Colloid oncotic pressure as a guide for the anesthesiologist in directing fluid therapy

Monitoring the critically ill equine patient

Measurements of physiologic parameters, such as blood pressure or lactate concentration, are useful to detect occult derangements, such as tissue hypoxia and dysoxia. These tools are also useful in determining the effects of therapy. Monitoring techniques are now widely available for the intensive management of critically ill horses and foals. A number of these have evolved into noninvasive or minimally invasive devices and procedures and provide information that can be used for earlier and more dynamic therapeutic intervention. The goal of increased monitoring is to improve the level of care in the ICU; L ultimately. increased survival of critical patients is the motivation behind enhanced monitoring of physiology, with particular attention being paid to trends or alterations over time. This review highlights practical and informative monitoring tools and techniques and provides normal reference values from the literature.

Effect of hydroxyethyl starch infusion on colloid oncotic pressure in hypoproteinemic horses

OBJECTIVE

To determine the effect of hydroxyethyl starch (HES) on colloid oncotic pressure (pi) during fluid resuscitation of hypoproteinemic horses and to evaluate the clinical usefulness of direct and indirect methods for determination of pi before and after infusion of a synthetic colloid.

DESIGN

Prospective clinical study.

ANIMALS

11 hypoproteinemic horses.

PROCEDURE

Horses received IV infusions of 8 to 10 ml of a 6% solution of HES/kg (3.6 to 4.5 ml/lb) of body weight during fluid resuscitation. Blood samples were obtained for determination of plasma measured colloid oncotic pressure (pi meas) and plasma total protein and albumin (A) concentrations. Plasma globulin concentration (G) was calculated as the difference between plasma total protein and albumin concentrations. Calculated values for colloid oncotic pressure (piA + G) were determined by use of a predictive nomogram previously developed for horses.

RESULTS

There was no significant difference between the means of pi meas and piA + G at the beginning of HES infusion. After HES infusion, the mean of pi meas was increased significantly from baseline for 6 hours. Mean plasma total protein and albumin concentrations and piA + G were decreased significantly from baseline for 24 hours. Differences between mean pi meas and piA + G after HES infusion were significant for 24 hours.

CONCLUSIONS AND CLINICAL RELEVANCE

There was good agreement between plasma pi meas and piA + G in blood samples obtained from hypoproteinemic horses immediately before infusion of HES. Use of a predictive nomogram did not, however, account for the oncotic effect of HES. Results of comparison of pi meas to piA + G after HES infusion suggest that a significant oncotic effect was maintained for 24 hours in the study horses.

[Clinical practice interpretation of oncotic pressure, serum albumin and protein determination and their ability for guiding therapeutics in cases of disturbances of capillary exchanges]

The osmotic pressure of the serum proteins, the colloid osmotic pressure (COP) or oncotic pressure, is one of the four Starling forces determining fluid movements across a capillary. Theoretical and empirical formulas considering plasma protein or albumin concentration have been used. However, such calculations are only approximations, being inadequate especially when protein fractionation pattern is abnormal or when artificial colloids are used. It is preferable to measure COP directly. In healthy standing adults COP is around 25 mmHg. It is influenced by age, position, blood pressure and pH. The interpretation of the COP must take into account, on the one hand the possible variations of the other factors involved in the Starling's equilibrium and, on the other hand the structural and functional differences between systemic and pulmonary circulations. When capillary permeability is not altered, a decreased COP is associated with an increased filtration rate across the capillaries, resulting in oedema in skeletal muscle, skin, subcutaneous tissue, intestine and heart. The lung is relatively insensitive to extravascular fluid shifts caused by lowering of the COP, due to the efficacy of the pulmonary lymph drainage and the high pulmonary interstitial protein concentration. The main determinant of fluid exchange in the lung is then the hydrostatic force, reflected by the pulmonary artery wedge pressure. When capillary permeability is altered, the decreased reflectance coefficient results in a smaller effect of the COP on interstitial oedema formation. Several studies demonstrated that oedema formation in the injured tissues seems unaffected by changes in COP. During fluid resuscitation, COP measurement is useful for assessing the risks of peripheral oedema formation and additional measurements of pulmonary artery wedge pressure for assessing the risks of pulmonary oedema. Therefore COP measurement can serve as a useful guide for selection of colloids or crystalloids in critically ill patients, avoiding expensive albumin administrations.