Biomedical Engineering Reference
In-Depth Information
tion in patients with many forms of acute respiratory failure
24
. For instance,
in patients with acute exacerbation of chronic obstructive pulmonary
disease (COPD) and hypercapnic respiratory failure, adding noninvasive
ventilation to standard therapy could decrease the need for endotracheal
intubation
9;12;8
, and reduce mortality
8
. In mechanically ventilated patients,
endotracheal intubation is the single most important predisposing factor
for developing nosocomial bacterial pneumonia and infections
13;17
and in-
creases the risk for sinusitis. Endotracheal intubation also injures the com-
pressed tracheal mucosa, inducing inammation, oedema and submucosal
hemorrhage. These conditions constitute the pathological basis of other
complications, such as airway stenosis
13
. The recent development of non-
invasive methods of ventilation has resulted from a desire to avoid compli-
cations of invasive mechanical ventilation during acute respiratory failure.
NIPSV is also used in a growing population with sleep apnea.
In NIPSV, the patient determines the inspiratory rate, time, volume
and ow for the pressure support ventilation. Volume delivered to the pa-
tient during NIPSV will be variable and related to pulmonary compliance,
resistance, inspiratory time, and ow rate. The patient triggers the venti-
lator; the ventilator delivers a ow up to a level of ventilatory support, for
example 10 cmH
2
O, depending on the desired minute volume. The patient
continues the breath for as long as desired, and ow cycles o when the
patient's inspiratory ow rate falls below a certain percentage of their peak
inspiratory ow (usually 25%). Tidal volumes may vary, just as they do in
normal breathing. NIPSV can be implemented with or without positive
end expiratory pressure (PEEP). On newer ventilators it is possible to
adjust the rate of pressurization and the point at which the ventilator cycles
o (as a percentage of peak ow). For many patients such adjustments are
unnecessary, but in a signicant fraction they can make a crucial dierence.
The inspiratory flow cut-off (the criteria for terminating inspiratory
pressure application) can aect patient-ventilator synchrony by causing a
neural-mechanical dyssynchrony. This is most often manifested as prolonged
inspiration, requiring active termination by the patient. Prolonged inspira-
tory time can also lead to ineective inspiration (triggering) eorts by the
patient. These eects are masked by positive pressure breath, prevent trig-
gering of the ventilator, and further contribute to patient-ventilator
asynchrony. There are also some limitations related to the specic trigger-
ing algorithms. These include low sensitivity, resulting in delayed or failed
triggering eorts. Excessive sensitivity may cause auto-triggering that can
give rise to hyperventilation and gas trapping.
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