QUARTERLY BEAT / OCTOBER 2024
QUARTERLY BEAT / OCTOBER 2024
HOW DOES THE HEART RATE AFFECT PERFUSION?
pressure is measured to aid in the assessment of patient well- being. When heart rate increases excessively, stroke volume is compromised resulting in decreased cardiac output and potentially low blood pressure. Inversely, low heart rates may decrease cardiac output resulting in low blood pressure when there is reduced systemic vascular resistance.
Heart rate is just one variable that influences the adequacy of cardiovascular circulation (Figure 1). A common reference range for canine heart rates is 60 – 160 beats per minute; however, this wide range of heart rates fails to ensure adequate perfusion across all canine patients in various clinical situations. 1 The heart rate and rhythm in tandem influence optimal stroke volume and cardiac output. Coupled with systemic vascular resistance, blood
MATTERS OF THE HEART: IMPORTANCE OF HEART RATE AND RHYTHM DURING ANESTHESIA
FIGURE 1. CARDIOVASCULAR CIRCULATION
AMANDA M. SHELBY RVT, VTS (Anesthesia & Analgesia)
Ever wonder why ALL your patients under sedation and anesthesia should have an ECG on (plus a pulse oximeter and blood pressure monitoring to boot!)? Read why in this VETgirl blog , where our newest VETgirl team member Amanda M. Shelby , RVT, VTS (Anesthesia & Analgesia) reviews the importance of heart rate and rhythm monitoring in our veterinary patients. Whether you’re a small animal or large animal veterinarian or veterinary technician, you’ll want to read more to learn about the importance of heart rate’s effect on cardiac output!
The body is a complex network of systems working in tandem to optimize survival. In sedated or anesthetized patients, monitoring of the cardiopulmonary systems is paramount. The pulmonary system functions optimally to provide ventilation (i.e., elimination of carbon dioxide) and oxygenation (i.e., oxygen saturation of blood) while the cardiovascular system provides circulation—movement of blood through the body. Working optimally, the cardiopulmonary systems provide perfusion,
the delivery of oxygen to tissues. Perfusion is challenging to directly measure in clinical settings. Subjectively, capillary refill time and mucous membrane color provide indications of perfusion and oxygenation. Objective methods of monitoring oxygenation, ventilation and circulation are provided in Table 1. Regardless, ensuring adequate perfusion to the tissues is a primary goal of patient monitoring and assessment, in conscious or unconscious patients.
Figure 1. Cardiovascular Circulation; Figure Courtesy of Amanda M. Shelby, RVT, VTS (Anesthesia & Analgesia)
DOES NORMAL BLOOD PRESSURE EQUAL ADEQUATE PERFUSION?
TABLE 1. MONITORING METHODSª
a compensatory increase in heart rate can also produce a state of low blood pressure and reduced perfusion. However, some reduction in systemic vascular resistance could increase blood flow to tissues, increasing perfusion. Blood pressure is often viewed as a quantitative variable, obtainable in clinical situations. However, given the many variables identified in Figure 1 that influence blood pressure, a qualitative approach using the data collected from the whole patient assessment requires interpretation by a dedicated, trained anesthetist to make conclusions on perfusion.
The assumption commonly made is that patients with normal blood pressure (i.e., systolic arterial pressure [SAP] over 90- 160 mmHg, mean arterial pressure [MAP] over 60-80 mmHg) have adequate perfusion. 1 Adequate blood pressure does not always ensure adequate perfusion. In states of excessive vasoconstriction, an increase in systemic vascular resistance occurs and low SpO2 values from the pulse oximeter are common. This demonstrates vasoconstriction could result in decreased perfusion. Conversely, excessive vasodilation resulting in decreased afterload, reduced stroke volume without
OXYGENATION
VENTILATION
CIRCULATION
INVASIVE NON-INVASIVE INVASIVE NON-INVASIVE INVASIVE
NON-INVASIVE
· Arterial blood · Gas
· MMC · Pulse
· Arterial or venous · Blood gas
· Capnography · RR & effort
· Direct arterial · Blood pressure · CO monitoring · CVP
· Non-invasive blood pressure · Oscillometric device · Doppler/Cuff/Sphygmomanometer · HR & Rhythm (ECG)
oximetry · RR & effort
· PR & Palpation · Capnography · CRT a MMC = mucous membrane color; RR = respiratory rate; CO = cardiac output; CVP = central venous pressure; HR = heart rate; ECG = electrocardiogram; PR = pulse rate; CRT = capillary refill time Table 1. Monitoring Methods; Table Courtesy of Amanda Shelby, RVT, VTS (Anesthesia & Analgesia)
KEY CONCEPT: BLOOD PRESSURE ≠ PERFUSION
BLOG HIGHLIGHTS
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VETGIRL BEAT EMAGAZINE | VETGIRLONTHERUN.COM
VETGIRL BEAT EMAGAZINE | VETGIRLONTHERUN.COM
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