Assessing Stroke Volume: Best Practices For Nurses

Stroke volume is a critical component of cardiac function and is defined as the volume of blood pumped by the heart during one minute or the volume of blood ejected from the left ventricle in a single heartbeat. It is influenced by several factors, including preload, afterload, and contractility of the left ventricle. To calculate stroke volume, a nurse would need to know the patient's cardiac output and heart rate, which can be determined using several methods. Some of these methods include measuring the amount of oxygen in the veins and arteries, comparing blood temperatures, and using portable devices or imaging techniques. Additionally, factors such as blood pressure, age, gender, and physical and mental condition can impact stroke volume calculations.

Calculating stroke volume

Stroke volume is a metric that serves as a key indicator of health and efficiency. It refers to the volume of blood pumped by the heart with each contraction. Assessing it involves intricate processes and considerations.

Understanding the Components

The main considerations for calculating stroke volume are:

• End-Diastolic Volume (EDV): The volume of blood in the ventricles at the end of diastole before contraction. It can be measured using imaging techniques like echocardiography or cardiac catheterization.
• End-Systolic Volume (ESV): The volume of blood in the ventricles at the end of systole after contraction. Like EDV, it is determined through imaging or invasive methods.

Calculation Methods for Stroke Volume

There are several methods to calculate stroke volume:

• Echocardiography: This method utilizes ultrasound to visualize the organ’s structure and measure blood flow. It allows for the assessment of EDV and ESV to derive the stroke volume.
• Cardiac Catheterization: An invasive procedure involving the insertion of a catheter into the heart chambers to directly measure pressures and volumes. It enables accurate determination of EDV and ESV.
• Thermodilution Technique: This method involves injecting a known quantity of a cold solution (usually saline) into a central vein and measuring the temperature changes as the injected fluid mixes with the blood in the pulmonary artery. This method is commonly used in intensive care settings.
• Doppler Techniques: Doppler ultrasound is used to measure blood flow velocities. This can be used in conjunction with the vessel's cross-sectional area to assess stroke volume.
• Impedance Cardiography: This technique measures changes in electrical impedance across the chest with each heartbeat, which correlates with changes in stroke volume.

Stroke Volume Formula

The stroke volume can be calculated using the following formulas:

• SV = EDV – ESV: Stroke volume is the difference between end-diastolic and end-systolic volumes.
• Cardiac Output (CO) = SV x Heart Rate (HR): Multiplying the stroke volume by the heart rate gives the cardiac output, which is another important metric in assessment.
• SV = CO / HR: Stroke volume can also be calculated by dividing the cardiac output by the heart rate, especially when CO and HR are measured directly or estimated through other means.

Factors Affecting Stroke Volume

It is important to note that stroke volume can vary based on an individual's heart health, physical activity, age, gender, and body size. It is also influenced by three main variables: contractility, preload, and afterload.

• Contractility: The force of myocyte contraction, referred to as the heart's inotropy. An increase in contractility generally leads to an increase in stroke volume.
• Preload: All factors contributing to passive muscle tension in the muscles at rest. An increase in preload generally causes an increase in stroke volume.
• Afterload: All factors contributing to total tension during isotonic systolic contraction. An increase in afterload generally leads to a decrease in stroke volume.

Using cardiac output

Stroke volume (SV) is the volume of blood ejected from the left ventricle in a single heartbeat. It is equal to the difference between the left ventricular end-diastolic volume and the left ventricular end-systolic volume.

Cardiac output (CO) is the volume of blood pumped by the heart during one minute. It is calculated by multiplying the heart rate (HR) by the stroke volume. This can be done using a cardiac output equation or formula.

SV is influenced by several factors, including preload, afterload, and contractility. Preload refers to the stretch on the ventricles before contraction, afterload refers to the resistance the heart must overcome to eject blood, and contractility refers to the strength of the contraction.

When determining stroke volume, a nurse can use several data points, including the patient's blood pressure, heart rate, age, sex, and physical and mental condition. These factors collectively influence the heart's pumping efficiency and help assess cardiovascular health.

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Cardiac output is a crucial metric in understanding the health and strength of the heart. It is calculated by multiplying the heart rate by the stroke volume. By using this formula, nurses and healthcare providers can gain valuable insights into the heart's performance and make informed decisions about diagnoses and treatments. Cardiac output is typically measured in litres per minute, with a normal range of 5 to 8 litres for a resting healthy individual.

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The stroke volume is influenced by several factors, including preload, afterload, and contractility. Preload refers to the amount of blood that fills the ventricles before contraction, and it can be increased by factors such as exercise or pregnancy. Afterload refers to the resistance that the heart must work against to pump blood, and it is influenced by factors such as high blood pressure or aortic stenosis. Contractility is the strength of the heart's contraction, and it can be affected by sympathetic stimulation or the release of norepinephrine.

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By understanding the factors that influence stroke volume, nurses can more accurately interpret cardiac output measurements. For example, if a patient has a high heart rate but a low cardiac output, it could indicate an issue with preload or afterload. Additionally, certain populations, such as athletes or the elderly, tend to have unique cardiac output profiles due to differences in stroke volume. Athletes may have higher stroke volumes due to increased contractility and venous tone, while the elderly may have lower stroke volumes due to reduced ventricular wall compliance.

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Nurses can utilise various methods to measure cardiac output, some of which are more invasive than others. One common method is to use a device that measures heart rate and stroke volume simultaneously, providing a continuous assessment of cardiac output. Another method is transthoracic echocardiography, which estimates stroke volume. In certain situations, such as during surgery or critical illness, optimising cardiac output may be more appropriate than aiming for normal values.

Measuring blood ejected from the left ventricle

Stroke volume is a key indicator of cardiac health and function. It is defined as the volume of blood pumped out of the heart's left ventricle during each systolic contraction.

Measuring the volume of blood ejected from the left ventricle is crucial for determining stroke volume and understanding cardiac function. Here are some methods used to measure this volume:

• External Doppler Imaging: This technique involves using an ultrasound probe along the chest wall cavity to measure blood flow. While it is a popular method, it is technically challenging and not commonly used for critically ill patients as it requires serial measurements.
• Esophageal Doppler Imaging: This method involves inserting a probe into the esophagus to collect Doppler signalling data from the descending aorta. This technique can be used for sedated patients in the operating room or intensive care unit.
• Endotracheal Bioimpedance: This approach measures stroke volume and cardiac output in intubated patients by assessing the impedance signal from the ascending aorta.
• Echocardiography: This non-invasive technique uses ultrasound to visualise and assess the heart. It is commonly used to measure the left ventricular ejection fraction, which is the proportion of blood ejected from the left ventricle during each contraction.
• Cardiac Catheterization: This invasive method involves measuring oxygen levels in the inhaled and exhaled breath and obtaining blood samples from an artery and a catheter in the pulmonary artery. This provides information about cardiac output and pressures inside the heart.
• Dye Injection: Injecting dye into the blood as it enters the right side of the heart and measuring the amount that comes out of the left side.

These methods allow healthcare professionals to calculate stroke volume and assess cardiac function, helping them make diagnoses, evaluate treatment effectiveness, and optimise patient care.

Assessing the patient's physical and mental condition

A patient's physical and mental health are closely intertwined, and so a nurse should assess both to determine the patient's overall health.

Physical Health Assessment

Physical health assessments can be done by a nurse or general practitioner

Estimating the patient's heart rate

Estimating a patient's heart rate is a simple and effective way to assess their health. It can help monitor their overall fitness level and identify potential heart conditions.

Manual Measurement

To manually measure a patient's heart rate, follow these steps:

• Place the pads/tips of your index and middle fingers on the patient's neck, just under the jawline. Alternatively, you can press them on the inside of the patient's wrist, below the base of the thumb.
• Gently press your fingers against the patient's skin.
• Count the number of beats you feel in 15 seconds. Use a stopwatch or timing device for accuracy.
• Multiply the number of beats by 4 to get the heart rate per minute.
• For a more accurate reading, repeat the process a few times and calculate the average.

Using a Device

You can also use devices such as an Apple Watch, Fitbit, or smartphone apps to measure heart rate. These devices use a patient's pulse to sense the pulse in their finger or wrist and provide a good estimate of their heart rate.

Factors Affecting Heart Rate

It is important to note that a person's heart rate can vary depending on their age,

Frequently asked questions