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Is the Patient able to Ventilate?
Alveolar ventilation is
adequate to keep the PaCO2 < 50 mmHg. The production of CO2 can be
controlled by reducing the carbon load in the diet (high fat), and
minimize agitation, pain, fever, shivering and muscle workload.
FACTORS
THAT MAY INTERFERE WITH WEANING
Neurological
Central:
prolonged sedation, with opioids (morphine, fentanyl, and benzodiazepines
(lorazepam, midazolam) reduces respiratory drive and prolongs ventilation.
The amount of sedatives used must be minimized and the patient wakened
daily. Psychological dependence on the ventilator follows prolonged usage,
and this needs to be addressed both holistically and pharmacologically.
A number
of factors will reduce central respiratory drive. As CO2 is the main
stimulus for ventilation, the patient’s PaCO2 must be returned to a level
normal for them. Metabolic or respiratory alkylosis reduces hydrogen ion
concentration in the brainstem, and thus the stimulus to breath.
For
weaning the patient must be awake and co-operative and able to protect
his/her airway.
Peripheral:
it is essential to rule out the possibility of a persistent neurological
injury such a phrenic nerve palsy, due to surgery. Neuromuscular blocking
(NMB) agents will prevent weaning, and it is important to ensure that full
reversal of blockade has taken place (by using a nerve stimulator). Some
drugs such as aminoglycosides can mimic NMBs.
Prolonged
critical illness may lead to the development of a critical illness
polyneuropathy, due to axonal degeneration.
Muscular
Muscular atrophy due to
malnutrition, prolonged muscle relaxants or critical illness myopathy may
limit weaning.
Anatomical Problems
Chest Wall – flail
chest: is the pain under control?
Does the patient have a
compliant chest wall?
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If the patient has to
work hard just to lift the chest wall - for example extensive edema,
large fat pads, tight dressings, increased abdominal pressure - due to
bowel swelling, packs, blood etc, then weaning will be very difficult.
Pleura – pleural
effusions – are they present, can they be drained? Does the patient have a
chest drain in – is there much coming in.
Airways – Is there any
form of reversible airway obstruction – mucus plugging, excessive
secretions or bronchospasm? Is there laryngeal edema – check for a cuff
leak.
Abdomen: does the patient
have a compliant (abdominal surface) chest wall. The presence of ascites,
distended bowel, abdominal hypertension, packs or tight surgical dressings
may interfere with ventilation.
Try to avoid being drawn into the "minute volume looks good
trap": the ability to ventilate is related to alveolar ventilation, not
minute ventilation. CLICK HERE FOR
INFORMATION
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