- In mammals with substantial hypovolemia, blood pressure decreases below a mean arterial pressure of 60 mmHg or a systolic pressure of 90 mmHg. Research indicates that birds have a baroreceptor response to shock similar to that seen in mammals.
- Crystalloids are the mainstay of rehydration and maintenance fluid therapy. Colloids, such as Hetastarch, whole blood, or plasma, are frequently indicated during fluid resuscitation to provide sustained intravascular volume expansion.
- Most debilitated birds benefit from initial administration of warmed [100-102°F (38-39°C)] crystalloids at 30 ml/kg given intravenously, intraosseously, or subcutaneously. When the bird appears stable, perform diagnostics including blood pressure monitoring and further treatment. One to three bolus infusions of crystalloids (10 ml/kg) and colloids (Hetastarch or Oxyglobin at 5 ml/kg) can be given in the same syringe IV or IO until blood pressure exceeds 90 mmHg systolic.
- Replacement fluid volume (ml) = estimated dehydration deficit (%) x body weight (kg) x 1000. Add daily maintenance fluid requirements (2 ml/kg/h or 48 ml/kg/day) to the fluid deficit volume. Replace the total volume over 24 hours.
- Provide maintenance fluids until the bird is eating on its own by dividing three bolus infusions over 24 hours.
Shock is defined as poor tissue perfusion due to low or unevenly distributed blood flow, which results in inadequate delivery of oxygen to the tissues. Fluid resuscitation of the patient in hypovolemic shock can be a challenge (Fig 1), and the clinician should understand the basic pathophysiology of shock, principles of perfusion, properties of and indications for different types of fluids and blood pressure monitoring techniques.
Hypovolemic shock is caused by either an absolute or relative decrease in blood volume. Etiologies of absolute hypovolemia include any potential cause of hemorrhage. Examples of relative hypovolemia include severe dehydration or extensive loss of plasma (e.g., in a burn patient).
The most common cause of hypovolemic shock is hemorrhage. When an animal begins hemorrhaging, there is a decrease in blood volume and venous return to the right side of the heart. The result is a decreased return to the left side of the heart and ultimately a decrease in cardiac output. With substantial hypovolemia, blood pressure decreases below a mean arterial pressure of 60 mmHg or a systolic pressure of 90 mmHg. Arterial baroreceptors detect a decrease in stretch due to decreased cardiac output, which in turn sends a neural signal to the vasomotor center in the medulla oblongata, resulting in inhibition of the vagal parasympathetic center and stimulation of the sympathetic center. The result is vasoconstriction of peripheral vasculature and an increase in the rate and strength of heart contractions. The humoral response is an increase in circulating catecholamines, which in turn stimulates renin release via adrenergic receptors on cells of the juxtaglomerular apparatus (specialized smooth muscle cells in the afferent arterioles of the kidney). Renin causes release of angiotensin 2, aldosterone and antidiuretic hormone. These hormones stimulate even stronger vasoconstriction and water retention, causing an increase in both intravascular fluid volume and blood pressure.
The pathophysiology of hemorrhagic shock is poorly understood in avian species. Acute blood loss of 30% to 40% of blood volume has been shown to result in 50% mortality (LD50) in mammals. Blood loss is better tolerated in birds than in mammals. In a recent study, the LD50 for acute blood loss in mallard ducks (Anas platyrhynnchos) was shown to be approximately 60% of the total blood volume. The study also documented an increase in heart rate and decrease in blood pressure following acute blood loss. Therefore, it is possible that birds have a baroreceptor response to shock similar to that seen in mammals.
It is difficult to assess hydration in the avian patient. The most reliable parameters may include elevations in hematocrit, total protein, and blood urea. Mucous membranes may be tacky and strands of mucus may be visible within the oropharynx. The skin on the dorsal surfaces of the feet or the upper eyelid may be gently tented. Severely dehydrated birds may also have sunken eyes.
Crystalloids, also called replacement fluids, are the mainstay of rehydration and maintenance fluid therapy, and they can be used together with colloids during resuscitation. Crystalloids are fluids containing sodium chloride and other solutes that are capable of distributing to all body fluid compartments. Replacement fluids have electrolyte concentrations that resemble extracellular fluid, whereas maintenance fluids contain less sodium (40-60 mEq/L) and more potassium (15-30 mEq/L). The most commonly used replacement fluids are 0.9% saline, lactated Ringer’s solution, Normosol-R, or Plasmalyte-A.
Buffered solutions are usually indicated for patients in shock, since administration of a highly acidic solution may worsen preexisting metabolic acidosis. Buffered solutions usually contain lactate, gluconate or acetate. The liver must metabolize lactate, whereas many cells in the body metabolize acetate and gluconate. Therefore, fluids containing lactate may not be tolerated well by animals with end-stage liver disease. There is evidence to support the use of 0.9% sodium chloride in animals with head trauma in order to avoid rapid changes in osmolality. Normal saline can also be used for conditions causing hyperkalemia (e.g., urinary obstruction and oliguric renal failure), hypercalcemia and hypochloremic metabolic alkalosis.
Crystalloids expand the intravascular space, but this effect is short-lived. Because approximately 80% of extracellular fluid is in the interstitial space, crystalloids rapidly redistribute. After approximately 1 hour, only 20% of administered volume remains in the circulation. Thus, crystalloids should be thought of as interstitial rehydrators, not intravascular volume expanders. This increase in interstitial fluid can lead to tissue edema (thus decreasing the ability of oxygen to diffuse to the cells). Interstitial edema may be extremely detrimental in cases of cerebral and pulmonary edema.
Colloids contain large molecular weight substances that are generally not able to pass through capillary membranes. They can be considered intravascular volume expanders. Since most patients in shock require sustained intravascular volume expansion, colloids are frequently indicated during fluid resuscitation. Examples include synthetic colloids such as hydroxyethyl starch (HES or hetastarch) and biologic colloids such as whole blood, plasma, and albumin.
Ideally, blood should be used for resuscitation if the patient has lost whole blood. The lower the hematocrit, the more important it is to ensure adequate oxygen delivery to the cells. The availability of blood products in sufficient quantities to meet the needs of the avian species is often the limiting factor in survival. Most hospitals do not have donors readily available for species-specific donation, and storage of avian blood for transfusion is not considered possible to date.
Hetastarch is a synthetic colloid that effectively increases oncotic pressure beyond that which can be obtained with infusion of blood products alone. Hetastarch expands the intravascular volume by about 1.4 times the volume infused and has a half-life of 25 hours. Synthetic colloids are administered with isotonic crystalloids to reduce interstitial volume depletion. In this case, the dose of crystalloid administered is only 40-60% of what it would be if crystalloids were used alone during resuscitation.
Editor’s Note: Hydroxyethyl starch (HES) solutions have been withdrawn from the market of some countries due to rising concern regarding acute renal injury in critically ill human patients. A direct association between synthetic starch colloids and acute kidney injury has not been reported in veterinary medicine however similar risks are possible in critically ill animals (Liu and Silverstein 2015, Nolan and Mythen 2013)
Hemoglobin-based oxygen carriers
Hemoglobin-based oxygen carriers (HBOC) are indicated during resuscitation when increased oxygen delivery to tissues is required. In addition, HBOC are effective colloids. The most commonly available HBOC has been Oxyglobin®, a purified polymerized bovine hemoglobin in a modified lactated Ringer’s solution, which is approved for use in dogs. The Oxyglobin® supply currently is very small and directed by the company to be sold to its best customers. Oxyglobin is an excellent fluid to use in animals suffering from acute blood loss secondary to trauma or surgical mishap and should be administered to patients that have become hemodiluted from crystalloids (PCV <15%-18%). Oxyglobin can also be administered to patients suffering from chronic anemia with PCV values less than 10%. The side effect of orange discoloration of the mucous membranes, urine and skin seen in mammals with Oxyglobin infusion does not occur in birds.
Temperature and volume
Any debilitated bird presenting for emergency care should immediately be placed in a warm incubator [85-90°F (29.4-32.1°C)] with oxygen supplementation for at least 2 to 4 hours. Active external hemorrhage must be stopped immediately. Most birds benefit from initial administration of warmed [100-102°F (38-39°C)] crystalloids at 30 ml/kg given intravenously (IV), intraosseously (IO), or subcutaneously. When the bird appears stable, diagnostics and further treatment for hypovolemia and dehydration can be performed. Blood pressure monitoring using Doppler and an electrocardiogram can be used during these procedures to monitor the patient. Bolus administration of crystalloids (10 ml/kg) and colloids (HES or Oxyglobin at 5 ml/kg) can be given IV or IO until blood pressure exceeds 90 mmHg systolic. Crystalloids and colloids are mixed in the same syringe. In the author’s experience, one to three bolus infusions are usually required.
In severely dehydrated birds that are not eating, an IV or IO catheter is placed for administration of replacement crystalloids. Estimation of the fluid deficit (ml) is based on the estimated level of dehydration and the patient’s body weight: estimated dehydration deficit (%) x body weight (kg) x 1000. Daily maintenance fluid requirements (2 ml/kg/h or 48 ml/kg/day) are added to the fluid deficit volume. The total volume replacement deficit is replaced over the first 24 hours.
After replacement of fluids for dehydration, maintenance requirements divided into three bolus infusions over 24 hours may be continued until the bird is eating on its own. In the author’s experience, there have been no side effects using crystalloids or colloids (HES or Oxyglobin) as bolus infusions in birds. The author has used as many as three bolus infusions of crystalloids and colloids without causing volume overload.
Intravenous or intraosseous fluids are the routes of choice for the treatment of the extremely dehydrated or hypovolemic bird. Accessible veins for catheter placement include the ulnar, medial metatarsal and jugular veins. IO fluids are indicated when veins are collapsed or too small to allow IV catheterization.
Subcutaneous (SC) or oral fluids are used for maintenance fluid therapy or mild dehydration. The usual site of SC fluid administration is in the inguinal region where the leg and the body wall meet. Subcutaneous fluids are not suitable for shock therapy or severe dehydration due to poor peripheral circulation, or for birds with severe hypoproteinemia. Oral fluids should not be given to birds that are laterally recumbent, seizuring, regurgitating, or have crop stasis.
|Table 1. Fluid requirements|
|Crystalloids||Colloids (HES** or Oxyglobin)|
|* BP: Blood pressure|
** HES: Hetastarch
(systolic BP* < 90 mmHg)
|Give 1-3 bolus infusions at 10 ml/kg mixed with colloids until systolic BP is > 90 mmHg||Give 1-3 bolus infusions at 5 ml/kg mixed with crystalloids until systolic BP is > 90 mmHg|
|Rehydration replacement||% dehydration x body weight (kg) x 1000 is added to maintenance requirements and administered over 24 hours|
|Maintenance requirements until eating||48 ml/kg/day divided into 3 bolus infusions|