Positive-Pressure Ventilation
Positive-pressure ventilation (see Chapter
75
) and positive end-expiratory pressure (PEEP) decrease RBF, GFR, sodium
excretion, and the urine flow rate.[105]
The extent
of depression of renal function depends on the mean airway pressure. Thus, the impact
on the kidney is greater with inverse ratio ventilation than with intermittent mandatory
ventilation (IMV) and greater with IMV than with spontaneous ventilation with PEEP
(continuous positive airway pressure).
These changes are largely mediated by transmission of airway and
intrapleural pressure to the intravascular space and lead to decreases in venous
return, transmural (i.e., effective) cardiac filling pressure, and cardiac output.
Transmission may actually be attenuated in acute lung injury because of poor lung
compliance. High levels of mean airway pressure may compress the pulmonary arterial
circulation, increase right ventricular afterload, and induce the intraventricular
septum to shift into the left ventricle and decrease its filling and cardiac output.
[106]
Positive-pressure ventilation increases inferior
vena cava pressure and renal venous pressure and may increase tubular sodium reabsorption
by increases in peritubular capillary pressure.
The decrease in cardiac output and systemic arterial pressure
results in a carotid and aortic baroreceptor-mediated increase in sympathetic nerve
tone in the kidney with subsequent renal vasoconstriction, antidiuresis, and antinatriuresis.
Volume receptors in the atria respond to decreased filling by reducing ANP secretion,
which results in increased sympathetic tone, renin activation, and AVP activity.
Salt and water retention during airway pressure therapy was originally
surmised to be due to an AVP effect,[107]
but it
is now thought that sympathetic responses are more important and that sodium retention
is largely the result of decreased sodium delivery to the tubules. The renin-angiotensin-aldosterone
system undoubtedly augments the renal responses to positive-pressure ventilation.
Annat and associates[108]
found that PEEP at 15
cm H2
O depressed cardiac output, RBF, GFR, and urine volume by 20% to
30% and was associated with increases in renin and aldosterone but not AVP. The
impairment in renal function induced by airway pressure therapy can be prevented
or reversed by preserving normal circulatory status, either by hydration[109]
or by the use of dopamine.[110]
