a normal left
Figure 32-35
Normal temporal relationships between the electrocardiographic,
central venous pressure (CVP), and left atrial pressure (LAP) traces. The LAP and
CVP waveforms have nearly identical morphologies, although the CVP a wave slightly
precedes the LAP a wave. (Redrawn from Mark JB: Atlas of Cardiovascular
Monitoring. New York, Churchill Livingstone, 1998, Fig. 2-9.)
atrial pressure waveform, thus suggesting that right atrial contraction is generally
more forceful than left atrial contraction and that the left atrium is less distensible
than the right atrium during passive systolic filling.[136]
Finally, the interval between right atrial contraction and right ventricular contraction
is longer by approximately 40 milliseconds than the interval between left atrial
contraction and left ventricular contraction.[136]
Consequently, a and c waves are seen more often as separate waves in a right atrial
pressure trace than in a left atrial pressure trace. In normal left atrial pressure
waveforms, the a and c waves merge into a composite a-c wave, although most clinicians
simply describe this pressure peak as the a wave. Similarly, the PAWP waveform generally
displays only a and v waves because the wedge pressure c wave is obscured further
as a result of the damping effect of the lung vasculature on the transmitted left
atrial pressure waves.
The terms pulmonary artery wedge pressure
and pulmonary artery occlusion pressure are used
interchangeably and refer to the same measurement obtained from the tip of a PAC
after balloon inflation and flotation to the wedged position. As already discussed,
this pressure provides an indirect estimate of mean left atrial pressure. In contrast,
the hydrostatic pressure in the pulmonary capillaries that drives edema formation
according to the Starling equation is a different pressure that must exceed left
atrial pressure to maintain antegrade blood flow through the lungs. This pulmonary
capillary pressure must not be confused with wedge pressure or left atrial
pressure, nor should these terms be used interchangeably.[441]
Continued use of the phrase "pulmonary capillary wedge pressure" rather than pulmonary
artery wedge pressure or occlusion pressure has perpetuated misconceptions about
these measurements. Although the magnitude of the difference between pulmonary capillary
pressure and wedge pressure is generally small, it can increase markedly when resistance
to flow in the pulmonary veins is elevated.[442]
In most situations, the major component of pulmonary vascular resistance (PVR) occurs
at the precapillary, pulmonary arteriolar level. However, rare conditions such as
pulmonary veno-occlusive disease may cause a marked increase in postcapillary resistance
to flow within the pulmonary veins. A similar situation arises in other conditions
that disproportionately increase pulmonary venous resistance, such as central nervous
system injury, acute lung injury, hypovolemic shock, endotoxemia, and norepinephrine
infusion.[441]
[443]
Under these conditions, measurement of wedge pressure will underestimate pulmonary
capillary pressure substantially and thereby underestimate the risk of hydrostatic
pulmonary edema. Although pulmonary capillary pressure can be measured at the bedside
by analyzing the decay in the PAP trace after inflation of the PAC balloon, these
techniques have not been widely adopted in clinical practice.[444]
[445]
[446]
[447]
[448]
[449]
To
avoid
confusion, the phrase "pulmonary capillary wedge pressure" should be abandoned because
it is imprecise and misleading.
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