Physiology

Rapid intravenous infusion of isotonic crystalloids causes continuous vascular volume expansion until the infusion stops, at which time most of the administered volume is in the vascular compartment. When 80 ml/kg was administered in 12 minutes to experimental, healthy dogs, blood volume was increased by 76% at the end of the infusion, 35% after 30 minutes, and 18% after 4 hours.⁵

Because of the fairly rapid distribution out of the vascular space, isotonic crystalloids must be administered at rapid rates to get the desired vascular volume expansion. Isotonic crystalloids have little effect on intracellular volume (see Chapter 64, Daily Intravenous Fluid Therapy).

Benefits

Isotonic crystalloids are inexpensive, readily available, and have a long track record of success as resuscitation fluids. The redistribution to the interstitium is beneficial for many dogs and cats in shock, because their underlying problem often involves preexisting severe salt and water losses that have depleted the extracellular volume (dehydration).

Adverse Effects

If isotonic crystalloids are delivered too slowly, the desired vascular volume expansion is not achieved but 75% of the administered volume is still distributed to the interstitium, predisposing the patient to interstitial fluid overload, possibly to pulmonary edema. However, the lung is richly supplied with lymphatic vessels, which, among other factors, can protect it from interstitial fluid overload. Large-volume isotonic crystalloid resuscitation did not cause increases in lung water or hypoxemia in a canine hemorrhagic shock model.⁶ Inappropriate and overzealous administration of isotonic crystalloids risks adverse effects of worsened pulmonary edema, increased intracranial pressure, and abdominal compartment syndrome.

Isotonic crystalloids dilute all plasma elements except those ions present in the administered fluid at plasma concentrations. Of major concern is dilution of albumin and therefore decrease in colloid osmotic pressure, and dilution of the red blood cell mass. With redistribution of crystalloid fluids this effect will be reduced.

Alterations in immunologic and proinflammatory states, such as neutrophil activation or increase in apoptosis, have occurred in experimental animals treated for shock with lactated Ringer’s solution.^7-9 Pulmonary apoptosis was increased when the D isomer of lactate was substituted for L isomer in lactated Ringer’s solution.¹⁰ Intestinal hyperpermeability was decreased in an experimental hemorrhagic shock model when Ringer’s ethyl pyruvate solution was used.¹¹

Fluid Prescription

The estimated dosage of isotonic crystalloids for treating shock is equal to the patient’s healthy blood volume (approximately 80 ml/kg for dogs, 50 ml/kg for cats). The actual fluid requirements of each patient will vary and must be prescribed individually. The total amount administered depends on the response to treatment and is usually one half to one blood volume (see Table 65-1). One strategy is to set up the fluid delivery system to deliver the prescribed amount in 20 minutes, and to slow or discontinue the infusion if the tissue perfusion parameters improve before the end of the infusion.

Pressure bags are helpful for rapid administration of large volumes of isotonic crystalloids. For cats, rapid infusion of fluids can be performed easily with a 60-ml syringe. This allows accurate measurement of the volume given and avoids the risk of inadvertent fluid overload. Patients are monitored continually throughout the resuscitation period, and rapid administration is stopped when the goals of resuscitation are accomplished.

Physiology

Synthetic colloids redistribute into the interstitial space at a much lower ratio than the isotonic crystalloid fluids. They are expected to increase serum colloid osmotic pressure, which will lead to movement of fluid from the interstitial space into the vascular space. Consequently, colloid administration will increase blood volume by an amount greater than the infused volume. This means that effective resuscitation may be possible by administering smaller volumes. When the full dose of 20 ml/kg was administered in 5 minutes to experimental healthy dogs, blood volume was increased by 25% at the end of the infusion, 36% after 30 minutes, and 26% after 4 hours.⁵ These fluids are isoosmotic, so there is little effect on intracellular volume (see Chapter 64, Daily Intravenous Fluid Therapy).

Benefits

Colloid solutions produce vascular volume expansion with less interstitial expansion than do crystalloid solutions. They support colloid osmotic pressure and are useful in patients who are symptomatic from their hypoalbuminemia. In theory, the larger molecular weight colloids, because of their molecular size and configuration, might seal capillary leaks in patients with capillary leak syndrome.¹² Some have postulated that capillaries have both small pores, which reflect colloids, and large pores, which do not. The capillary leak syndrome that occurs in critical illness causes a change in the number of these pores but not in their size. For this reason, the molecular weight distribution of hydroxyethyl starch would not be a determining factor.¹³ Some studies have supported the benefits of colloids in capillary leak,^14,15 others have not.^13,16

Adverse Effects

The most important adverse effect of synthetic colloids is on coagulation. High molecular weight hydroxyethyl starch (the only available formulation in the United States) and dextran at dosages greater than 20 ml/kg q24h are likely to prolong clotting times.^17-19

The causes of these changes in coagulation are many, including decreases in factor VIII and von Willebrand factor. Clinically significant bleeding associated with these products is unpredictable, probably because concurrent disease affects coagulation in many critically ill patients. They should be used in caution with patients already showing blood clotting abnormalities or in those in which blood clotting ability is a major concern, such as in those undergoing emergency surgery.

Colloids dilute red blood cell mass and albumin, as do crystalloids, and this effect is long lasting in proportion to the persistence of colloids in the vascular space. Allergic reactions to hydroxyethyl starch, dextran, and gelatin are reported but are extremely rare.