4 - Ventricular Function II (Stroke Work, Cardiac Output)

Question 1

Equation: The Fick Principle

How is it often used?

Answer 1

X_tc = Q([X]_a - [X}_v)

X = Tissue consumtpion rate

Q = Blood Flow

Xa = Concentration in artery

Xv = Concentration vein

Oxygen consumption by the myocardium (part of mechanical efficiency of heart)

Question 2

Equation: How do you apply Fick’s Principle to measure myocardial O2 consumtion?

Answer 2

V O₂ by specific organ = Q (blood flow)

O₂ content in Artery / Vein used

VO₂ = Q ([aO₂] - [vO₂])

Question 3

Equation: How do you apply Ficks Principle to measure Cardiac Output (CO)

Where do you collect venous samples?

Answer 3

Cardiac Output = Flow

Q = C.O.

Solve equation for Q:

CO = ( Whole body O₂ Consumption) / ( [aO₂] - [mixed vO₂])

Right Ventricle or Pulmonary Artery

Question 4

Clinical: Measuring Cardiac Output

Thermodilution

Answer 4

Use of Swan Ganz Catheter

Inserted into systemic vein, threaded to right side of heart

Cold saline solution injected to right atrium; measured again at pulmonary artery

Can calculate Right Ventricular Cardiac output

Question 5

Clinical: Measuring Cardiac Output

Imaging Techniques

Answer 5

1. Use time-dependent images of heart to estimate differences between EDV and ESV
   - - -

Require radiolabeled blood components

2. Echocardiography - realtime, 2D images used to estimated changes in ventricular volume

Question 6

Cardiac Index

Answer 6

Cardiac Output per square meter of body surface area

Method to NORMALIZE C.O. for large population

C_index = C_output / BSA

Question 7

Clinical: Pericardial Effusion

Answer 7

Abnormal accumulation of excessive fluid in the pericardial cavity

Causes: Inflammation of pericardium, intrapericardial hemorrhage

Question 8

Clinical: Cardiac Tamponade

Answer 8

Compression of the heart chambers fro pericardial fluid

Symptoms: Hypotension, Reflex Tachycardia

Question 9

What are two purposes of the pericardium?

Answer 9

1. Limits excessive movement of the heart within the mediastinum
2. Limits heat distensibility should there be sudden increase in venous return

Question 10

Equation: Measurement of Stroke Work?

Answer 10

Stroke Work (SW) = Work done by ventricle to eject volume of vlood.

S_work = S_volume * MAP

(MAP = mean arterial pressure)

You can measure this at Mean Aortic Pressure for Left Ventricle, or Mean Pulmonary Artery for Right Ventricle

Question 11

Equation: Cardiac Work

Answer 11

C_work = S_work * HR

Cardiac Work = Stroke Work x HR

Question 12

What does the Pressure-Volume Area (PVA) diagram of the heart represent?

What represents Elastic Potential Energy?

Equation: Pressure Volume Area?

Answer 12

Total Mechanical Energy generated by a single heart beat

Area to left underneath ESPVR, above EDVR

PVA = SW + PE

Question 13

What defines the Mechanical efficiency of the heart?

Equation?

Answer 13

Ration of useful energy produced by the heart (stroke work) to myocardial oxygen consumption

Normal ~ 25%

Cardiac Efficiency = PVA / MVO₂

(MVO2 = Myocardial O2 consumption, calculate with Fick Principle)

(PVA = SW + PE)