When Fluids Aren T Enough: Next Steps in Treating Hypotension

When Fluids Aren’t Enough: Next Steps in Treating Hypotension Megan Brashear BS, CVT, VTS (ECC) VCA Northwest Veterinary Specialists, Clackamas OR

Blood pressure measurement is becoming more and more common in veterinary practices as medicine becomes more advanced. Many of us are practicing goal-oriented treatment and using blood pressure to steer treatments. But what happens when fluid therapy, the old standby for treating hypotension, doesn’t work to increase the blood pressure? Where do we go?

What is blood pressure? Blood pressure is Cardiac Output x Systemic Vascular Resistance (having to do with blood viscosity and vascular tone). Cardiac Output can be defined as Heart Rate x Stroke Volume (the amount of blood ejected with each heart beat). Blood pressure measures the ability of the heart to push blood throughout the body, and the resistance of the blood vessels against the heart. The blood pressure of our patients can be altered by one or more of the following:

 Heart rate (tachycardia or bradycardia)  Volume status (overhydration or hypovolemia)  Vasodilation  Vasoconstriction

When measuring blood pressure, it is important to your success as a technician and as a hospital to have each employee aware of how to measure a blood pressure, and where that blood pressure is measured. There are multiple machines, techniques, and cuff placement options, and different employees can and will get different readings on the same patient. Record as much information as possible on the medical record to ensure accurate results, including which machine was used on the patient.

Placing an indwelling arterial catheter and using a transducer to measure direct arterial pressure is the gold standard for blood pressure measurement, but is expensive and labor intensive. Non-invasive blood pressure can be measured by using a Doppler unit or by using an oscillometric machine. The following are normal values for non-invasive blood pressure readings:  Systolic: 90-140mmHg Dogs; 80-140mmHg Cats (contraction phase of the heart)  MAP: 60-100mmHg Dog and Cat (mean arterial pressure)

When using a Doppler to measure blood pressure, the number obtained falls somewhere between the patient’s systolic and mean arterial pressure. The mean arterial pressure is what the animal’s organs are experiencing on a continuous basis and should not be allowed to drop below 60mmHg to ensure proper organ perfusion. Proper training of all medical staff is important to ensure not only accurate and consistent results, but also to ensure proper reporting of abnormal results to the veterinarian.

In a 2007 ACVIM Consensus Statement, proper technique for obtaining blood pressure was documented on both dogs and cats. According to this paper – in cats, using a Doppler machine yielded the most accurate results. There was not preferred recumbency for the patient, and using the tail or limbs was recommended; whichever was preferred by the patient. In dogs, an oscillometric reading yielded more accurate results, with the patient in lateral recumbency and the blood pressure cuff placed at the level of the heart. Patients should (if possible) be allowed to acclimate to the environment before attempting to measure their blood pressure. At minimum, four readings should be obtained, the first reading tossed out, and the remaining averaged to get an accurate blood pressure. If the readings vary greatly (>20%), toss out the outliers and continue to measure until you have three readings appropriate to average together. It is also important to look at trends in your blood pressure results. A blood pressure reading is a snapshot of one moment. Look at your patient and relate the results back to them. If the reading is different than what you expected, repeat the measurement a number of times over the next few minutes and watch the trend. Look at other vital signs and relate your results. Consider walking the patient and allowing them to urinate, sitting with them to allow them to acclimate to you, or administering pain management and repeating the blood pressure reading after. Record the cuff size, placement, patient recumbency, and blood pressure machine used on the treatment sheet so the measurements are as standard as possible for that patient. If the information is recorded, monitoring trends becomes more valuable.

The sympathetic nervous system is one of the main controllers of blood pressure; pressure is controlled via receptors within the vascular system. These receptors will dilate or constrict vessels as needed, as well as increase heart rate in an attempt to increase cardiac output. These receptors are sensitive to chemicals within the body and during normal function will elevate and decrease blood pressure as needed. The body will make significant sacrifices in an attempt to regulate and maintain blood pressure, but when this system fails a patient, we can step in and supply the body with assistance in an attempt to maintain the blood pressure.

As mentioned previously, a blood pressure reading gives us an idea of how well the body is circulating blood and perfusing organs. Blood flow equals oxygen flow, and oxygen is vital to life. Hypotension means that your patient is not receiving adequate oxygen delivery to their organs, and if hypotension is persistent that equals cell death and eventually organ dysfunction. Shock is the result of inadequate oxygen delivery to tissues and your ability to recognize the signs of shock in your patient may alert you to a crisis before you get a low blood pressure reading. The causes of shock are numerous, but the common clinical signs are tachycardia, pale mucus membranes, prolonged capillary refill time, poor pulse quality, and cold extremities.

Causes of hypotension in veterinary patients are numerous and varied but important to understand because they will need to be addressed and treated in order to correct the hypotension.

 Dehydration  Hemorrhage  Sepsis  Anaphylaxis  GDV/ATE (obstructive process)  Cardiac disease  Effect of drugs given

When faced with a hypotensive patient it is important to begin treatment as soon as possible, and identify and treat the underlying cause of that hypotension. The first determination to make is whether the underlying problem is cardiac. Patients in cardiogenic shock are treated quite differently than hypovolemic shock so this determination must be made early.

The mainstay of hypovolemic (the most common) hypotension treatment is fluid therapy. Crystalloid fluid therapy will increase preload giving the heart more to pump out and circulate throughout the body. Relatively easy to start and monitor, every veterinary clinic has the supplies necessary for treating hypotension with IV fluids. Goals are set (reduction in heart rate, increase in blood pressure) and fluids are administered until endpoints are reached.

Crystalloids, in the form of balanced electrolyte solutions, are administered first. Crystalloids will provide a quick bump in intravascular volume, but remember that they will shift out of the intravascular space and into the interstitial space about 30 minutes after administration. Remember this when monitoring a hypotensive patient – one normal blood pressure measurement does not equal fixed; if fluid shifts are occurring the blood pressure may drop again and needs to be monitored continuously. If crystalloid fluid therapy is not working to correct the hypotension, it can be because not enough fluids have been administered (depending on the patient and their disease state, large quantities are needed before a change is noted), the patient has cardiac dysfunction, or the patient is battling vasodilation and may need additional drugs to combat this.

Colloids are made of larger molecules than crystalloids and will remain in the intravascular space longer than crystalloids. They will also help to draw fluids towards them thereby increasing intravascular volume. Blood products are also colloids. When dealing with normovolemic hypotensive patients colloid therapy becomes more common. Patients that are hypoproteinemic may also be on colloid therapy as a way to decrease edema. Colloids are not a benign drug, and increasing research from human medicine has introduced doubt into their use in critically ill patients. They still have a place in veterinary medicine, especially blood products, but should not be introduced without appropriate thought.

With some patients, their hypotension is not caused by a loss of fluid or blood, but by a systemic illness. Inflammation causes hormone release that leads to vasodilation and hypotension. Septic patients can be hypotensive and normovolemic, and correcting their hypotension with fluid challenges can be detrimental to their recovery. SIRS, anaphylaxis and cardiac disease patients can present the same challenge when treating their hypotension. Continue to treat the underlying cause, but fluid therapy will not correct the hypotension in these patients. In these situations we will add drug therapy.

In normally functioning patients, the sympathetic nervous system (the fight or flight response) will control alpha and beta receptors in the vasculature and heart to vasoconstrict and increase heart rate to compensate for what the animal is experiencing. The body performs this adaptive response to stimulus and keeps cells oxygenated and blood pressure maintained. When this system is not able to keep blood pressure maintained we can administer drugs that will work on those alpha and beta receptors to give the desired result of increased blood pressure.

 DOPAMINE: Dopamine will stimulate adrenergic receptors (to stimulate the sympathetic nervous system) and stimulate the myocardium, so by giving dopamine you are causing vasoconstriction and increased cardiac contractility to increase blood pressure. The effects are dose dependent with higher doses risking arrhythmias, extreme tachycardia, and hypoxia due to elevated myocardial needs for oxygen. Dopamine has a very short half-life and so it must be administered as a CRI and the dose changed according to patient response. Most patients will receive a dose range of 5-10mcg/kg/min but can go >10mcg/kg/min.  DOBUTAMINE: Dobutamine will stimulate myocardial receptors to increase cardiac output without increasing the heart rate and having a minimal effect on vascular resistance. At high doses, tachycardia and arrhythmias can occur. Cats on dobutamine have a higher incidence of CNS signs (tremors and seizures) and doses higher than 5mcg/kg/min. Dobutamine has a very short half-life and must be administered as a CRI at a dose range of 5-20mcg/kg/min (dogs).  NOREPINEPHRINE: Norepinephrine plays an important role in the fight or flight response of the body. When administered IV, it causes potent vasoconstriction without significantly affecting cardiac output and heart rate. Because of this, it is best to use norepinephrine when you know the patient’s cardiac function, as heart disease can be exacerbated by norepinephrine administration. Norepinephrine is the first-line choice for treating hypotension related to sepsis as it will not affect cardiac output or heart rate. It can cause sloughing if given outside the vein so careful attention must be paid to the infusion site. It is only given as a CRI and the effects are dose dependent and titrated until the desired outcome is reached. The dose is started at 0.5mcg/kg/min.  VASOPRESSIN: Vasopressin is antidiuretic hormone in the body. Its role is to conserve body water in times of need. There are vasopressin receptors located throughout the body, and when activated, will cause potent vasoconstriction. It is administered as a CRI with a dose range of 0.1-0.4U/kg/min; higher doses can lead to coronary and splanchnic vasoconstriction, decreased cardiac output, and death. While on any of the aforementioned drugs, patients must be closely monitored to gauge their response. CRI rates are adjusted up and down and the technician needs to be watching blood pressure readings at least every 5 minutes. Patients are given 15-20 minutes to respond on the medications before titrating up the rate 20% at a time. Direct arterial monitoring is ideal but if not available, closes indirect monitoring is necessary.

While blood pressure monitoring is vital to therapy, there are other parameters to watch as well to determine how the patient is handling fluid and drug therapy. When we are dealing with these critical patients we cannot overlook their other needs.

 Body Weight: Blood pressure challenges often mean fluid challenges and high rates of many different types of fluids. Body weight should be measured in these patients at least twice a day as an indicator of fluid loss and gain. Even small changes can be significant to the patient.  Mucus membranes: Pay attention to tacky vs. moist, as this is a simple indicator of hydration and how well the patient is using the fluids.  Skin Turgor: Noticing when a patient goes from normal to gelatinous is an important step to preventing edema and the clinical signs of fluid overload.  Urine Output: Measure ins and outs on these patients. Be sure the DVM is aware when ins are far outpacing outs.  Urine SpGr: As well as volume, measure how concentrated the urine is. The outs are behind the ins, but is it because the patient is still dehydrated?  CVP: Trending CVP values may help in determining a patient’s intravascular volume and help mark progress in hydration.  Lactate: Lactate measurements can help you determine perfusion and if the patient is able to circulate the volume that it has. Increasing lactate values tell you that more work needs to be done.

It is important to know what drugs to turn to when blood pressures aren’t where they should be, but with these critical patients it’s not only drugs. Looking at disease process, all of their vital signs and monitoring them closely is important.

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