Morgan & Mikhail Chap 51(Fluid Management+Blood Therapy) Flashcards
Evaluation of IV Volume
Intravascular volume can be estimated using patient history, physical examination, and laboratory analysis, often with the aid of sophisticated hemodynamic monitoring techniques. Regardless of the method employed, serial evaluations are necessary to confirm initial impressions and to guide fluid, electrolyte, and blood component therapy. All parameters are indirect, nonspecific measures of volume; therefore, reliance upon one parameter may lead to erroneous conclusions.
Signs of fluid loss (hypovolemia
HEMODYNAMIC MEASUREMENTS
The maximum, minimum, and mean SV are calculated for a set period of time by the various measuring devices. During spontaneous ventilation, the blood pressure decreases on inspiration. During positive pressure ventilation, the opposite occurs.
Normal SVV is less than 10% to 15% for patients on controlled ventilation. Patients
with greater degrees of SVV are likely to be responsive to fluid therapy. In addition to
providing a better assessment of volume and hemodynamic status than that obtained with CVP monitoring, these noninvasive modalities avoid multiple risks associated with
central venous and pulmonary artery catheters.
Consequently, we rarely employ
pulmonary artery catheters to guide hemodynamic therapy.
Intravenous Fluids
CRYSTALLOID SOLUTIONS
Because most intraoperative fluid losses are isotonic, isotonic crystalloid solutions such as normal saline or balanced electrolyte solutions (low-[Cl–] crystalloids, which
have preserved ionic “balance” by replacing Cl– with lactate, gluconate, or acetate)
such as lactated Ringer’s solution or PlasmaLyte are most commonly used for
replacement
Normal saline, when given in large volumes, produces hyperchloremic metabolic acidosis because of its high chloride content and lack of bicarbonate (see Chapter 50). In addition, chloride-rich crystalloids such as normal saline may contribute to perioperative acute kidney injury.
Therefore, we prefer
balanced salt solutions for most perioperative uses.
Composition of plasma, 0.9% saline, and commonly used balanced
crystalloids
COLLOID SOLUTIONS
The osmotic activity of high-molecular-weight substances in colloids tends to maintain these solutions intravascularly. While the intravascular half-life of a
crystalloid solution is 20 to 30 min, most colloid solutions have intravascular half-lives between 3 and 6 h. The relatively greater cost and occasional complications associated with colloids may limit their use.
Perioperative Fluid Therapy
Perioperative fluid therapy aims to replace normal losses (maintenance requirements) and correct preexisting fluid deficits and surgical losses (including blood loss).
PREEXISTING DEFICITS
Patients presenting for surgery after a traditional overnight fast without any fluid intake
will have a preexisting deficit proportionate to the duration of the fast. The deficit can be estimated by multiplying the normal maintenance rate by the length of the fast. For the average 70-kg person fasting for 8 h, this amounts to (40 + 20 + 50) mL/h × 8 h, or 880 mL (Table 51–3).
Abnormal fluid losses frequently contribute to preoperative deficits. Preoperative bleeding, vomiting, nasogastric suction, diuresis, and diarrhea are often contributory.
SURGICAL FLUID LOSSES
Blood Loss
The most commonly used method for estimating blood loss is the measurement of blood in the surgical suction container and a visual estimation of the blood on surgical
sponges (“4 by 4’s”) and laparotomy pads (“lap sponges”). A fully soaked “4 × 4” is generally considered to hold 10 mL of blood, whereas a soaked “lap” may hold 100 to
150 mL. More accurate estimates are obtained if sponges and laps are weighed before and after use, which is especially important during pediatric procedures.
Other Fluid Losses
Many surgical procedures are associated with obligatory losses of fluids other than blood. Such losses are due mainly to evaporation and internal redistribution of body
fluids
INTRAOPERATIVE FLUID REPLACEMENT
Intraoperative fluid therapy should supply basic fluid requirements and replace residual preoperative deficits as well as intraoperative losses (blood loss, fluid redistribution, evaporation).
Replacing Blood Loss
The transfusion point can be determined preoperatively from the hematocrit and by estimating blood volume (Table 51–5). Patients with a normal hematocrit should
generally be transfused only after losses greater than 10% to 20% of their blood volume. The timing of transfusion initiation is based on the patient’s procedure,
comorbid conditions, and rate of blood loss. The amount of blood loss necessary for the hematocrit to fall to 30% can be calculated as follows:
1. Estimate blood volume from Table 51–5.
2. Estimate the RBC volume (RBCV) at the preoperative hematocrit (RBCVpreop).
3. Estimate RBCV at a hematocrit of 30% (RBCV30%), assuming normal blood volume
is maintained.
4. Calculate the RBCV lost when the hematocrit is 30%; RBCVlost = RBCVpreop –
RBCV30%.
5. Allowable blood loss = RBCVlost × 3.
Therefore, transfusion should be considered only when this patient’s blood loss exceeds 800 mL. Increasingly, transfusions are not recommended until the hematocrit decreases to 24% or less (hemoglobin <8.0 g/dL), but transfusion decisions must be made on an individualized basis and take into account the potential for further blood loss, rate of blood loss, and comorbid conditions (eg, cardiac disease).
Clinical guidelines for commonly used transfusions include: (1) transfusing 1 unit of RBCs will increase hemoglobin 1 g/dL and the hematocrit 2% to 3% in adults, and (2) a 10-mL/kg transfusion of RBCs will increase hemoglobin concentration by 3 g/dL and
the hematocrit by 10%.
Average blood volumes
Transfusion
