Page 72 - Clinical Application of Mechanical Ventilation

38 Chapter 2

mechanism whereby required substances that are filtered by the glomerulus are re-
claimed by the cells lining the renal tubule and are ultimately reabsorbed into the
blood. Some of these substances are reabsorbed down to an equal concentration gra-
dient; thus an enhanced concentration gradient could lead to increased reabsorption.
As cardiac output is reduced by mechanical ventilation, renal blood flow and,
thus, urine volume, are also reduced. As the urine becomes more concentrated,
the drugs in the glomerular filtrate also become more concentrated. This causes an
increase in the reabsorption gradient of the drugs in the filtrate. Some of the drugs
used in critically ill patients include aminoglycosides, theophylline, and phenobar-
bital (Perkins et al., 1989). Decreased renal perfusion causes a higher reabsorption
rate of these drugs back into the circulation.

HEPATIC CONSIDERATIONS

Hepatic perfusion accounts for about 15% of the total cardiac output. Positive
hepatic perfusion: Blood flow to
the liver. It is decreased when the pressure ventilation alone does not alter the blood flow to the liver to any significant
blood volume or cardiac output degree. When PEEP is added to mechanical ventilation, the blood flow to the liver
is low.
is noticeably reduced (Bonnet et al., 1982).

PEEP and Hepatic Perfusion

The rate of hepatic blood flow is inversely related to the level of PEEP. In one study,
the hepatic blood flow decreased 3%, 12%, and 32% at PEEP of 10, 15, and 20 cm
H O, respectively (Bonnet et al., 1982). The decrease in hepatic blood flow is solely
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caused by a reduction in cardiac output as a result of PEEP. This inference is made
because the ratio of hepatic blood flow to cardiac output remains unchanged at
15% during mechanical ventilation without PEEP (Perkins et al., 1989).
Indicators of Liver Dysfunction
Impairment of
liver function is likely when
prothrombin time is .4 sec, Liver dysfunction may be monitored by measuring the prothrombin time and bili-
bilirubin level is Ú50 mg/L,
or albumin level is …20 g/L. rubin and albumin levels (Kraus et al., 1993). Impairment of liver function is likely
when coagulation time is increased (prothrombin time .4 sec over control, biliru-
bin level is increased (Ú50 mg/L), or albumin level is decreased (…20 g/L).

Effects of Decreased Hepatic Perfusion
on Drug Clearance

A decrease in hepatic blood flow may diminish the drug clearance mechanism of the
Hypoperfusion of the liver. Drugs most likely to be affected by changes in hepatic blood flow are agents whose
liver may affect the rate of
drug clearance and lead to a clearance relies on the liver and its perfusion. Examples of such drugs commonly used
higher drug concentration in in the intensive care unit include lidocaine, meperidine, propranolol, and verapamil
the circulation.
(Perkins et al., 1989). When hepatic perfusion is inadequate, use of these drugs may lead
to a relatively higher serum concentration due to diminished drug clearance.

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