Diastolic dYsfunctioN AssessMent in critICally Ill Patients

Bedside echocardiography plays a critical diagnostic role in patients with heart failure, being other examinations (i.e., physical examination, electrocardiogram, and chest radiograph) unable to provide key information to discriminate between diastolic and systolic heart failure. Transthoracic echocardiography (TTE) can assess diastolic function by means of the doppler filling pattern, and can non-invasively measure intracardiac pressure at the bedside.

During sinus rhythm, diastolic flow shows the E and A waves, which reflects early diastolic filling and atrial contraction in the late diastole, respectively. Velocity of blood flow across the mitral valve depends on the trans-mitral pressure gradient; then, E-wave velocity is influenced both by the rate of early diastolic relaxation and by the left atrial pressure. Changes of the velocity pattern can suggest left ventricular diastolic function (LVDD) and prognosis, although mitral inflow patterns are highly susceptible to loading conditions (mainly left atrial pressure). LVDD is quite is a condition quite common in the population. A cross-sectional survey of over 2000 randomly selected Minnesota residents aged 45 years or older found an incidence of LVDD almost five times higher than LV systolic dysfunction (28% vs. 6%, respectively), which was a strong predictor of mortality (hazard ratio ranging from 8.3 for mild LVDD to 10.2 for at-least-moderate LVDD).

In a randomly recruited population sample (n = 539; 50.5% women; mean age, 52.5 years), the prevalence of the LVDD in those patients older than 50 y.o. was of about 50%5.

Doppler echocardiography could be a noninvasive tool for the detection of weaning-induced left ventricular filling pressure elevation. Transmitral flow can help measure peak Doppler velocities of E and A waves. E/A ratio has been proposed to estimate the left ventricular filling pressure. Tissue Doppler imaging can measure early diastolic mitral annular velocity (E'), which is a load-independent indicator of myocardial relaxation. The combination of tissue Doppler imaging and pulsed Doppler transmitral flow can allow the computation of the E/E' ratio, which is one of the best echocardiographic estimate of left ventricular filling pressure.

The role of LVDD in critically ill patients is probably greatly underestimated by intensivists, probably because in the past diastolic function was difficult to evaluate at the bedside6. The LVDD in intensive care unit (ICU) patients can affect the outcome of weaning from mechanical ventilation, especially in septic patients. In fact, cardiogenic pulmonary edema has been recognized as a highly incident cause of weaning failure. Unsuccessful weaning from mechanical ventilation occurs in approximately 20% of patients and is related to prolonged mechanical ventilation, length of stay in the intensive care unit, and increased morbidity and mortality. It has been proved that a spontaneous breathing trial (SBT) increases the left ventricular filling pressure, leading to cardiogenic pulmonary edema and impaired gas exchange.

However, studies focused on the impact of that medical condition on weaning showed conflicting results. Moreover, heterogeneous definitions and measurements of LVDD have been provided, associated to different form of SBTs. Furthermore, the majority carried out a cross-sectional assessment of the LVDD before and after the SBT, not including changes in the diastolic function during the ICU stay. All these variables limit the comparability and the clinical applications of these studies.

Moreover, LV diastolic function depends on myocardial relaxation, LV stiffness, and filling pressures, which are frequently impaired during the ICU stay (e.g., progression of the disease, volume resuscitation, positive end-expiratory pressure, administration of inotropes, vasopressors, etc). It is still uncertain whether the SBT failure could be affected by pre-existing conditions (i.e. a LVDD present at the ICU admission) or by a worsening of the cardiac function after the ICU admission.

As a matter of fact, the assessment of LVDD at the bedside is not routinely performed before the SBT and the role of the degree of the LVDD in still not defined. Moreover, it unclear whether the outcome of the weaning process could be affected by a pre-existing LVDD (before ICU admission), or by the worsening of a chronic pattern, or by a de-novo LVDD presentation.