Page 291 - Clinical Application of Mechanical Ventilation

Monitoring in Mechanical Ventilation 257

Hypoventilation. Acute hypoventilation causes CO retention (increased PaCO )
Acute hypoventilation 2 2
causes respiratory acidosis. and respiratory acidosis. Without supplemental oxygen, hypoventilation leads to
hypoxemia as well. This type of hypoxemia should not be treated with oxygen
alone as the underlying condition can only be corrected by improving the alveolar
ventilation.
ventilation/perfusion (V/Q)
mismatch: An abnormal
distribution of ventilation and Ventilation/Perfusion Mismatch. When PaO is decreased with little or no change in
2
pulmonary blood flow. High V/Q is PaCO , ventilation/perfusion (V/Q) mismatch or intrapulmonary shunt should
related to dead space ventilation, 2
whereas low V/Q is associated with be suspected. Hypoxemia caused by ventilation/perfusion (V/Q) mismatch is char-
intrapulmonary shunting. acterized by a normal or low PaCO , and this type of hypoxemia responds well to
2
moderate levels of supplemental oxygen.
intrapulmonary shunting: Pul-
monary circulation that does not Intrapulmonary Shunting. Hypoxemia caused by intrapulmonary shunting does
come in contact with ventilated not respond well to high concentrations of oxygen. This is because shunted blood
alveoli.
does not come in contact with ventilated (oxygenated) alveoli. The PaCO is usually
2
normal or low because the peripheral chemoreceptors respond readily to hypoxemia
by increasing the minute ventilation (West, 2011).
When the PaO 2 is
decreased with little or Positive end-expiratory pressure (PEEP) in conjunction with oxygen are usually
no change in PaCO 2 , V/Q
mismatch or intrapulmonary required to correct hypoxemia caused by intrapulmonary shunting. If hypoventila-
shunt should be suspected. tion is present as documented by an increased PaCO , ventilatory assistance may
2
also be necessary.
Diffusion Defects. Diffusion abnormalities can cause hypoxemia by three mecha-
Hypoxemia caused by
intrapulmonary shunting nisms: (1) low oxygen pressure gradient, (2) increased alveolar-capillary thickness
does not respond well to high or diffusion gradient, and (3) decreased alveolar surface area.
concentrations of oxygen.
A low alveolar-arterial oxygen tension gradient is usually due to reduction in al-
veolar PO . Oxygen therapy increases the alveolar PO and alveolar-arterial PO
2
2
2
tension gradient. It is therefore very effective in correcting hypoxemia caused by
PEEP and oxygen are
usually required to correct uncomplicated diffusion defect.
hypoxemia caused by intra- Increased alveolar-capillary thickness or diffusion gradient can be seen in condi-
pulmonary shunting.
tions such as pneumonia and pulmonary and interstitial edema. In mild and un-
complicated cases, hypoxemia may be corrected by oxygen therapy.
Decreased alveolar surface area can be seen in emphysema due to destruction of
the lung tissues (Tobin, 1990). This type of structural defect is not reversible, but
the diffusion problem may be partially managed by oxygen therapy.

Limitations of Blood Gases

Blood gas analysis and monitoring is not without its limitations. Arterial blood
sampling is a procedure requiring the puncture of an artery or placement of an
arterial catheter. Inaccurate results can occur with introduction of air bubbles,
dilution with excessive heparin, or with faulty handling of the sample itself.
It is also important to keep in mind that blood gas values reflect an isolated
measurement in time rather than a trend. They should be used to correlate and
document trends established by noninvasive monitoring devices such as pulse
oximetry. Finally, arterial blood gas measurements are generally a late indicator
of respiratory failure and of limited use as an early warning sign (Tobin, 1990).

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