VETgirl October 2024 Beat e-Magazine

QUARTERLY BEAT / OCTOBER 2024

EFFECT OF HEART RATE & RHYTHM ON BLOOD PRESSURE The heart is a smart organ. Baroreceptors present in the heart respond to changes in variables within the Figure 1 equation. For example, in states of hypotension resulting from hypovolemia (i.e., reduced afterload and preload), the heart rate increases to increase cardiac output and maintain blood pressure—think about patients in hypovolemic shock. The heart’s electrical rhythm influences its mechanical function. Arrhythmias are disruptions in the electrical pathway with or without heart rate abnormality. During arrhythmias patients are at increased risk for reduced cardiac contractility, cardiac output, blood pressure, and potential perfusion. As the heart rate increases, the phase of diastole is minimized. When diastole is shortened, perfusion to the myocardium, ventricular filling, and resulting stroke volume during systole are reduced. This decreased cardiac output because of tachycardia (high heart rates), with or without a tachyarrhythmia, contributes to hypotension and potentially poor perfusion. Similarly, bradycardia (low heart rates), with or without bradyarrhythmias, can also result in decreased cardiac output and blood pressure. Figure 2 demonstrates the relationship of bradyarrhythmia impact on blood pressure in a cat.

HEART RATE & ANESTHETIC DRUGS Most drugs used in the peri-anesthetic period have a dose- dependent effect on heart rate. Table 2 details common peri-anesthetic drugs and their effect on cardiovascular variables seen in Figure 1. As heart rate increases so does myocardial oxygen consumption, along with the incidence of tachyarrhythmias. Extreme increases in heart rate decrease ventricular filling time and stroke volume resulting in decreased cardiac output and blood pressure. Figure 3 is a video that shows a patient in a tachyarrhythmia with resulting pulse pressure variance on the arterial waveform and plethysmograph. Although not hypotensive, stroke volume for some of the cardiac contractions is compromised. Anticholinergics increase heart rate and as a result, increase cardiac output and blood pressure. Conversely, most opioids cause bradycardia. When bradycardia is excessive, cardiac output decreases resulting in reduced blood pressure. Buprenorphine, a partial mu opioid agonist, does not have profound cardiovascular effects at clinical doses. Alpha2 adrenergic agonists cause a reduction in heart rate as a reflex response to an increase in systemic vascular resistance, vasoconstriction. While blood pressure increases due to vasoconstriction, perfusion often decreases. When alpha2 adrenergic agonists are used as premedication agent prior to halogenated inhalants, which cause a decrease in contractility and vasodilation, and opioids, which augment bradycardia, this results in a reduced cardiac output and hypotension. Acepromazine causes a reduction in systemic vascular resistance and a resulting decrease in blood pressure with a potential increase in perfusion. In combination with other peri-anesthetic drugs, hypotension could result. The important concept to recognize is that blood pressure and perfusion do not have a linear relationship. WHAT IS THE ‘IDEAL’ HEART RATE UNDER ANESTHESIA? The ideal heart rate under anesthesia for dogs and cats is a heart rate that maintains blood pressure and optimizes tissue perfusion. The author prefers to perform a pre-anesthetic assessment of each patient to identify the patient’s specific resting heart rate and blood pressure prior to the administration of any peri-anesthetics, when possible, to maintain a heart rate within 20-30% of the patient’s specific resting heart rate. With the use of alpha2 adrenergic agonists, this general guideline often requires allowances for heart rates lower than 20-30% less than the patient’s specific resting heart rate. When this occurs, blood pressure is used to assist the anesthetist in

TABLE 2: COMMON PERI-ANESTHETIC DRUGS USE IN CATS AND DOGS ON CARDIOVASCULAR PARAMETERS (2)

EFFECT ON CARDIOVASCULAR PARAMETERS*

PERI-ANESTHETIC DRUG CLASSIFICATION

PERI-ANESTHETIC

CONTRACTILITY

SVR

Atropine/glycopyrrolate

Anticholinergic

NC**

/NC

Morphine, hydromorphone, methadone, fentanyl

Full agonist opioid

Butorphanol 3

Opioid agonist (kappa)/ antagonist(mu)

/NC

Acepromazine 4

Phenothiazine

Dexmedetomidine/ medetomidine/xylazine 5

Alpha2 agonist

/NC

Propofol 6

Non-barbiturate sedative/ hypnotic

/NC /NC

Aflaxalone 7-9

Neurosteroid, non- barbiturate sedative/ hypnotic

Ukn***

Ketamine

Dissociative agent

Midazolam/diazepam

Benzodiazepine

Etomidate 9

Imidazole derivative, non-barbiturate sedative/ hypnotic

/NC /NC

/NC

Isoflurane/sevoflurane

Halogenated inhalants

*A drug effect on cardiovascular parameters is often dose dependent where higher doses generally produce more significant change; NC** = no change; Ukn*** = unknown Table 2. Common Peri-Anesthetic Drugs Use in Cats and Dogs on Cardiovascular Parameters. Table courtesy of Amanda M. Shelby, RVT, VTS (Anesthesia & Analgesia)

determining if the heart rate requires a treatment response. If a patient experiences hypotension with a low heart rate, an anticholinergic may be used to increase heart rate and cardiac output resulting in improved blood pressure. Conversely, when bradycardia is associated with hypertension because of administration of an alpha2 adrenergic agonist, treatment is often not warranted. When tachycardia with hypotension under anesthesia is experienced, identifying the contributing cause of hypotension is prioritized. Rarely does the tachycardia require direct treatment with a beta blocker or calcium channel blocker.

Regardless of brady- or tachycardia, electrocardiographic (ECG) evaluation should be performed. Heart rates with accompanying dysrhythmias often have a negative impact on cardiac output, blood pressure, and perfusion. With arrhythmias, priority is given to identification of the arrhythmia, resolving the underlying cause, and returning to normal electrical rhythm and rate.

Figure 2. Impact of Bradyarrhythmia on Blood Pressure; Photo courtesy of Amanda M. Shelby, RVT, VTS (Anesthesia & Analgesia)

BLOG HIGHLIGHTS

VETGIRL BEAT EMAGAZINE | VETGIRLONTHERUN.COM

Made with FlippingBook - Online Brochure Maker