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What other factors will influence weaning?
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Cardiovascular
– pulmonary edema due to left ventricular failure or volume overload
decreases lung compliance and will make weaning more difficult. When
mechanical ventilation is discontinued, significant physiological
changes occur which will influence cardiovascular performance: change
from positive pressure to negative pressure ventilation, reduced mean
intrathoracic pressure, increased preload and afterload. This may lead
to critical loading of myocardial fibers and provoke ischemia – failure
and edema.
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Gastroinestinal
– recurrent aspiration pneumonitis, ascites or abdominal wounds leading
to diaphgramatic splinting. Abdominal distension or hypertension, for
any reason (massive fluid resuscitation, surgical packs etc), will reduce
chest wall compliance and lead to failure to ventilate.
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Nutrition
-protein malnutrition leading to muscular atrophy, which affects the
diaphragm and intercostals.
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Acid base
– metabolic alkalosis, particularly due to use of diuretics reduces
respiratory drive. Conversely,
muscles perform poorly in an acidic environment.
Metabolic acidosis is caused by excessive amounts of measured anions
(chloride) or unmeasured anions (lactate - from hypoperfusion), ketones
and renal acids.
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Electrolytes–
hypophosphatemia, hypomagnesemia, hypokalemia, hypocalcemia: these all
affect muscular function and protein metabolism.
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Endocrine
– muscle weakness due to hypothyroidism or steroid induced myopathy.
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Oxygen delivery capacity
– the circulating hemoglobin concentration: anemia increases respiratory
drive and cardiac output in order to maintain oxygen delivery.
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Pain control
– it is very difficult to wean patients who are in pain, particularly
from upper abdominal or thoracic surgery or injuries. If a patient has a
flail chest, it may be necessary to insert a thoracic epidural prior to
extubation
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