Page 416 - Clinical Application of Mechanical Ventilation

382 Chapter 12

ventilation, but it must be corrected by improving alveolar ventilation. Arterial
Alveolar ventilation may PaCO is the best indicator of a patient’s ventilatory status. When hypoxemia is
be improved by c ventilator 2
frequency or V T or c sponta- caused by hypoventilation (i.e., low PaO and high PaCO ), ventilation alone may
2
2
neous frequency or V T .
be sufficient to correct this type of hypoxemia. Ventilation can be provided by
increasing the ventilator frequency or tidal volume, or by increasing the patient’s
spontaneous tidal volume or frequency.
Alveolar ventilation may also be improved by reducing the deadspace volume.
Alveolar ventilation may Endotracheal intubation and tracheostomy are both effective in reducing the ana
be improved by T the ana-
tomic, mechanical, or alveolar tomic deadspace. Mechanical deadspace of an endotracheal tube may be decreased
deadspace. by cutting it shorter than the original length. If a high V/Q mismatch (ventilation in

excess of perfusion) exists, alveolar deadspace may be reduced by improving pulmo-
nary perfusion.

Improve Circulation

Adequate pulmonary blood flow is necessary for proper gas exchange. If perfu-
Hypoperfusion due to sion is too low relative to ventilation, deadspace ventilation (high V/Q) results.
congestive heart failure may
be corrected by improving the If perfusion is too high, pulmonary hypertension becomes the potential prob-
myocardial function. lem. In order to maintain a normal ventilation-perfusion relationship, the
hemodynamic values should be monitored regularly. Hemodynamic monitor-
ing may include invasive procedures such as pulmonary artery catheter and
#
noninvasive procedures such as esophageal Doppler ultrasound and VCO
2
monitoring.
When hypovolemia occurs due to volume loss, fluid replacement is necessary. If
In relative hypovolemia the cause of hypovolemia is shock (i.e., relative hypovolemia; loss of venous tone),
(loss of venous tone), fluid
replacement should be done fluid replacement should be done with extreme caution because of the potential
with extreme caution because for fluid overload when vascular tone returns to normal. Vasopressors are useful to
of the potential for fluid
overload when vascular tone provide quick relief from hypovolemia due to shock. The ultimate solution to this
returns to normal.
type of hypovolemia is to find and correct the causes of shock.

Maintain Normal Hemoglobin Level

Monitoring of the PaO alone for assessment of oxygenation status may be
2
inadequate when a patient’s hemoglobin level is below normal. This is because
PaO measures the amount of oxygen dissolved in the plasma, whereas a vast
2
majority (.98%) of the oxygen in the blood is combined with and carried
by the hemoglobins. During arterial blood gas sampling and analysis, CO-
oximetry should be run to evaluate the arterial oxygen content and the hemo-
globin levels. Anemia (hemoglobin less than 10 g/100 mL) should be reported
along with blood gas results.
Treatment of anemia must be specific to the cause. For example, anemia due
to excessive blood loss should be treated by stopping the blood loss and replacing
the blood volume. Anemia caused by insufficient hemoglobin should be treated by
blood transfusion. Once the hemoglobin level is restored, the arterial oxygen con-
tent should return to normal.

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