Page 417 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

Perioperative Management of Fluid Therapy 407

400 RBC transfusion should be based on the patient’s risk
Hb sat— 99%@Pao = 100 mm Hg for complications of inadequate oxygenation.
2
350 = 500 mm Hg 3. The use of a single hemoglobin “trigger” for all
Hb sat— 100%@Pao 2
patients and other approaches that fail to consider
Cardiac index (mL/kg/min) 250 restrictive versus liberal transfusion practices and could
300
all important physiologic and surgical factors affecting
oxygenation are not recommended.
A review in the Cochrane database examined the use of
200
identify no adverse effects of the use of transfusion
150
84
Although hemoglo-
triggers in the 7 to 9 g/dL range.
bin concentration is reported on the complete blood
100
count, it is more common for veterinarians to evaluate
the hematocrit, which usually is approximately three
50
times the hemoglobin concentration (expressed in
0 g/dL). A scoring system for the rational use of packed
3 4 5 6 7 8 9 10 11 12 13 14 15 RBCs in dogs was developed in an attempt to decrease
Hemoglobin concentration (g/dL) unnecessary use. 102 However, this scoring system did
Figure 17-1 The graph indicates the alteration in cardiac index not account for blood transfusions under conditions of
needed to provide an oxygen delivery of 15 mL/kg/min when the rapid blood loss and failure to maintain blood pressure.
PaO 2 is increased from 100 to 500 mm Hg, assuming that It is important to assess anemic dogs and cats carefully
hemoglobin saturation increases from 99% to 100%. Note that the and to estimate the likelihood of blood loss during the
increased PaO 2 begins to make a difference only when the
hemoglobin decreases below about 5 g/dL. procedure. A dog with a hematocrit of 18% and a healthy
cardiovascular system about to undergo a noninvasive
diagnostic procedure may be a candidate for anesthesia
without previous transfusion. A patient with the same
hemoglobinconcentrationandthecardiacindexassuming hematocrit but with clinically relevant mitral regurgita-
a constant saturation of hemoglobin (99%) to deliver oxy- tion and about to undergo an exploratory laparotomy
gen at a given rate (15 mL/kg/min). The second line for an undefined abdominal mass would be more likely
shows the same relationship for a PaO 2 of 500 mm Hg to require a preoperative blood transfusion.
assuming a hemoglobin saturation of 100%. In acute ane-
mia, the animal may have been able to increase cardiac out- POLYCYTHEMIA
put, but there has not been sufficient time for changes in Patients with polycythemia are at risk for complications
hemoglobin affinity to occur, and the delivery of oxygen because of the increased viscosity of their blood. High vis-
is likely to be decreased further. What is a “critical” hemo- cosity increases myocardial work and may lead to inade-
globin concentration? In many experiments, carried out in quate flow in some capillary beds, especially if the
9
dogs, the critical hemoglobin concentration is defined as animal becomes hypotensive. The hematocrit should
the point at which oxygen delivery fails to keep up with tis- be reduced to at least 65% by removal of blood and
sue oxygen demand. In the healthy, lightly anesthetized replacement with an isotonic crystalloid before the
dog, this concentration appears to be approximately polycythemic patient is anesthetized. Animals with poly-
3 g/dL but varies with the anesthetic used and increases cythemia caused by chronic hypoxia (e.g., tetralogy of
substantially at deeper planes of anesthesia. 164,166 Many Fallot) must be monitored carefully for signs of inade-
human patients are anesthetized and survive with hemo- quate oxygen delivery when such hemodilution is
globinconcentrationsaslowas3to4 g/dL,butanesthesia undertaken.
is not recommended in this situation unless great care is
taken to ensure that the patient has adequate cardiovascu- HYPOPROTEINEMIA
larreserveandunlesstechniquescanbeusedthatminimize Many drugs given during anesthesia are highly protein
reduction in cardiac output. 8,32 The ASA guidelines are bound, and hypoproteinemia may result in a greater frac-
based on the available literature in human medicine. 143 tion of the anesthetic being available. More profound
The ASA recommendations for use of packed red cells depression thus may occur from a given dose in the
include the following: hypoproteinemic patient. Most drugs bind to albumin,
1. Transfusion is rarely indicated when the hemoglobin is and it is this fraction of the proteins that is of greatest
greater than 10 g/dL and is almost always indicated importance. However, if the drug is titrated to effect,
when it is less than 6 g/dL, especially when the anemia the increased free fraction of drug is accounted for by
is acute. close monitoring of anesthetic induction. Thus concerns
2. The determination of whether intermediate hemoglo- about hypoproteinemia are greater when using intramus-
bin concentrations (6 to 10 g/dL) justify or require cular injection or bolus dose techniques.