FDIC 2024 Preview: C.O. Equals HR x SV: The Important Formula for EMS

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In every EMS class from EMT to paramedic to PHTLS to AMLS and even CPR, we hear about cardiac output. Proper cardiac output is essential to life, but what does it really mean? How many classes have you attended where cardiac output was talked about but never really explained? And how much have you heard about how dynamic the formula can be? How can we use the formula in the Fire/EMS field to get an idea of a patient’s cardiac output?

Initial Considerations

Let’s first look at cardiac output as a measurement of both the amount of blood moving through the heart and how quickly it is moving. This also includes the blood moving through the circulatory system to the organs. In most situations, it is approximately 5L. One part of the formula is heart rate. If the heart rate increases, and the stroke volume remains the same, cardiac output increases. Here’s another way to look at it: If your heart rate lowers and the stroke volume remains the same, your cardiac output is lower.

Heart Rate

For the adult, a normal resting heart rate is 60 to 100 beats per minute (BPM). For our well-trained athletes, a normal resting heart rate could be below 60—possibly in the 40s. Many factors influence heart rate, including the following:

  • Age.
  • Fitness and activity levels.
  • Smoker status.
  • Cardiacvascular disease.
  • High cholesterol.
  • Diabetes.
  • Body temperature.
  • External temperature.
  • Caffeine consumption.
  • Alcohol consumption.
  • Emotional state.
  • Body size.
  • Medications.
  • Any abnormalities in the conduction system of the heart.

High Heart Rate (Tachycardia)

Our heart can increase its BPM. Under normal conditions, the heart responds to the body’s higher demands. The heart can, from an abnormality, become dangerously high. In this situation, we see a decrease in the time allowed to let the ventricles, mainly the left ventricle, fill with blood and then contract, sending blood out of the ventricles. In high-heart rate situations, the heart may be beating so fast that a pulse will not be generated because the blood is not being ejected.

Low Heart Rate (Bradycardia)

For an adult in a resting state, anything below 60 BPM is defined as bradycardia. This can have a variety of causes including heart problems, medicines, or electrolyte imbalances. This low heart rate can be life threating if the heart is unable to maintain a rate that pumps enough oxygen-rich blood to itself and throughout the body. As we noted earlier, if you are a well-conditioned athlete, your resting heartbeat could be below 60 to the mid-40s. For low heart rate situations, the condition may be fixed with pharmacology then electrical therapy, from EMS or in a hospital to heart surgery.

Stroke Volume

The definition of stroke volume is the volume of blood pumped out of the left ventricle of the heart during each systolic cardiac contraction. Three variables affect stroke volume, including preload, contractility, and afterload. Here’s a closer look at each one:

  • Preload: The filling pressure of the heart at the end of diastole.
  • Contractility: The inherent vigor of contraction of the heart muscles during systole.
  • Afterload: The pressure against which the heart muscle must work to eject blood during systole. The normal stroke volume range is 60 to 120 ml. The illustration below is how to calculate stroke volume.

Stroke Volume Formula

SV =  EDV (end diastolic volume) – ESV (end systolic volume)

Stroke Volume is Different than Ejection Fraction

Stroke volume is the difference between the EDV – ESV. The ejection fraction is a measurement, expressed as a percentage, of how much blood the left ventricle pumps out with each contraction. An ejection fraction of 65% means that 65% of the total amount of blood in the left ventricle is pushed out with each contraction. This means that 35% of blood is in the left ventricle when the ESV has ended. A normal heart’s ejection fraction is between 55% and 70%. An ejection fraction measurement higher than 75% could indicate a heart condition, such as hypertrophic cardiacmyopathy. The most widely used test to obtain the ejection fraction is an echocardiacgram.

Cardiac Output Affects Blood Pressure

The formula to calculate blood pressure is blood pressure (BP) = cardiac output x total peripheral resistance (TPR)/systemic vascular resistance. Total peripheral resistance is an important concept to understand because it plays a vital role in the establishment and manipulation of blood pressure. Peripheral resistance is determined by three factors: sympathetic activity constricts peripheral arteries and Parasympathetic activity dilates peripheral arteries naturally in the body. We can provide pharmacologic agents to mimic the body chemistry for constriction or dilation of the vessels.

Cardiac Output Mean Arterial Pressure

Mean arterial pressure (MAP) is the average arterial pressure throughout one cardiac cycle, systole, and diastole. MAP is influenced by cardiac output and systemic vascular resistance, each of which is influenced by several variables. A normal MAP is between 70 and 100 mmHg. If the MAP drops below 60 mmHg, there is a concern there won’t be enough pressure to perfuse organs including the brain and heart. If MAP is above 100 mmHg, the patient may be experiencing a high artery pressure which can contribute to increased oxygen demand by the heart, ventricular remodeling, vascular injury, end organ damage, and stroke. MAP is tightly regulated in order to maintain appropriate perfusion of vital organs.

Cardiac Output Affects Your Kidneys

The heart’s job is to send a continuous supply of oxygenated blood around the body. The kidney filters the blood, extracting waste in the form of urine, and helps regulate the water and salt levels to control blood pressure. Reduction in cardiac output results in a disproportionate reduction in renal perfusion, which consequently leads to a diminished glomerular filtration rate. The amount of blood flowing through the kidneys is huge relative to their size. Renal blood flow is about 1L/min. This constitutes 20% of the resting cardiac output.

Conclusion

When you look at cardiac output, you could be assessing a medical patient or a trauma patient. The cardiac output formula is the foundation for other formulas to be calculated. When cardiac output is not in its normal range, it may cause devastating results within the body. This is why C.O. = HR x SV is such an important formula in EMS.

More: https://www.fdic.com/

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