A 60 year old man was electively admitted to the intensive care unit following a combined kidney pancreas transplant. Diabetes mellitus was the cause for his end stage renal failure. He was admitted for overnight HDU care, and was discharged the following day. He had delayed graft function thought to be related to a prolonged cold ischaemic time of the kidney. He would need dialysis until the function of the transplanted kidney improved. Four days later whilst on the ward he became hypotensive, became unconscious and suffered a cardiac arrest. He was successfully resuscitated and was readmitted to ICU.

In the ICU he required a blood transfusion as his haemoglobin level had dropped. He was taken to theatre for a re-laparotomy and a graft pancreatectomy was performed and all bleeding was stopped.

He continued to suffer from delayed kidney graft function and needed intermittent dialysis. After two days he was on minimal respiratory support and on sedation hold was deemed ready for extubation. He was extubated successfully and remained so for the next 12 hours. He then had an episode of bradycardia and had a markedly reduced cardiac output. He was re-intubated and stabilised. A temporary pacing wire was inserted to control potential episodes of bradycardia.

His condition remained stable over the next day and was again extubated. His oxygenation needs increased over the next 12 hours and was placed on non- invasive ventilation. This stabilised him over the next 12 hours but he suffered from retained secretions and was re-intubated. He then suffered with an ileus and had abdominal distension which complicated his respiratory function. He had a tracheostomy placed and remained on mechanical ventilation for 2 weeks. He was difficult to wean as he suffered set backs related to acute sputum retention and a ventilator associated pneumonia.

This patient had been extubated twice with some degree of morbidity associated with it as he had to be reintubated. It would also be reasonable to assume that this increased his length of stay on the ICU slowed down his ICU discharge. Deciding when to extubate a patient seems to be still a difficult decision to make in some cases and the experience of senior clinicians remains an important role.

For those who have not accumulated this level of clinical experience are there tools available to help them in deciding when and who could be weaned and extubated from mechanical ventilation?

James Day

Weaning from mechanical ventilation involves patient selection for who would be suitable; reduction of respiratory support; removal of an airway and satisfactory maintenance of spontaneous ventilation.

There are many publications describing putative weaning predictors but there has always been scepticism over their validity. This is practically demonstrated as no one method is used or at all despite description in the literature for the last 35 years. There has been an inability to produce reproducible results. Most predictors work as a screening test and most suffer from low specificity but reasonable sensitivity.

Traditional weaning parameters look at indicators of gas exchange and indices of chest mechanics or breathing pattern.

Gas exchange criteria include a PaO2 of 60 mmHg with a FiO2 of 0.4 and a PEEP of less than 5 cmH2O. Other parameters for oxygenation include PaO2/ PAO2 >0.35, A-a O2 <350 mmHg or a PaO2/ FiO2 >200 (1). The predictive values for these indices of a successful weaning trial are low.

Indices for chest mechanics during spontaneous ventilation include vital capacity greater than 10-15 ml/kg, tidal volume > 5ml/kg, respiratory rate < 35/min, minute ventilation < 10-15 L/min, maximum voluntary ventilation at least twice minute ventilation and maximal inspiratory mouth pressure -15 to -30 cmH2O.

These all suffer from poor specificity and positive predictive value. The idea of a clinical tool to select patients suitable for weaning and extubation remained appealing so groups tried to improve the function of weaning tests.

They did this by grouping variables or by seeking more physiological relevant correlates. The rapid shallow breathing index (RSBI) was developed by Yang and Tobin (2). They found that patients who failed weaning rapidly became tachypnoeic with reduced tidal volumes. They measured the respiratory rate 1 minute after discontinuation of ventilatory support compared to the tidal volume. The threshold was set at 105 breaths/min/L. The sensitivity was 97% and the specificity was 64%.

Pooled data from six studies showed similar sensitivity but lower specificity. It was found that sensitivity for RSBI was reduced as ventilation time increased (2) and specificity was influenced by the disease state (3). Specificity of RSBI in COPD patients was 65% while in patients with neurological disease or acute respiratory failure of varying aetiology the specificity was 26 and 28% respectively (3). RSBI has been reported to be different in different patient groups necessitating a threshold change dependent on patient population. Females and patients with smaller endotracheal tubes may need a higher threshold (4). Elderly may need a threshold of 130.

Other indexes have been devised such as the CROP index, airway occlusion pressure ratios and the occlusion pressure 100 milliseconds after airway occlusion. No one test seems to be a better predictor than another. Subsequent papers have suggested that various combinations of the above may improve predictive power.

New techniques include the oxygen cost of breathing which is the difference in oxygen consumption during spontaneous breathing and on mechanical ventilation. A difference of <30% is a promising indicator (5). The use of gastric tonometry to measure the change in intra-mural pH reflects diversion of blood from splanchnic circulation to supply increasing workload to respiratory muscles. A fall in pH of <7.3 or a fall of >0.09 during spontaneous breathing suggests likely weaning failure (6).

Lessons Learnt

There is some resistance to the increasing use of protocols in medicine to drive clinical decision making. It is felt that the judgment of an experienced clinician is better than an algorithm and is tailored for individual patients.

It seems unlikely that one screening test will satisfactorily produce accurate enough predictions to be widely used. The idea of combing different screening tools in parallel runs the risk of becoming increasing more complex and difficult to calculate. It is important to realise the in built errors in measurement can be magnified if many variables are involved.

The suggestions that RSBI could be used, but in a tailored fashion may have some merit. If it were used regularly as a first line screen it would mean that patients clearly able to wean would not be kept ventilated unnecessarily i.e. the false negative rate is low as sensitivity is high. As the specificity is low, i.e. false positive rate is high, if it were taken at face value then you potentially harm patients who would fail extubation.

RSBI could be used as an indicator to progress to spontaneous weaning trials and the results of these two tests in series improves the positive predictive value of successful weaning and extubation.

References:

1. Lessard MR, Brochard LJ. Weaning from ventilatory support. Clin Chest Med 1996; 17(3):475-489.
2. Yang KL, Tobin MJ. A prospective study of indexes predicting the outcome of trials of weaning from mechanical ventilation. N Eng J Med 1991; 324(21):1445-1450.
3. Vallerdu I, Calaf N, Subirana M, Net A, Benito S, Mancebo J. Clinical characteristics, respiratory functional parameters, and outcome of a two hour T piece trial in patients weaning from mechanical ventilation. Am J Resp Crit Care Med 1998; 158:1855-62.
4. Epstein SK, Ciubotaru RL. Influence of gender and endotracheal tube size on preextubation breathing pattern. Am J Respir Crit Care Med 1996; 154(6 Pt 1):1647-1652.
5. Miwa K, Mitsuoka M, Takamori S, hayashi A, Shirouzu K. contiuous monitoring of oxygen consumption in patients undergoing weaning from mechanical ventilation. Work of breathing: reliable predictor of weaning and extubation. Respiration 2003; 70:623-630.
6. Hurtado FJ, Beron M, Olivera W, et al. Gastric intramucosal pH and intraluminal PCO2 during weaning from mechanical ventilation. Crit Care Med 2001; 29:70-76.