Cardiac Output | Preload and Afterload EXPLAINED!

Dr Matt & Dr Mike
20 Apr 202214:19
EducationalLearning
32 Likes 10 Comments

TLDRIn this informative video, Dr. Mike Turdovich delves into the critical concept of cardiac output in cardiology, explaining its direct impact on blood pressure—a significant health concern. He defines cardiac output as the volume of blood ejected from the heart per minute, which is determined by the stroke volume (blood volume per contraction) and heart rate (contractions per minute). On average, the stroke volume is about 70 milliliters, and the heart rate is around 70 beats per minute, resulting in a cardiac output of approximately 4.9 liters. Factors influencing stroke volume include venous return, preload (the stretch on ventricle walls when the heart is maximally filled), and afterload (the resistance blood faces as it leaves the ventricle). The autonomic nervous system, particularly the sympathetic and parasympathetic systems, regulates heart rate. Positive ionotropic agents increase calcium availability for stronger contractions, while calcium blockers or electrolyte imbalances can decrease it. The video is an insightful exploration of the mechanics of the heart and how various factors can affect its performance.

Takeaways
  • 💓 **Cardiac Output Definition**: Cardiac output is the volume of blood ejected from the heart per minute, which is crucial for blood pressure regulation.
  • 🔢 **Components of Cardiac Output**: It is determined by two factors - stroke volume (the volume of blood ejected per heart contraction) and heart rate (the number of contractions per minute).
  • 📏 **Stroke Volume Calculation**: Stroke volume is calculated as the difference between the end diastolic volume (EDV, around 130 milliliters) and the end systolic volume (ESV, around 60 milliliters), averaging around 70 milliliters.
  • 🏃 **Venous Return Influence**: Exercise and muscle contractions can increase venous return, which fills the heart more and can increase stroke volume and cardiac output.
  • 🧘‍♂️ **Preload and Frank-Starling Mechanism**: The preload, or the stretch on the ventricle walls at EDV, is directly related to the force of heart contraction and stroke volume due to the Frank-Starling mechanism.
  • 🚫 **Calcium Role in Contraction**: The heart's contraction is calcium-dependent, with ionotropic agents either promoting or inhibiting calcium levels to affect contractility.
  • 🚿 **Afterload and Ejection**: Afterload, the resistance blood faces as it leaves the ventricle, is inversely correlated with stroke volume. Factors like atherosclerosis can increase afterload and decrease stroke volume.
  • ❤️ **Heart Rate Variability**: Heart rate can be influenced by the autonomic nervous system, with the sympathetic nervous system increasing it and the parasympathetic nervous system decreasing it.
  • ⚖️ **Ionotropic and Chronotropic Effects**: Ionotropic agents affect the force of heart contraction, while chronotropic agents influence the timing (rate) of contraction.
  • 🔄 **Regulation of Cardiac Output**: Cardiac output can be regulated by altering stroke volume or heart rate, which in turn can be influenced by factors such as venous return, preload, calcium levels, and afterload.
  • 👨‍⚕️ **Importance of Cardiac Output**: Understanding cardiac output and its determinants is vital for medical professionals as it contributes significantly to overall cardiovascular health and blood pressure management.
Q & A

  • What is cardiac output and why is it important in cardiology?

    -Cardiac output is the volume of blood ejected from the heart per minute. It is important because it contributes to blood pressure, which is a significant factor in cardiovascular health and a leading cause of death.

  • What are the two primary factors that determine cardiac output?

    -The two primary factors are stroke volume, which is the volume of blood ejected in a single contraction, and heart rate, which is the number of contractions per minute.

  • How is stroke volume defined in the context of the heart's function?

    -Stroke volume is the amount of blood ejected from the heart every time it contracts, which is determined by the end diastolic volume minus the end systolic volume.

  • What is the average stroke volume in milliliters?

    -The average stroke volume is about 70 milliliters.

  • What is the average heart rate in beats per minute?

    -The average heart rate is about 70 beats per minute.

  • How is cardiac output calculated using stroke volume and heart rate?

    -Cardiac output is calculated by multiplying the stroke volume by the heart rate. For example, if the stroke volume is 70 milliliters and the heart rate is 70 beats per minute, the cardiac output would be 4.9 liters per minute.

  • What is venous return and how does it affect cardiac output?

    -Venous return is the amount of blood returning to the heart via the veins. It affects cardiac output by influencing the amount of blood available to be ejected with each heartbeat, thereby affecting stroke volume.

  • What is preload in the context of the heart's contraction?

    -Preload is the stretch that the blood places on the walls of the ventricles when the heart is maximally filled at the end diastolic volume. It is directly related to stroke volume due to the Frank-Starling mechanism, which states that the more the heart is stretched, the greater the reflexive contraction and the more blood that gets ejected.

  • How does calcium influence the contraction of the heart and stroke volume?

    -Calcium is essential for muscle contraction. Positive ionotropic agents increase the availability of calcium, promoting stronger contractions and higher stroke volume. Conversely, negative ionotropic agents decrease calcium availability, leading to weaker contractions and lower stroke volume.

  • What is afterload and how does it impact stroke volume?

    -Afterload is the resistance that the blood experiences as it leaves the ventricle, often due to the diameter of the arteries. An increase in afterload, such as from atherosclerosis, increases the resistive force on the blood, leading to a decrease in stroke volume. Afterload is inversely correlated with stroke volume.

  • How do the sympathetic and parasympathetic nervous systems affect heart rate?

    -The sympathetic nervous system, responsible for the fight or flight response, increases the heart rate by innervating the sinoatrial and atrioventricular nodes. The parasympathetic nervous system, particularly the vagus nerve, decreases the heart rate by signaling the heart to slow down. These effects are known as chronotropic effects.

Outlines
00:00
😀 Understanding Cardiac Output and Its Importance

Dr. Mike Turdovich introduces the concept of cardiac output as a critical factor in cardiology, explaining how it affects blood pressure. He defines cardiac output as the volume of blood ejected from the heart per minute, which depends on two factors: stroke volume (the volume of blood ejected per contraction) and heart rate (the number of contractions per minute). The video further breaks down the stroke volume as the difference between the end diastolic volume (EDV) and the end systolic volume (ESV), with an average stroke volume of about 70 milliliters. Factors that can affect stroke volume are also mentioned, setting the stage for a deeper discussion in subsequent paragraphs.

05:01
🏃‍♂️ Factors Influencing Stroke Volume and Cardiac Output

The second paragraph delves into the calculation of cardiac output by multiplying stroke volume by heart rate, with an average heart rate of about 70 beats per minute, leading to an average cardiac output of 4.9 liters per minute. Dr. Turdovich then discusses three main factors that can change stroke volume: venous return, the contraction of the heart, and ejection of blood (afterload). He explains how exercise can increase venous return and preload, which affects stroke volume through the Frank-Starling mechanism. The importance of calcium for heart contraction is highlighted, with ionotropic agents being able to either increase or decrease the availability of calcium. Lastly, afterload, the resistance the blood encounters as it leaves the heart, is introduced as inversely correlated with stroke volume.

10:02
🚑 Autonomic Nervous System's Impact on Heart Rate

The final paragraph focuses on heart rate and its regulation by the autonomic nervous system. It differentiates between the sympathetic and parasympathetic nervous systems, explaining how the former can increase heart rate through its effects on the sinoatrial and atrioventricular nodes, while the latter, primarily through the vagus nerve, can decrease it. These effects are referred to as chronotropic effects, with 'chrono' relating to time and thus impacting the timing of heart contractions. The paragraph concludes by summarizing the factors influencing cardiac output and inviting viewers to engage with the content and the creator on social media.

Mindmap
Keywords
💡Cardiac Output
Cardiac output refers to the volume of blood pumped by the heart per minute. It is a critical concept in cardiology as it directly contributes to blood pressure, which is a significant health concern. In the video, Dr. Mike Turdovich explains that cardiac output is determined by the product of stroke volume (the volume of blood ejected per heartbeat) and heart rate (the number of heartbeats per minute). The average cardiac output is about 4.9 liters per minute.
💡Stroke Volume
Stroke volume is the amount of blood ejected from the heart with each contraction. It is a key component in calculating cardiac output, as it represents the volume of blood that is pushed out into the circulation with each heartbeat. The script mentions that the average stroke volume is about 70 milliliters, which, when multiplied by the heart rate, gives the cardiac output.
💡Heart Rate
Heart rate is the number of times the heart beats per minute. It is one of the two factors, along with stroke volume, that determine cardiac output. The video states that the average heart rate is about 70 beats per minute. Changes in heart rate can directly affect cardiac output, with an increase in heart rate leading to an increase in cardiac output.
💡Diastole
Diastole is the phase of the heart cycle when the heart is at rest and filling with blood. It is a critical period for venous return, where the heart's relaxation allows blood to flow back into it. The end of the diastole phase is marked by the end diastolic volume, which is the maximum volume of blood the heart can hold when fully relaxed.
💡Systole
Systole is the phase of the cardiac cycle during which the heart contracts and pumps blood out into the arteries. It follows diastole and is essential for the ejection of blood from the ventricles. The end of systole is marked by the end systolic volume, which is the remaining volume of blood in the ventricle after contraction.
💡End Diastolic Volume (EDV)
End diastolic volume is the amount of blood that maximally fills the heart at the end of the relaxation phase, or diastole. It is an important measure as it represents the volume of blood available for ejection during the subsequent systole. The script mentions that on average, the EDV is about 130 milliliters.
💡End Systolic Volume (ESV)
End systolic volume is the volume of blood remaining in the heart after it has finished contracting (systole). It is used to calculate stroke volume by subtracting ESV from EDV. The script indicates that the average ESV is about 60 milliliters, which, when subtracted from the EDV, gives an average stroke volume of 70 milliliters.
💡Venous Return
Venous return is the flow of blood back to the heart via the veins. It is influenced by factors such as exercise, which can increase venous return by squeezing veins and promoting blood flow back to the heart. Venous return is directly related to preload, the stretch on the ventricles' walls when the heart is maximally filled with blood, and thus affects stroke volume.
💡Preload
Preload refers to the stretch or tension on the heart muscle at the end of diastole when the ventricles are fully filled with blood. It is a determinant of stroke volume due to the Frank-Starling mechanism of the heart, which states that the more the heart is stretched, the stronger the subsequent contraction and the more blood is ejected with each beat.
💡Frank-Starling Mechanism
The Frank-Starling mechanism is a physiological principle that describes the heart's ability to increase the force of contraction in response to an increased volume of blood filling the heart. In the context of the video, it explains how preload affects stroke volume, as a greater preload leads to a stronger contraction and more blood being ejected with each heartbeat.
💡Afterload
Afterload is the resistance that the ventricle must overcome to eject blood into the circulation. It is influenced by the condition of the blood vessels, such as in atherosclerosis, where plaques narrow the arteries and increase resistance. An increased afterload means the heart has to work harder to pump the same volume of blood, which can decrease stroke volume.
💡Autonomic Nervous System
The autonomic nervous system is responsible for the body's involuntary functions, including the regulation of heart rate. It consists of two parts: the sympathetic nervous system, which increases heart rate during 'fight or flight' responses, and the parasympathetic nervous system, which decreases heart rate during 'rest and digest' states. These systems have chronotropic effects on the heart, affecting the timing of contractions.
Highlights

Cardiac output is a critical concept in cardiology as it contributes to blood pressure, a major health concern.

Cardiac output is defined as the volume of blood ejected from the heart per minute.

Two factors determine cardiac output: stroke volume (blood volume per contraction) and heart rate (contractions per minute).

Stroke volume is the amount of blood ejected with each heart contraction, averaging around 70 milliliters.

Heart rate is the number of heartbeats per minute, with an average of about 70 beats per minute.

Cardiac output can be calculated by multiplying stroke volume by heart rate, resulting in an average of 4.9 liters per minute.

Stroke volume can be influenced by factors such as venous return, heart contraction, and ejection of blood.

Exercise increases venous return, which in turn increases stroke volume and cardiac output.

Preload, the stretch on the ventricle walls at maximum filling, is directly related to stroke volume due to the Frank-Starling mechanism.

The Frank-Starling mechanism states that the more the heart is stretched, the greater the reflexive contraction and the more blood is ejected.

Contraction of the heart is calcium-dependent, and ionotropic agents can either promote or inhibit calcium availability.

Afterload is the resistance blood experiences as it leaves the ventricle, and it is inversely correlated with stroke volume.

Atherosclerosis can increase afterload by narrowing artery diameter, reducing the amount of blood ejected from the heart.

Heart rate is regulated by the autonomic nervous system, with the sympathetic nervous system increasing and the parasympathetic decreasing heart rate.

Chronotropic effects refer to the influence on the timing of heart contractions, with the sympathetic nervous system accelerating and the parasympathetic slowing it down.

Factors contributing to stroke volume include venous return, heart contraction due to calcium, and afterload.

The autonomic nervous system plays a key role in regulating heart rate, impacting the fight or flight response and rest and digest functions.

Transcripts

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CardiologyCardiac OutputBlood PressureHeart HealthMedical EducationHealthcareHeart RateStroke VolumeFrank-StarlingVenous Return