Hemodynamic instability encompasses hypotension, low or (pathologically) high cardiac output, and abnormal filling pressures. Assessing hemodynamic instability is one of the most important indications for echocardiography in the operating room and the ICU.
Evaluation of hemodynamic instability should include assessment of (1) hemodynamic parameters (atrial pressures, pulmonary arterial pressure, and cardiac output), (2) preload and fluid responsiveness, (3) LV afterload, (4) ventricular function (LV systolic function, LV diastolic function, and RV function), (5) the presence of tamponade, (6) valvular function, and (7) LVOT obstruction.
A system for rapidly assessing hemodynamic instability in the ICU is provided in Chapter 18 .
Hemodynamic parameters
Right atrial pressure
RA pressure is usually measured directly via a central venous catheter. Only infrequently is echocardiographic assessment required during the perioperative period.
Motion of the atrial septum
In ventilated patients with normal atrial pressures, the atrial septum bows predominantly rightward throughout the cardiac cycle, with a brief reversal in midsystole during both inspiration and expiration ( Figure 19-1 ). Increased RA pressure, above LA pressure, causes leftward bowing of the atrial septum throughout the cardiac cycle. This sign is unreliable in the presence of severe mitral and tricuspid regurgitation, as these jets influence the motion of the atrial septum. The motion of the atrial septum is best appreciated in the apical (TTE) or midesophageal (TEE) four-chamber view.
Respiratory effects on the inferior vena cava
In spontaneously breathing patients, the expiratory diameter and the extent of inspiratory collapse of the IVC, measured from the subcostal view (TTE), can be used to estimate RA pressure ( Table 19-1 ). This technique requires patient cooperation, is not valid in the presence of positive pressure ventilation or respiratory distress, and has limited applicability in the perioperative period.
| Inferior Vena Cava Diameter in Expiration (cm) | Collapse During Inspiration (%) | Right Atrial Pressure (cm H 2 O) |
|---|---|---|
| <1.5 | 100 | <5 |
| 1.5–2.0 | >50 | 5–10 |
| >2.0 | >50 | 10–15 |
| >2.0 | <50 | 15–20 |
Tricuspid E/E′ ratio
With TTE, the ratio of the transtricuspid E wave to the tricuspid annular E′ wave (obtained using TDI of the lateral tricuspid annulus obtained from an apical four-chamber view) has been used to assess RA pressure in ventilated patients. This is analogous to the mitral E/E′ ratio used for assessing LA pressure; described later. A tricuspid E/E′ greater than 4 predicts an RA pressure greater than 10 mm Hg, with a sensitivity of 88% and a specificity of 85%. This measure is unreliable in patients early after cardiac surgery.
Pulmonary arterial pressure
Pulmonary arterial systolic pressure
Application of the simplified Bernoulli equation allows RV systolic pressure (P RVsystolic ) to be obtained from the peak tricuspid regurgitant jet velocity ( V TR ) and RA pressure (P RA ) (see Chapter 21 ):
P RVsystolic − P RA = 4 ( V TR ) 2 .
In the absence of pulmonary stenosis, pulmonary arterial systolic pressure (P PAsystolic ) is equivalent to P RVsystolic . Thus,
P PAsystolic = 4 ( V TR ) 2 + P RA .
The peak tricuspid regurgitant jet velocity can be obtained using CW Doppler from an apical (TTE) or midesophageal (TEE) four-chamber view. A peak tricuspid regurgitant velocity greater than 2.5 cm 2 yields systolic pulmonary arterial pressure of 25 mm Hg (+P RA ) and thus provides a useful cutoff for defining pulmonary hypertension. A further clue to the presence of pulmonary hypertension is a rapid peaking spectral Doppler waveform (<80 msec) obtained from the PA ( Figure 19-2 ).
Pulmonary arterial diastolic pressure
Pulmonary arterial diastolic pressure (P PA-end diastolic ) can be measured from the end-diastolic pulmonary regurgitation waveform ( V PRed ) using CW Doppler ( Figure 19-3 ). In the absence of pulmonary stenosis, at end diastole, the pressure difference between the PA (P PA-end diastolic ) and the right ventricle (P RV-diastolic ) is given by
P PA-end diastolic − P RV-diastolic = 4 ( V PR-end diastolic ) 2 .
In the absence of disease of the TV, at end diastole, the pressures in the right atrium and the right ventricle are the same. Thus,
P PA-end diastolic = 4 ( V PRend diastolic ) 2 + P RA .
With TTE, a pulmonary regurgitation waveform may be obtained from a parasternal AV short-axis view. With TEE, a pulmonary regurgitation waveform can sometimes be obtained from the upper esophageal aortic arch short-axis view (see Figure 13.2 ) or the transgastric RV inflow–outflow view (see Figure 13-3 ). However, for several reasons, the technique has limited clinical applicability during the perioperative period. First, pulmonary regurgitation is frequently not present in sufficient severity to obtain an adequate Doppler signal. Second, image quality and jet alignment with the Doppler beam are often poor, particularly with TEE. Third, pulmonary arterial diastolic pressure is most useful clinically as a surrogate for LA pressure, and LA pressure is more easily estimated by other means, as described later.
Left atrial pressure
Motion of the atrial septum
The normal motion of the atrial septum was described earlier (see Figure 19-1 ). Raised LA pressure is indicated by loss of the normal midsystolic reversal and exaggerated rightward displacement of the atrial septum. Loss of midsystolic reversal predicts LA pressure of 15 mm Hg, with a sensitivity of 89% and a specificity of 95%. This sign is unreliable in the presence of severe mitral and tricuspid regurgitation, as these jets influence the motion of the atrial septum.
Excessive motion and systolic buckling of the atrial septum indicate low LA pressure. However, care must be taken to not misdiagnose excessive septal motion as low LA pressure when the actual cause is an aneurysmal atrial septum (see Figure 6-10 ).
Peak mitral regurgitant jet velocity
Application of the simplified Bernoulli equation allows LA pressure (P LA ) to be obtained from the peak mitral regurgitant jet velocity ( V MR ) and the LV systolic pressure (P LVsystolic ) (see Chapter 21 ):
P LVsystolic − P LA = 4 ( V MR ) 2 .
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