Weaning From Mechanical Ventilation Using Permissive Hypercarbia

The primary objective of this research is to assess the utility of permissive hypercarbia
(PH) in the weaning of patients with risk factors for difficult weaning from mechanical
ventilation. This pilot study seeks to validate the concept that PH will significantly
improve the ability of patients to be weaned from mechanical ventilation and change the
approach that physicians take to ventilator weaning for many patients.

Null Hypothesis: Patients with PH will not be weaned from mechanical ventilation more rapidly
than those not receiving PH.

Patients with significant lung disease fail weaning from mechanical ventilation either as a
result of an inability to oxygenate the blood or to adequately remove carbon dioxide. This
results in longer Intensive Care Unit stays and increased morbidity and mortality as these
patients have prolonged periods of mechanical ventilation. This study will focus on patients
who have failure to adequately remove carbon dioxide (ventilatory failure) and endeavors to
evaluate a method for enhanced carbon dioxide elimination by altering the acid-base status of
the patients utilizing permissive hypercarbia (PH). PH will be used specifically during
weaning from mechanical ventilation to enhance carbon dioxide elimination during this period.
PH has used by some intensivists for this purpose but has not been critically evaluated in a
clinical study.

Mechanical ventilation using PH maintains an increased CO2 with either partial or complete
metabolic compensation. Therefore, the blood pH is either normal or slightly reduced. In
patients who are adequately sedated, the reduced pH is well tolerated without respiratory
distress or air hunger. However, if PH is to be used in awake patients as proposed here, full
metabolic compensation would be required to avoid air hunger while being weaned from
ventilator support. The metabolic compensation is produced either as a result of a renal
compensation or, more typically in the ICU setting, iatrogenically. Metabolic alkalosis can
be produced through the administration of alkalinizing agents or as a result of gastric
suctioning or from contraction alkalosis resulting from the use of diuretics.

PH enhances CO2 elimination by increasing the concentration of CO2 in the exhaled gas. The
pulmonary capillary pCO2 is elevated and therefore so is the alveolar CO2 and the exhaled or
end-tidal CO2. Thus more CO2 is exhaled in an equal volume of exhaled gas. This increased CO2
concentration reduces the minute ventilation required to exhale the CO2 produced by the
patient. PH will decrease the minute ventilation requirement even in patients with an
increased dead space and will facilitate weaning these patients from mechanical ventilation
by the same mechanism as just described.

Inclusion Criteria:

- Adult (> 18 yo) patients in the intensive care units at the Hershey Medical Center who
are on mechanical ventilation

- and whose clinical course permits weaning from mechanical ventilation

- and are felt to have risk factors for difficult weaning. (Risk factors will include
patients with rib fractures, pulmonary contusions, obesity (BMI >30), or history of
COPD or other significant lung disease.)

- all patients will have failed (rapid shallow breathing index >105) a 30 minute
spontaneous breathing test.

Exclusion Criteria:

- Overt signs of sepsis including: a temperature > 38.5F, white blood cell count >
14000, systolic blood pressure < 100 mmHg,

- intracranial hypertension (ICP > 18 or based upon clinical exam and/or history),

- or pulmonary hypertension (by history or PAsys > 40 mmHg).

- or history of recent(past 6 mos)MI