Download 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:
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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.
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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.
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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.
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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 0.10.4U/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.
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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.
References:
Brown, S., C. Atkins, R. Bagley, A. Carr, L. Cowgill, M. Davidson, B. Egner, J. Elliott, R. Henik, M.
Labato, M. Littman, D. Polzin, L. Ross, P. Snyder, and R. Stepien. "Guidelines for the Identification,
Evaluation, and Management of Systemic Hypertension in Dogs and Cats." Journal of Veterinary Internal
Medicine 21.3 (2007): 542-58. Web.
Creedon, Jamie M. Burkitt., and Harold Davis. Advanced Monitoring and Procedures for Small Animal
Emergency and Critical Care. Chichester, West Sussex: Wiley-Blackwell, 2012. Print.
Norkus, Christopher L. Veterinary Technician's Manual for Small Animal Emergency and Critical Care.
Chichester, West Sussex, UK: Wiley-Blackwell, 2012. Print.
Shih, Andre, Sheilah Robertson, Alessio Vigani, Anderson Da Cunha, Luisito Pablo, and Carsten Bandt.
"Evaluation of an Indirect Oscillometric Blood Pressure Monitor in Normotensive and Hypotensive
Anesthetized Dogs." Journal of Veterinary Emergency and Critical Care 20.3 (2010): 313-18. Web.
Silverstein, Deborah C., and Kate Hopper. Small Animal Critical Care Medicine. St. Louis, MO:
Saunders/Elsevier, 2009. Print.