Compensatory Intravascular Volume Expansion
Intravascular volume usually must be supplemented to compensate
for the venodilation and cardiac depression caused by anesthesia. Sustaining adequate
oxygen delivery in relation to oxygen consumption is an important goal of fluid therapy.
Tissue oxygen delivery depends on hemoglobin concentration, oxygen tension, organ
perfusion pressures, and organ vascular resistance. Organ perfusion pressures depend
on systemic arterial pressure and
TABLE 46-22 -- Volume and composition of gastrointestinal fluids
|
Fluid Source |
24-Hour Volume (mL) |
Na+
(mEq/L) |
K+
(mEq/L) |
Cl-
(mEq/L) |
HCO3
-
(mEq/L) |
|
Saliva |
500–2000 |
2–10 |
20–30 |
8–18 |
30 |
|
Stomach |
1000–2000 |
60–100 |
10–20 |
100–130 |
0 |
|
Pancreas |
300–800 |
135–145 |
5–10 |
70–90 |
95–120 |
|
Bile |
300–600 |
135–145 |
5–10 |
90–130 |
30–40 |
|
Jejunum |
2000–4000 |
120–140 |
5–10 |
90–140 |
30–40 |
|
Ileum |
1000–2000 |
80–150 |
2–8 |
45–140 |
30 |
|
Colon |
— |
60 |
30 |
40 |
— |
the higher of organ venous pressure or tissue pressure. Arterial pressure depends
on cardiac output and systemic vascular resistance. Cardiac output is related to
stroke volume and heart rate, and stroke volume depends on preload, contractility,
and afterload. Most general and regional anesthetics cause arteriolar and venous
dilation, expanding the vascular capacity. The latter reduces peripheral venous
pressure and therefore venous return and cardiac output. Fluid must be administered
to expand the blood volume to compensate for venodilation. General anesthetics produce
myocardial depression (see Chapter
7
). Increasing cardiac preload by infusing fluid intravascularly to take
advantage of the Starling mechanism often returns stroke volume to an acceptable
range. Postoperatively, venodilation and myocardial depression rapidly subside when
administration of the anesthetic is stopped. Patients with impaired cardiac or renal
responses may then become acutely hypervolemic. CVE with 5 to 7 mL/kg of balanced
salt solution must occur before or simultaneous with the onset of anesthesia.
