Hemodynamic Instability
When a sudden, severe change in hemodynamics occurs, qualitative
estimates of LV filling and ejection serve as a practical guide for the administration
of fluids and inotropes. With some ultrasonographs, computer technology converts
the echocardiographic images from one cardiac cycle into digital code and then plays
this cycle repetitively to allow side-by-side comparison with other cardiac cycles
captured at other times during surgery. With this technology, an experienced observer
can consistently detect decreases in preload before they result in greater than a
10% decrease in blood pressure.[57]
This technology
also facilitates qualitative assessment of changes in LV ejection and prompt differential
diagnosis of the etiology of hypotension. For example, severe hypovolemia is easily
recognized as a marked decrease in LV end-diastolic area with a marked increase in
LV FAC, and LV failure is easily recognized as a marked increase in LV end-diastolic
area with a marked decrease in LV FAC. During hypotension, arterial vasodilatation,
aortic regurgitation, mitral regurgitation, and ventricular septal defects can manifest
the same LV filling and ejection pattern at the TG mid SAX cross section: adequate
LV end-diastolic filling area with an increased LV ejection fraction. Fortunately,
distinguishing these etiologies of hypotension is not difficult with the use of other
cross sections and color. Table 33-4
summarizes the most common causes of hypotension and their echocardiographic characteristics
at the TG mid SAX cross section.
TABLE 33-4 -- Origin of hypotension
*
|
EDA |
EF |
Cause |
|
↓↓ |
>0.8 |
Hypovolemia |
|
↑↑ |
<0.2 |
LV failure |
|
Normal |
>0.5 |
Low SVR or severe MR, AR, or VSD |
|
AR, aortic regurgitation; EDA, end-diastolic cross-sectional
area; EF, ejection fraction; LV, left ventricular; MR, mitral regurgitation; SVR,
systemic vascular resistance; VSD, ventricular septal defect. |
|
From Cahalan MK: Intraoperative Transesophageal Echocardiography:
An Interactive Text and Atlas. New York, Churchill Livingstone, 1996. |
*Obviously,
other problems can cause hypotension, but the ones listed here include the most common
occurring in the operating room. Two important caveats need to be mentioned. First,
low systemic vascular resistance and hypovolemia sometimes occur simultaneously (for
instance, in sepsis), and until hypovolemia is treated, low systemic vascular resistance
will not be apparent. Second, one cannot assume that just because the papillary
muscles meet during systole (i.e., LV cavity obliteration), the patient is hypovolemic.
This finding can occur in the setting of either low systemic vascular resistance
or hypovolemia. To make this differential diagnosis correctly, one must determine
whether diastolic filling is adequate.
Prospective studies have established the value of this TEE application,
sometimes called "rescue TEE."[58]
[59]
[60]
[61]
[62]
[63]
[64]
For example,
in 60 consecutive patients with severe, persistent hypotension after cardiac surgery,
TEE corrected the presumed diagnosis in almost half of these patients.[59]
In two patients, TEE prompted emergency surgery, and in five others it prevented
unnecessary reoperations. In another study, unstable cardiac surgical patients in
the operating room (n = 57) or intensive care unit
(n = 83) underwent emergency TEE.[59]
Based on the TEE findings alone, 22 of these patients had urgent surgical interventions.
The average time to diagnosis was 11 minutes. In critically ill surgical patients,
TEE is more cost-effective than transthoracic echocardiography because the latter
so often fails to reveal diagnostic images.[60]
Even in the setting of prolonged cardiopulmonary resuscitation, TEE may reveal crucial
diagnostic information.[61]
