Weaning frominvasive ventilation

The Big Asthma Lie

Natural Asthma Treatment Ebook

Get Instant Access

Weaning is the progressive reduction in respiratory support as respiratory failure improves. The respiratory muscles become deconditioned during prolonged ventilation so an abrupt withdrawal of ventilatory support is often unsuccessful. Weaning is appropriate when the underlying

20 10

20 10

Pressure control (no spontaneous effort)

Pressure control (no spontaneous effort)

20 10

Time

Pressure support (set to 10 and 0 cm H2O)

Pressure support (set to 10 and 0 cm H2O)

Spontaneous breath triggers ventilator

CPAP Expiration

SIMV

(spontaneous breaths in between)

Patient's spontaneous breath

BiPAP

Patient's spontaneous breath

BiPAP

Inspiratory and expiratory times can be adjusted (e.g. longer time allowed for expiration in wheeze)

Spontaneous breath triggers ventilator

Inspiratory and expiratory times can be adjusted (e.g. longer time allowed for expiration in wheeze)

Inspiration

Figure 4.4 Pressure waveforms of different ventilation modes (cm H2O)

cause of respiratory failure has been treated, other organ systems are stable, an appropriately low level of FiO2 is required and PEEP is 5 cm H2O or less. Some ventilatory modes are better than others during the weaning process, although limited evidence exists. There is usually a reduction in pressure support and a trial of breathing unsupported before extubation occurs. The rapid shallow breathing index has been studied extensively and is the best measurement to predict successful weaning. It measures the respiratory rate:tidal volume ratio during a 1-minute trial of breathing unsupported through a T-piece. Patients who develop more rapid and shallow breathing are unlikely to manage without ventilatory support. The cut-off figure is 105 - so a patient with 20 breaths per minute and tidal volumes of 0 5 litres has a rapid shallow breathing index of 40 and is likely to wean successfully, whereas a patient with 40 breaths per minute and tidal volumes of 0 25 litres has an index of 160 and is unlikely to manage unsupported.

Mini-tutorial: intubation in acute severe asthma

Intubation can be a life-saving intervention. Therefore it is important that it is performed early if indicated, particularly in acute severe asthma. A few minutes preparation beforehand is time well spent, particularly in those who are most unstable, as cardiovascular collapse can occur from uncorrected volume depletion, the abolition of catecholamine responses, and vasodilatation when general anaesthesia is given. Patients are volume loaded prior to intubation and a vasopressor (for example, ephedrine) is kept ready to treat hypotension. Anaesthetic drugs are given cautiously to minimise any vasodilatory effect and drugs that cause histamine release are avoided if possible. In life-threatening asthma, maximum medical therapy includes less commonly used bronchodilators, such as nebulised or subcutaneous adrenaline, intravenous salbutamol, magnesium sulphate, ketamine, and volatile anaesthetic agents. Once intubated, the patient is ventilated to allow a long expiratory time and this may mean only 6-8 breaths per minute is possible. "Permissive hypercapnia" is the term used when the PaCO2 is allowed to rise in such situations, in order to prevent "stacking". In asthma, if the next positive pressure is delivered before there has been enough time for expiration to occur, the lung volume slowly expands, reducing venous return and leading to a progressive fall in cardiac output. The resulting hypotension which occurs is corrected by disconnecting the ventilator and allowing passive expiration to occur (which can take several seconds).

Was this article helpful?

0 0
Coping with Asthma

Coping with Asthma

If you suffer with asthma, you will no doubt be familiar with the uncomfortable sensations as your bronchial tubes begin to narrow and your muscles around them start to tighten. A sticky mucus known as phlegm begins to produce and increase within your bronchial tubes and you begin to wheeze, cough and struggle to breathe.

Get My Free Ebook


Post a comment