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When faced with a borderline blood gas and a request for ICU
admission and possible mechanical ventilation, it is essential to deduce
what part of the respiratory apparatus is malfunctioning.
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Is it failure to
ventilate (is the PCO2 > 50mmHg), or failure to oxygenate (is the PO2
<50mmHg)? Remember that a low O2 is much more significant than a high
PCO2, but is frequently easier to treat.
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If it is ventilatory
failure, where is the injury in the brain (the medulla), in the spinal
cord, in the peripheral nerves, at the neuromuscular junction, in the
muscle itself or in the chest cage?
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If the problem is
oxygenation failure, where is the injury: is it in the blood supply, at
the alveolar-capillary interface or in the upper, middle or lower
airways?
When
evaluating these patients it is important that we remind ourselves of what
is required to maintain a respiratory system:
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A mechanism for telling the body that it is time to breath:
this involves CO2 sensors in the brainstem, which signal diaphragmatic
movement via the cervical nerves.
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The phrenic nerves (and on occasion the intercostals nerves)
signal, via acetylcholine, the diaphragm to contract it does so by
increasing the volume of the thorax, by moving down into the abdomen,
making the intrapleural and intra-alveolar pressure more negative,
creating a pressure gradient between the atmospheric and the alveoli, and
allowing air to pass down through a series of ever narrowing bronchi into
the alveoli.
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The alveoli and the pulmonary capillary network, derived from
the main pulmonary arteries, are in apposition: oxygen and carbon dioxide
diffuse across the concentration gradient out of and into the alveoli
respectively. The diffusion of CO2 is more effective due to its higher
solubility.
In essence the problem is one or more of the following:
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The chest cage is not effective in guaranteeing adequate
minute ventilation.
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Air is not able to pass effectively from the upper to the
lower airway increased airway resistance.
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Gas is unable to pass effectively from alveoli to capillaries
due to some obstruction in the interstitial space.
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Ventilation is being wasted alveoli are being ventilated
but not perfused: dead space ventilation or more air than the blood can
utilize (high ventilation/perfusion (V/Q) ratio).
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Blood flow is inadequately utilized and blood is passing
through the lungs without coming into contact with aerated alveoli:
perfused but not ventilated shunt or ventilation falls behind blood
flow (low V/Q ratio).
Copyright Patrick Neligan
2001-2002 |
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