Benjamin M. Brainard, VMD, DACVAA, DACVECC
In the ICU or emergency room setting, patients with hemorrhage or at risk for hemorrhage are commonly encountered. Hemorrhage creates shock because it decreases the total number of red blood cells (RBCs) in circulation, thus decreasing the ability of the blood to deliver oxygen to tissues. In addition to discovering and stopping the source of blood loss, patients must be stabilized and resuscitated, which may include administration of blood products such as RBCs, fresh whole blood, or plasma. Once patients have been stabilized, it is necessary to continue to monitor for additional episodes of bleeding so that prompt intervention may be made.
Patients who have hemorrhage merit close observation. Because the hemorrhage may not always be apparent, nursing staff and doctors must use surrogate measures that can alert them to continued or new hemorrhage. Physical examination should be repeated at regular intervals so that acute changes may be recognized. In patients with hemorrhage, the physical exam may note pale mucous membranes, instead of the normal pink color. A delayed capillary refill time (CRT; > 2.5 seconds) may indicate an overall loss of circulating blood volume, or the inability of the heart to pump adequately. As an animal develops hypovolemic or hemorrhagic shock, the compensatory mechanisms of the body will result in an increased heart rate (tachycardia) and also peripheral vasoconstriction. Peripheral vasoconstriction may be manifested by pale mucous membranes, or by cool extremities. In extreme cases, vasoconstriction and hypovolemia will cause a decrease in rectal temperature due to lack of perfusion. Palpation of pulses in a patient with hypovolemia or hemorrhage will usually reveal a weak, fast pulse that may be difficult to find initially.
The tachycardia that develops during hypovolemic shock is caused by increased sympathetic tone, and so monitoring of an ECG is expected to show a sinus tachycardia. In some animals, especially those who have low oxygen delivery to the heart, or disease in the heart muscle itself, ventricular arrhythmias or ventricular tachycardia may be present. For this reason, ECG monitoring is beneficial in these cases.
Although palpation of pulses can give subjective information about the arterial blood pressure in our animals, measurement of the blood pressure can give objective data regarding the patient. There are many methods for blood pressure measurement; the two most common noninvasive methods are using a Doppler ultrasonic flow probe and using an automated oscillometric blood pressure device. The Doppler flow probe is used to amplify the pulse sound, and combined with a cuff and an aneroid manometer, a blood pressure that may represent either the mean or systolic blood pressure is obtained. The correlation of the Doppler blood pressure to the actual blood pressure is not consistent over studies, and some have found it to more closely represent the systolic arterial blood pressure (SAP) and others the mean arterial blood pressure (MAP). The safest approach is to assume that the Doppler is representing the SAP, so that hypotension is not undertreated. The placement of a catheter directly into an artery may be used to measure direct arterial blood pressure (DABP), and this is considered the gold standard for accuracy of pressure measurements. Direct arterial blood pressure gives accurate measures for SAP, MAP, and also diastolic blood pressure (DAP), but requires some skill and advanced monitors that can read the pressure transducer output.
Other monitoring that can be done for patients with hemorrhage is an assessment of the respiratory effort and ability. Measurement of respiratory rate and quantification of any change in respiratory effort or respiratory noise is an important metric to monitor serially. Additionally, an assessment of oxygenation status may be obtained by using a pulse oximeter on any mucosal surface. The pulse oximeter generates a value for SpO2, which correlates with and estimates the PaO2. In general, the SpO2 should read above 95% in patients at sea level to be normal. Any values lower than 94% should be treated aggressively, with a search for the cause of hypoxemia and the provision of supplemental oxygen if indicated. Patients with hypoxemia may have cyanotic (bluish) mucous membranes. In patients with hemorrhage that already have a decreased oxygen-carrying capacity in the blood, hypoxemia must be avoided to prevent any additional strain on the body. The PaO2 can also be measured directly, by analyzing a sample of arterial blood (arterial blood-gas). The benefit of blood gas analysis is that the clinician and nurse can obtain information about both the oxygenation (reflected by PaO2) and the ventilation (reflected by PaCO2) status of the patient. Blood-gas analysis also gives a pH reading and a calculated value for bicarbonate ion, both of which may help direct the clinician to the underlying cause of disease.
Laboratory Monitoring and Diagnostics
In patients at risk of hemorrhage or those with active bleeding, measurement of the RBC number is critical to monitor the effectiveness of transfusion therapy and of other therapies to stop bleeding. The RBC concentration and hematocrit (Hct) may be obtained using many commercial CBC machines, but it is easy and straightforward to measure a spun packed cell volume (PCV) to gauge this value. PCV correlates to the Hct and is measured using a small amount of blood, a microhematocrit tube, and a centrifuge that can spin the tube. Most of the glass PCV tubes are coated with anticoagulant (usually marked with a red color stripe on the top of the tube), although there are others (marked with a blue stripe) that do not contain anticoagulant. It is important to use tubes with anticoagulant if the blood is not already anticoagulated. In addition to the PCV, following centrifugation, the tube can be broken and the plasma portion analyzed with a refractometer for the plasma total protein concentration. In acute bleeding, the PCV may remain close to normal (30–45%) while the total protein drops. This is due to splenic contraction, and if a patient appears to have hypovolemic shock but a normal PCV, the total protein concentration must be checked as well, as it will be low (< 5 g/dL) with blood loss.
A blood smear is another helpful diagnostic that can easily be performed in a bleeding patient; verifying the presence of circulating platelets will help rule out thrombocytopenia as a cause of the hemorrhage. If blood is being drawn, a sample placed in 3.2% citrate can be used to check coagulation times (this depends on the machine that is used; some machines run prothrombin time (PT) using whole blood alone. It is important to make sure the correct amount of blood is placed in the citrate; if the tube is overfilled or underfilled, the results may be inaccurate. In concert with this blood work, the blood type of the patient should be determined in case a transfusion is required. This is particularly important in cats, where cats with type B blood will have preexisting antibodies against type A blood. A mismatched transfusion can lead to hemolysis and inflammation, in addition to wasted blood. In dogs, the most antigenic RBC antigen is the 1.1 antigen, and dogs are generally classified as 1.1 negative or positive. Dogs who are 1.1 positive may receive transfusions from dogs that are either positive or negative for 1.1. Dogs who are negative should not receive blood from 1.1 positive donors. In general, animals do not require a crossmatch for the first transfusion, but should be crossmatched to the donor if subsequent transfusions are necessary more than 3 days after the first transfusion.
Monitoring for Blood Transfusions
Even with proper blood typing, transfusion reactions may occur. The most common transfusion reactions are febrile, non-hemolytic reactions, but the possible reactions can range from hemolytic reactions, where the recipient destroys the donor cells through an immunologic mechanism, to allergic reactions (manifested by facial swelling, hives [urticaria], pruritis, fever) or anaphylaxis. The possible reactions are classified as acute or delayed, and immunologic or non-immunologic. Acute reactions are identified during or immediately following a transfusion. Delayed reactions may not become apparent until days after the transfusion. Immunologic reaction describes any reaction caused by antigen-antibody interaction (these may be on RBCs, white blood cells, or plasma proteins and are generally a type I or II hypersensitivity reaction). Acute immunologic reactions include hemolysis, febrile non-hemolytic reactions, allergic reactions, and anaphylaxis. Acute non-immunologic reactions are generally a result of improper storage, collection, or use of the RBC product, and are usually seen as hemolysis from RBC damage in the storage bag, or systemic infection from bacterial growth in the bag. Delayed immunologic reactions are generally delayed immune-mediated hemolysis. Delayed non-immunologic reactions include infectious disease transmission (e.g., Babesia, Ehrlichia) and the development of hemochromatosis.
Because of the risk of transfusion reactions, all transfusions should be started at a slow rate that is gradually increased to a final rate over 30 minutes. Physical exam, blood pressure, heart rate and blood pressure (if indicated) should be measured every 5 to 10 minutes for the first 30 minutes until it does not appear that the patient will have a transfusion reaction, after which the rate can be increased. Due to the danger of bacterial growth in the transfusion products, it is recommended that an entire transfusion be completed within 4 hours of spiking the blood product container. In the emergency setting, if an animal presents with near-fatal anemia, blood products may be bloused, using either a syringe or a pump set. Some pumps may cause trauma and hemolysis to the RBCs so the compatibility of the pump should be verified before use in this context.
Fresh frozen plasma (FFP) is indicated for replacement of coagulation factors in patients with hemorrhage due to inadequate or non-functional factors. While transfusion reactions from FFP are less common than those from RBCs, the monitoring and rate adjustments should be the same with FFP transfusions.
Therapy for Transfusion Reactions
Most febrile transfusion reactions are self-limiting and respond to slowing of the transfusion rate. If this does not resolve the reaction, or if the patient shows signs of pruritis, urticaria, or edema, it may be necessary to discontinue the transfusion and administer diphenhydramine or corticosteroids. Anaphylaxis should be treated with immediate discontinuation of the blood transfusion and may require epinephrine (adrenaline), IV fluid therapy, oxygen supplementation, or vasopressors.