Ultrasonographic Evaluation of Lung and Heart in Predicting Successful Weaning in Mechanically Ventilated Neurosurgical Patients

Ultrasonography has become a ubiquitous feature of Intensive care nowadays, with its influence ranging from various diagnostics to various therapeutic interventions. It is readily available at the bedside and is non-invasive, making it an ideal tool in the hands of the intensivist. It has excellent safety profile, and hence can be performed repeatedly. These days it has become an indispensable tool in the intensive care units owing to its potential utility in the process of weaning a critically ill patient from mechanical ventilator support.

A neurosurgical patient is different from any other post-operative or critically ill patients. Their altered cerebral physiology, specific goals of therapy, varied response to usual management protocols, put them in a different league whole together. The primary aim of care for these patients is to detect and prevent any secondary neurological insult while supporting systemic and neurological homeostasis. Hypoxia and hypercarbia are factors which need to be absolutely avoided while caring for such patients. A good proportion of these patients will have respiratory instability, airway compromise and altered sensorium, which makes them prone to hypoxia and hypercarbia. To avoid these secondary insults to the neurological system, endotracheal intubation and mechanical ventilation is instituted in patients who are at high risk. Mechanical ventilation is continued until the patient is clinically stabilized and primary neurological damage has been taken care of. Subsequently the transition from control ventilation to spontaneous ventilation begins

The weaning process from mechanical ventilation involves sequential reduction of ventilatory parameters, assessment of readiness of the patient for extubation and when all these criterias are acceptable, then finally extubation. Daily, meticulous evaluation of clinical and neurological conditions and completion of spontaneous breathing trial (SBT) should be considered in order to recognize and facilitate the process of withdrawal of the mechanical ventilation. Extubation is considered as a success when the ventilator prosthesis is removed after the patient passed the SBT and there is no need for reinstitution of the MV in the next 48 hours. The entire process of weaning can be categorised as a six step process:

1. Taking care of the primary event
2. Deciding whether to start weaning
3. Assessing the readiness to wean
4. Spontaneous breathing trial (SBT)
5. Extubation
6. Assessment of probable reintubation6

Several parameters have been instituted for assessing the capability of weaning. These include: Rapid Shallow Breathing Index, which is the ratio of respiratory frequency to tidal volume (RSBI=f/VT), Pulmonary gas exchange (like: PaO2/FiO2, PaCO2), Vital Capacity (VC), Minute Ventilation and Static Compliance. Weaning may not always have a successful outcome. Difficult weaning may in fact be due to different or mixed etiologies, the diagnosis of which requires meticulous monitoring of various physiologic and objective parameters. Assessment of lung aeration by ultrasonography is rapidly gaining significance in weaning protocol. Apart from lung ultrasonography, the role of transthoracic echocardiography in successfully predicting weaning capability have been investigated in the recent times. Cardiac related weaning failure may be due to systolic LV dysfunction or isolated diastolic dysfunction. By this study we are trying to evaluate the scope of ultrasonography in detection of lung aeration and cardiac systolic and diastolic function in mechanically ventilated neurosurgical patients undergoing weaning; and whether they can be used as a good diagnostic tool to detect those who are likely to fail weaning in this specific subset of patient population.