VOL: 101, ISSUE: 25, PAGE NO: 55
Joanne Withers, RN, DipHE, wrote this article while a staff nurse in the cardiac critical care unit, North Staffordshire Royal Infirmary. She is now a senior staff nurse, cardiac services, Royal Wolverhampton NHS Trust, Wolverhampton
Mechanical ventilation is a process by which gases are moved into and out of the lungs by means of a ventilator, a machine that delivers a controlled flow of gas to a patient’s airway. The reasons for mechanical ventilation include respiratory arrest, acute lung injury, critical illness and respiratory support following surgery.
There is general agreement that weaning a patient off a ventilator quickly reduces the risk of nosocomial infection and improves mortality (Thorens et al, 1995; Marelich et al, 2000). Crocker (2002) notes that weaning time is directly linked to the length of stay in intensive care and hospital and to adverse events arising from ventilation.
Weaning is the process of reducing ventilator support so that patients can establish and maintain their own respirations (Price, 2001). Several methods of weaning have been suggested (Adams and Osbourne, 1997) and there is debate about which is the most effective (Witta, 1990; Esteban et al, 1995). Short-term weaning of cardiac patients following surgery is discussed below (Edwards and Hess, 1996).
The aim of a nurse-led weaning protocol
A nurse-led weaning protocol aims to reduce the number of hours that a patient is ventilated (Marelich et al, 2000; Durbin, 1996), a process that is recommended by the NHS Modernisation Agency Critical Care Programme (NHS Modernisation Agency, 2002). Some authors imply that it is the quality of nursing that is significant in the weaning process rather than the protocol itself (Norton, 2000; Cull and Inwood, 1999).
Simeone et al (2002) suggest that the ventilation period can be shortened using a weaning protocol for patients who are undergoing cardiopulmonary bypass surgery.
Using the nurse-led weaning protocol
The weaning process used at the North Staffordshire Royal Infirmary has general but not prescriptive phases to assist the nurse. The patient is weaned off the ventilator using synchronised intermittent mandatory ventilation (SIMV). The ventilator rate is reduced in steps of two to four breaths a minute, to allow the patient to begin to breathe spontaneously. Patients should be re-evaluated if they fail to breathe or if they cannot synchronise their breathing with the ventilator.
If patients are breathing well on SIMV, positive pressure support (PPS) ventilation can be used, which means they trigger all their breaths and are given variable levels of support by the ventilator. This is useful for patients who are difficult to wean; for example, those with chronic bronchitis. Ventilatory support is gradually reduced until the patient is ready for a continuous positive airway pressure circuit.
Mr Jones came to the intensive care unit following three coronary artery bypass grafts. Once his cardiovascular status was stable and he met the weaning criteria (Box 1) his sedation was stopped to allow him to wake and begin to make some spontaneous respiratory effort.
Initially, his ventilator was set on SIMV and used in conjunction with PPS. This mode requires the ventilator to deliver a pre-set number of breaths a minute. If the patient wishes to take a breath or number of breaths, the ventilator synchronises pre-set breaths with the patient’s spontaneous ones. Patients can also breathe spontaneously between ventilator breaths at their own rate and depth. If they fail to trigger the ventilator, a mandatory breath will be delivered by the machine. The aim is to increase patient comfort while gradually reducing the SIMV rate.
Evidence suggests that patients weaned on SIMV and PPS experience shorter weaning periods than those on SIMV alone owing to a reduction in the effort required to breathe and a reduction in muscle fatigue (Cull and Inwood, 1999).
For Mr Jones, the ventilator was pre-set initially to deliver 12 mandatory breaths a minute, an oxygen level of 40 per cent, a positive end-expiratory pressure (PEEP) of 5cm H2O, tidal volume (the volume of air breathed in and out in any one breath) between 6 and 8ml of gas per kg of patient weight (Jevon and Ewens, 2002), and a PPS of 10cm H2O. It is customary to have 5cm H2O of PEEP set as this substitutes for the loss of the physiological PEEP of normal breathing, and prevents the alveoli from collapsing at the end of expiration, thus improving oxygenation (Cull and Inwood, 1999).
Although Mr Jones was attempting to make some spontaneous effort, this was minimal, so the mandatory respiratory rate of ventilation was reduced to 10 breaths a minute to encourage him to increase his respiratory effort.
An arterial blood gas sample was taken 20 minutes after the ventilator setting was changed to ensure there was no deterioration in respiratory function. The respiratory rate was then reduced to eight breaths a minute to encourage Mr Jones’ own spontaneous effort. Inwood (2002) confirms that this method prevents weakening or atrophy of the pulmonary musculature.
As Mr Jones became more oriented and made more spontaneous effort, the ventilator was changed to the PPS mode, which meant that all the breaths were initiated by the patient and supplemented with PPS. This improved Mr Jones’ spontaneous tidal volume and reduced respiratory effort (Cull and Inwood, 1999).
It is thought that PPS ventilation helps a patient to breathe in synchronisation with the ventilator. However, there is evidence that desynchronisation may still occur in some instances with this method (Fabry et al, 1995).
When the weaning process was introduced for Mr Jones the PPS was set at 10cm H2O. Normally it is not lower than this, because at this point the positive pressure does not overcome the resistance/pressure of the ventilator tubing circuit (Cull and Inwood, 1999). If Mr Jones had initially struggled to maintain adequate tidal volumes, the PPS would have been increased to aid support and then reduced. However this was not necessary, as his tidal volumes were adequate.
Once Mr Jones’ arterial blood gas results were acceptable, and the extubation criteria were met (Box 2), he was extubated and given 40 per cent humidified oxygen via a facemask. Inwood (2002) recommends that the oxygen delivered following extubation should be equal to that delivered before the endotracheal tube is removed. Extubation was achieved within five hours of Mr Jones being returned from theatre.
The protocol used for weaning a postoperative patient provides clear guidelines that standardise the extubation plan and ensure consistency in patient care. It is clear that there are benefits from early extubation.