Cor pulmonale

Key words: acute cor pulmonale; adult respiratory distress syndrome; echocardiography; interventicular septum; massive pulmonary embolism; PEEP

Abbreviations: ACP=acute cor pulmonale; EDA=end-diastolic area; IVS=interventricular septum; LV=left ventricle PEEP =positive end-expiratory pressure; PFO=patent foremen ovale; RV=right ventricle

A cute cor pulmonale (ACP) may be described as a clinical setting in which the right ventricle (RV) is suddenly and exceedingly afterloaded. RV afterloading results from acute obstruction of the pulmonary vasculature by a potentially reversible process, involving the part of the pulmonary vascular bed that extends from the main pulmonary artery to the alveolar vessels. In this setting, impairment in RV ejection results in an increase in RV end-sytolic volume, and in turn, an increase in RV end-diastolic volume, and different mechanical and geometric consequences for the right heart cavities. Acute changes in right heart cavities also affect left heart cavities, particularly the left ventricle (LV), since it is enclosed in the same stiff pericardial space.

USEFULNESS OF BEDSIDE DOPPLER ECHOCARDIOGRAPHY FOR DIAGNOSING ACP

Over the past few years, this noninvasive technique has been used extensively in several ICUs, and thus makes it possible to clearly describe the Doppler echocardiographic pattern of ACP.

Pulmonary Arterial Hypertension

Pulmonary arterial hypertension is usually evidenced by Doppler flow velocity recording in the RV outflow tract. Whereas in a normal subject, Doppler velocity tracing from the pulmonary artery exhibits a peak ejection velocity occurring close to midejection, in a patient with significant pulmonary hypertension, the shape of the envelope resembles more a systemic curve with a shortened acceleration time. In addition, the ejection curve may acquire a biphasic configuration, with a midsystolic reduction in velocity (Fig 1). In most patients with ACP, RV systolic pressure (used as pulmonary artery systolic pressure) may also be measured using the tricuspid regurgitation signal recorded with continuous-wave Doppler. RV systolic pressure may be inferred because peak velocity of the regurgitant jet is proportional to the systolic pressure gradient between the RV and the right atrium (Fig 2). When the quality of the tricuspid regurgitant flow envelope does not allow an accurate measurement, it may be enhanced by saline solution injection.[1] In preagonic ACP, sometimes observed in patients with massive pulmonary embolism, pulmonary hypertension may be lacking because major RV failure with inefficient RV contraction is responsible for markedly low cardiac output.[2]

[Figure 1 to 2 ILLUSTRATION OMITTED]

RV Enlargement

RV enlargement is best evidenced on a parasternal short-axis or an apical four-chamber view. On the parasternal short-axis view, RV enlargement is associated with a leftward septal shift (Fig 3). On the apical four-chamber view, the RV appears enlarged and deformed, with the apical region losing its triangular shape for a more rounded shape (Fig 3). RV end-diastolic area (EDA) may be measured on this apical view, and the RVEDA/LVEDA ratio is normally lower than 0.6. A ratio ranging from 0.6 to 1 indicates mild RV dilatation, whereas a ratio from 1 to 2 denotes severe RV dilatation. We occasionally observed a ratio greater than 2 in the most severe forms of massive pulmonary embolism. In the apical four-chamber view, RV enlargement, is associated with a reduction in RV fractional artea contraction and right atrial enlargement.[3] Tricuspid regurgitation is usually present, easily detected by color-Doppler echocardiography. RV dilatation may be less apparent in some cases of ACP, when absolute or relative hypovolemia is present. A typical example of the latter condition is provided by RV changes occurring during positive end-expiratory pressure (PEEP) therapy: RV afterloading is associated with a reduction in venous return, which acts as a relative hypovolemia and precludes major RV enlargement.[4, 5]

[Figure 3 ILLUSTRATION OMITTED]

Septal Flattening and Paradoxic Septal Motion

Normally, the interventricular septum (IVS) is oriented and contracts such that it entirely forms a part of the left ventricle. On a LV short-axis view at the mitral valve or papillary muscle level, septal thickening during systole produces a regular motion toward the center of LV, whereas systolic thickening of the posterior LV wall produces a simultaneous and symmetric motion toward this center, resulting in a homogeneous and regular shortening of the LV cavity. During diastole, the sequence is reversed: the septum and the posterior wall move symmetrically away from the LV center, producing a homogeneous and regular enlargement of LV cavity. An M-mode recording of this sequence may be obtained from a parasternal short- or long-axis view. RV afterloading produces an abnormal and characteristic septal motion, due to changes in RV contraction that become stronger and longer than normal.[6] As a result, when LV starts relaxing at end-sytole/onset of diastole, RV contraction continues and thereby reverses the transseptal pressure gradient and causes the IVS to bulge toward the LV (Fig 4). During diastole, abnormal septal position is maintained, because of equalization or even reversal of diastolic pressures (Fig 4). However, at the onset of systole, the sudden increase in LV pressure produced by LV contraction restores the normal transseptal pressure gradient and pushes the IVS in the opposite direction, toward the RV cavity (Fig 4). As a result, a paradoxic septal motion is created, with the IVS and LV posterior wall moving parallel on an M-mode record.

[Figure 4 ILLUSTRATION OMITTED]

Diastolic LV Impairment

Because the two ventricles are enclosed within the relatively stiff pericardium, the sum of diastolic ventricular dimensions has to remain constant.[7] Thus, the acute RV dilatation observed in ACP can take place only if it is associated with an acute and proportional reduction in LV diastolic dimension (Fig 3). Septal displacement impairs LV relaxation and consequently, Doppler mitral flow is modified (Fig 5), so that the A wave is much higher than the E wave.[8]

[Figure 5 ILLUSTRATION OMITTED]

RV Hypertrophy

Using a parasternal approach, RV wall thickness can be measured by M-mode echocardiography in short- or long-axis views. Normal diastolic RV thickness is usually about 4 mm.[9] RV hypertrophy can be considered if RV thickness is greater than 6 mm.[9] Unlike patients with chronic respiratory disease with chronic cor pulmonale in whom RV hypertrophy is obvious, the lack of RV hypertrophy may be useful for the differential diagnosis. In chronic cor pulmonale, diastolic RV thickness averages 10 mm, approximating that of the LV wall, and this markedly hypertrophic RV may elevate the pulmonary artery pressure up to the level of systemic pressure, which contrasts with the moderate pulmonary hypertension observed in ACP.10 Moreover, RV hypertrophy is heterogeneous, usually more pronounced at the apex, which seems virtual, and on a short-axis view, the presence of marked intracavitary muscle trabeculations is highly suggestive. Severe RV hypertrophy is never observed in ACP, but mild RV hypertrophy is often present, with an increased RV thickness at about 6 mm, and clear visualization of intracavitary muscle trabeculations.

Airway Pressure and ACP

In the pulmonary vascular bed, intra-alveolar vessels are submitted externally to the pressure of distal airways, which is normally close to the atmospheric pressure (the zero reference level for vascular pressures) in an open airway. However, the pressure distending the distal airways may be increased either if the airway pressure becomes positive or if pleural pressure becomes markedly negative. Both conditions increase pulmonary vascular resistance and afterload the RV.

During mechanical ventilation, intermittent lung inflation is produced by an intermittent increase in airway pressure, resulting in an intermittent RV afterloading.[11-13] During expiration however, loading conditions exerted on the RV normalize.[11, 12] However, if a PEEP is added, airway pressure is continuously increased, and some RV afterloading persists during the expiratory phase.[4, 5] High PEEP level, which is no longer used in ICU, was in the past a privileged clinical setting to observe paradoxic septal motion and the resultant change in RV shape during acute RV afterloading:[5] on simultaneous RV and LV pressures recording during application of PEEP, RV peak systolic pressure was increased and delayed while LV peak systolic pressure decreased, allowing RV pressure to transiently exceed LV pressure at the onset of LV relaxation (Fig 6). This sudden reversal of the transseptal pressure gradient shifted the IVS toward the LV cavity (Fig 6), resulting in an acute change of the RV geometry, from a crescentic to a more circular shape.[5] During diastole, reduced LV filing and increased RV afterload with PEEP caused diastolic pressure equalization (Fig 6), and persistent IVS displacement resulted in an increase in RV diastolic volume without an increase in transmural diastolic pressure.5 At the onset of systole, LV pressure suddenly exceeded RV pressure (Fig 6), and the IVS returned toward the RV cavity (Fig 6).

During acute asthma, pleural pressure is deeply depressed during inspiration,[14] resulting in an increased RV load at this respiratory time.[14, 15] An intermittent and only inspiratory ACP is thus present during acute asthma.[14, 15] Inspiratory RV dilatation is still further increased by the mechanical effect on venous return of the negative swing in pleural pressure.[14, 15]

ACP and Microvascular Obstructions

Because ARDS causes microvascular obstruction in the lung,[16] some degree of RV afterloading had been evidenced as soon as 1977.[17] Using two-dimensional echocardiography, we described the typical features of ACP in ARDS patients.[18] In fact, this hemodynamic condition appears quite infrequently: during the last 10 years, we investigated 196 ARDS patients by two-dimensional echocardiography and found a definite pattern of ACP in 17 cases only. In the course of ARDS, ACP exceptionally occurred at the onset of respiratory failure,[19] but usually was delayed, after 7 to 15 days of aggressive respiratory support. It probably results from a progressive destruction of the lung by an excessive airway pressure or volume.[20] Since we use permissive hypercapnia to reduce barotrauma, we never observed delayed ACP in ARDS patients. In our experience, hypercarbia per se, despite elevated pulmonary artery pressure, does not induce an echocardiographic pattern of ACP.

ACP and Primary Lactic Acidosis

Pulmonary hypertension, increased pulmonary vascular resistance, and reduced cardiac output have been reported in primary lactic acidosis.[21] In the last 10 years, we observed 3 cases of ACP in a context of circulatory failure and primary lactic acidosis. Clinical presentation permitted a correct diagnosis in two cases, but in one patient, pulmonary angiography was necessary to rule out massive pulmonary embolism. It is noteworthy that circulatory failure in this setting does not appear directly related to a reduction in LV preload, as in massive pulmonary embolism, which is associated with an RV dilated to a greater extent and a small LV.

ACP and Massive Pulmonary Embolism

ACP is constant in massive pulmonary embolism (ie, embolism involving two or more lobar arteries), whereas it may be absent in submassive pulmonary embolism (ie, embolism involving less than two lobar arteries). During the last 10 years, we investigated 104 patients free of prior cardiopulmonary disease for suspected pulmonary embolism. ACP was diagnosed by echocardiography in 75 of them. Massive pulmonary embolism demonstrated by angiography was present in 70 patients, submassive pulmonary embolism in 4 patients, and a normal angiogram was found in 1 patient who had an episode of primary lactic acidosis. On the opposite, of the 29 patients who had normal echocardiographic studies, submassive pulmonary embolism was present at angiography in 5 of them, and the remaining 24 had a normal angiogram.

Direct visualization of the thrombus in the right heart cavities or in the pulmonary artery remained uncommon[22] until transesophageal echocardiography became widely used. In our experience, visualization of a thrombus in the main pulmonary artery or in the initial part of the right pulmonary artery is not rare (Fig 7).

[Figure 7 ILLUSTRATION OMITTED]

The severity of pulmonary vascular obstruction, assessed by angiography, is significantly correlated with the extent of RV enlargement and the decrease in RV fractional area contraction.[23] LV preload impairment also correlates with RV dilatation,[3] and systemic hemodynamic consequences of pulmonary embolism are directly related to the extent of vascular obstruction, indirectly reflected by the degree of RV enlargement.

Spontaneous fibrinolysis leads to a progressive reduction in right heart dilatation. Echocardiographic studies usually return to a normal pattern within 12 to 20 days.[3] A more rapid disappearance of echocardiographic abnormalities is achieved by using a thrombolytic agent. We observed echocardiographic normalization within 24 h after recombinant tissue plasminogen activator infusion in several patients with severe ACP. However, to our knowledge, thrombolysis agents, which may induce hemorrhagic complications, have never been shown to improve the final outcome.

ACP and Patient Foramen Ovale

Autopsy studies have shown that a patent foremen ovale (PFO) is found in approximately 25% of the population. This anatomic abnormality has no physiologic consequence in normal conditions, namely when the pressure in the left atrium is higher than in the right atrium. This transatrial pressure gradient maintains the valve of the foremen in the closed position.[24] However, in pathologic states in which right atrial pressure overcomes left atrial pressure, the foremen may reopen leading to right-to-left shunting[25, 26] which worsens hypoxemia in all settings associated with acute[27] or chronic[28] pulmonary hypertension. The most serious complication is paradoxic embolism in the systemic circulation.[29, 30]

Detection of a PFO requires contrast echocardiography (Fig 8), and has greatly been improved by transesophageal echocardiography[31] associated with maneuvers that transiently reverse the transatrial pressure gradient.[32, 33] Color Doppler echocardiography with the same approach may also be useful and easier done than contrast echocardiography.

[Figure 8 ILLUSTRATION OMITTED]

Conclusion

The classic clinical description of ACP has to be revisited and completed by a more modern approach, based on echocardiographic changes of the size and function of the RV and also of the LV. These echocardiographic features allow us to definitely assess the hemodynamic consequences of acute respiratory diseases, which in the past, were clinically evaluated or required an invasive procedure. The finding of a normal echocardiographic study can ruled out the diagnosis of ACP and confirm that pulmonary hypertension, if any, is well tolerated. Such information, which involves important prognostic implications, was not available until recently. However, when pulmonary embolism is suspected, a normal echocardiographic study does not permit us to exclude this diagnosis, because a small pulmonary embolism that is not associated with hemodynamic consequences may well precede a larger life-threatening embolism. Thus, diagnostic procedures such as angiography or ventilation-perfusion scanning should be performed even in the absence of ACP.

References

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(*) From the Respiratory Intensive Care Unit and the Department of Cardiology, Hospital Ambroise Pare, Boulogne Cedex, France. Manuscript received April 11, 1996; revision accepted May 31. Reprint requests: Dr. Jardin, Hopital Ambroise Pare, 9 avenue Charles de Gaulle, 92104, Boulogne Cedex, France

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