Assessment of Hemodynamic Changes During Weaning From Mechanical Ventilation With Flo-Trac Vigileo (TM) Monitor.

In the process of liberation or discontinuation from mechanical ventilation (also commonly referred as "weaning") the goal is to achieve spontaneous breathing and ultimately extubation. Weaning failure occurs in up to 25% of patients who meet criteria for weaning, especially in those with underlying COPD or heart failure (1). Weaning failure is well known to prolong intensive care unit (ICU) stay, and to increase morbidity and mortality in critically ill patients. Early identification and treatment of causes of weaning failure is of paramount importance to improve outcome in difficult to wean patients.

Although respiratory system failure is the most common cause of weaning failure, occult cardiovascular insufficiency occurs very often in critically ill patients, and it can contribute to, or be the only cause of weaning failure in many patients (2). Recognition of cardiovascular origin of weaning failure is crucial since the use of vasodilators and/or diuretics may result in successful weaning (1).

Cardiovascular instability during liberation from mechanical ventilation was first described more than 30 years ago by Beach et al (3) in post-operative cardiac surgery patients. Since then, a few other studies have described the hemodynamic changes both in patients who underwent successful and unsuccessful weaning form mechanical ventilation (1,4,5,6,7). Lemaire et al. (4) emphasized the use of a pulmonary artery catheter as the "gold standard" to detect left ventricular dysfunction during spontaneous breathing trial (SBT). They reported a markedly increased in pulmonary artery occlusion pressure (PAOP) in patients who were suffering from COPD or heart failure who failed to wean from mechanical wentilation. Jubran et al. (5) reported an increase in PAOP, decreased mixed venous oxygen saturation (SvO2) and an increase in the oxygen extraction ratio in a group of patients who failed to wean from mechanical ventilation. In contrast, De Backer et al. (6) described an increase in cardiac index in a group of patients who were successfully weaned after cardiac surgery. A recently published study by Frazier et al. (7) corroborated previous data by comparing the hemodynamic changes between a group of patients who were successfully weaned and a group of patients who failed to wean from mechanical ventilation. The cardiac output and stroke volume were estimated by the differential Fick partial rebreathing technique, with carbon dioxide as the indicator substance. The group of patients who were successfully weaned had a statistical significant increase in both stroke volume and cardiac index, unlike the group that failed weaning.

The physiologic changes that explain the above mentioned findings in hemodynamic parameters during weaning are complex and not fully understood (1). It is well known that weaning can place an excessive burden on the heart and it can be considered a form of "exercise" (2). As studied by exercise physiologists, the increase in oxygen consumption during exercise in healthy individuals is matched both by an increase in cardiac output and oxygen extraction. In patients with overt heart failure, the increase in cardiac output is inadequate, and the predominant response constitutes and increase in oxygen extraction. Return to spontaneous breathing induces alteration in intrathoracic pressures that influence cardiovascular function, producing, among other changes, an increase in venous return. Other postulated physiologic changes include increase sympathetic tone, weaning-induced myocardial ischemia and increased ventricular interdependence (2).

The recognition of cardiac origin of weaning failure is of vital importance. Unfortunately, the clinical signs of patients who fail weaning because of acute heart failure (tachypnea, tachycardia, anxiety, etc) are often difficult to distinguish from signs of respiratory failure. Reliable bedside minimally invasive monitoring tools and tests need to be developed to help the clinician in identifying these patients and help titrate cardiovascular therapies during weaning trials. Currently, the list of such tools is short and the validity of each technique in different situations is unknown (2).

Cardiovascular dysfunction may be the missing piece in predicting weaning success. Cardiovascular monitoring can be useful in identifying and managing patients who fail to wean due to lack of cardiac reserve.

We propose the use of Flo-Trac/VigileoTM, a semi-invasive technique that estimates cardiac output from the arterial pressure waveform, as a tool for cardiac monitoring during the process of weaning from mechanical ventilation.

References:

1. Richard C and Teboul JL. Weaning failure from cardiovascular origin. Intensive Care Med 2005;31:1605-1607
2. Pinsky MR. Breathing as exercise. Intensive care Med 2000;26:1164-1166
3. Beach T, Millen E, Grenvik A. Hemodynamic response to discontinuation from mechanical ventilation. Crit Care med 1973;1:85-90
4. Lemaire F, Teboul JL, Cinotti L, Giotto G, Abrouk F, Steg G, Macquin-Mavier I and Zapol W. Acute left ventricular dysfunction during unsuccessful weaning from mechanical ventilation. Anesthesiology 1998;69:171-179
5. Jubran A, Mathru M, dries D and Tobin MJ. Continuous recordings from Mixed Venous Oxygen Saturation during Weaning from Mechanical Ventilation and the Ramifications Thereof. Am J Respir Crit Care Med 1998:158:1763-1769
6. De Backer D, El Haddad P, Preiser JC and Vincent JL. Hemodynamic responses to successful weaning from mechanical ventilation after cardiovascular surgery. Intensive Care Med 2000;26:1201-1206
7. Frazier SK et al. Hemodynamic changes during discontinuation of mechanical ventilation in medical intensive care unit patients. Am J Crit Care 2006;15:580-593