Weaning / Discontinuation of  Mechanical Ventilation

What about this particular patient?

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Following this system it is possible to identify a number of reasons why there may be difficulty weaning this patient:

1.  CNS – he remains sedated with long acting agents – lorazepam and fentanyl (which tends to accumulate), these will both reduce levels of consciousness and impair central respiratory drive. These agents must be aggressively weaned.

2.  PNS – although he has only been ventilated for two weeks, he is doing little work himself, and may have some muscular atrophy. Moreover, the long duration of aminoglycoside therapy may cause some neuromuscular blocking effects. In addition, the combination of a low serum potassium, magnesium and phosphate does not augur well for muscular function. These need to be supplemented.

3.  CVS – the patchy infiltrates on CXR and the history of myocardial ischemia are worrisome, this indicates that this patient will not easily tolerate the autotransfusion associated with moving from positive to negative pressure ventilation. It is essential to image his heart with an echocardiogram, assess cardiac performance and consider the use of an agent that remodels the ventricle and reduced preload and afterload – an ACE inhibitor. I would be cautious in this circumstance with the history of renal failure. Alternative therapies would be the introduction of either nitrates or dobutamine in the hours peri-extubation.

4.   Renal – renal function is reasonably good now, although the high urea to creatinine ratio suggests over enthusiastic diuresis, confirmed with the metabolic alkalosis (which may also indicate sodium bicarbonate use – seen in the high serum sodium). This alkalosis can be corrected with judicious use of sodium chloride (the chloride will correct the alkalosis by returning to electro-neutrality) or increasing enteral free water delivery.

5. Gastrointestinal/Abdomen – a tense tight abdomen will interfere with diaphragmatic excursion, and thus respiratory mechanics. We have little control over this. It is worth asking, nonetheless, with a tense abdomen and nothing draining, if the drains are blocked. Does the patient have ascites? If so, it may be worth draining this to reduce intra-abdominal pressure.

6.   Extremities – the combination of peripheral edema and a low serum albumin does not make me feel confident about an early extubation, as the patient probably also has soggy lungs, from sepsis induced capillary leak (low oncotic pressure and fluid extravascation). There is little that can be done about this edema, the fact that it is resistant to diuretics is interesting. Has the patient been given adequate prophylaxis against deep venous thrombosis and pulmonary embolism?

7. Pulmonary function – the x-ray findings indicate a distinct mechanical disadvantage, patchy areas of consolidation (difficult to oxygenate) and a pleural effusion (difficult to ventilate). The effusion can be drained if necessary. I am concerned about a possible nosocomial pneumonia – I note a persistent leucocytosis, lung infiltrates and a low grade temperature. The patient has not been covered for pseudomonas or MRSA pneumonia, and it is essential to out rule this possibility by performing a broncho-alveolar lavage at this time. Is there any indication of infection on tracheal aspiration (mucopurulent sputum)? How long have the patient’s lines been in – is that the source?

This patient will probably tolerate a spontaneous mode of ventilation (such as pressure support) fairly well, although his electrolytes and acid base status require correction. If there is no movement towards minimal ventilator settings within 48 hours, a prolonged wean is probably likely (due to low physiological reserve) and the patient will require a tracheostomy.

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