EXAM #1: CARDIAC OUTPUT Flashcards
What is the definition of CO?
Volume of blood ejected from the heart per minute
What is the Stroke Volume?
Volume of blood ejected from the heart per beat
What is the equation for CO?
CO= HR x SV
What is the normal CO?
5 L/min
What is the Cardiac Index?
CO x SV x m2
m2= unit of body surface area
What is the normal Cardiac Index?
3 L/min/m2
What are the major contributions to the EDV?
1) Atrial kick (20%)
2) Ventricular filling (80%)
How does a-fib affect CO?
Loss of atrial kick lowers CO
What is the preload?
End diastolic volume
What are the three major determinants of SV?
1) EDV
2) Ventricular contractility
3) Afterload
What is preload?
Tension exerted on the cardiac ventricular muscle when it begins to contract
Directly related to EDV
Draw and label the ventricular pressure volume loop.
N/A
What is afterload?
Pressure the ventricle has to expel blood against
Draw the ventricular pressure volume loop with an increase in afterload.
N/A
What is the End Systolic Volume (ESV)?
Volume of blood left in the ventricle after systole
What are the three major determinants of ESV?
1) Preload (determines contractility)
2) Ventricular contractility
3) Afterload
What is the effect of increased preload on stroke volume?
Increased preload will INCREASE stroke volume TO A POINT
What is the Frank-Starling Law of the Heart?
Ventricular contractility is determined by EDV
- Increased EDV= increased contractility
- Decreased EDV= decreased contractility
What is the mechanism of the Frank-Starling Law of the heart?
Myocardial sarcomere length determines the sensitivity to Ca++
- Increased length= earlier contraction
- Decreased length= later contraction
What happens with inadequate preload?
Decreased CO and hypotension
What can happen with excessive preload?
Decompensation and decreased CO
Remember, the contractility only increased with increased EDV TO A POINT
What happens to SV with increased afterload?
SV DECREASES with increased afterload
What is the ANREP effect?
Aortic pressure elevates abruptly and leads to a POSITIVE INOTROPIC effect within 1-2 minutes
Think the hand grip maneuver
What is the mechanism of the ANREP effect?
- Increased LV wall tension
- Increases Na+
- Increases Ca++ sensitivity
- Increased myocardial contraction
What happens to SV with increased contractility?
Increased SV
What does a sympathetically activated state i.e. excersicse do to the Frank-Starling Curve?
Shifts up and left
Less stretch/volume is needed for the same CO
How is the Frank-Starling curve shifted in HF?
Down and to the right
More stretching/ volume is needed compared to normal to produce the same increase in CO
How does hypoxia/ hypercapnia alter myocardial contractility?
Reduces contractility
What is the force-frequency relationship or the heart called?
1) “Bowditch Phenomenon”
2) Treppe Phenomenon
Increased HR= increased contractility
What factor opposes the Bowditch Phenomenon?
Decreased ventricular filling
Note that up to 180 bpm CO will be normal despite decreased ventricular filling
How is CO measured in practice?
Pulmonary Artery Catheterization (PAC) thermodilution with cold fluid bolus
How is PAC with thermodiluation performed?
- Catheter into the PA
- Cold saline injected and exits a hole in catheter in RA
- Temp sensor in PA
Time it takes for temp. to return to normal is how the CO is determined
Is a small curve or a large curve with the PCA thermodiluation indicative of a larger CO?
Small curve= larger CO
