CO and venous return Flashcards
What are two equations to calculate CO?
CO = SV x HR
CO = MAP/TPR
How is CO maintained during the early stages of exercise?
An increase in HR and an increase in SV
How is CO maintained during the late stages of exercise and why?
HR only as SV plateaus
What are the invasive methods for measuring CO?
Fick principle
Indicator dilution method
Thermodilution method
What are the non-invasive methods of measuring CO?
Doppler US
Impedence cardiography
Electrical cardiometry
MRI
What are the 2 equations related to the Fick principle?
CO = rate of O2 consumption / (arterial O2 content/venous O2 content)
O2 content or AV difference = 0.0136 x Hgb x O2 saturation difference
What is the principle behind the dilution methods for measuring CO?
Rate of blood flow can be determined from the rate of change in the concentration of a substance after a known amount of it has been added to the bloodstream
What also increases with increased contractility of the heart?
SV
What increases contractility of heart?
Catecholamines
Insulin and glucagon
Growth hormone
Thyroid hormone
Increased intracellular Ca++
Decreased activity of Na/Ca exchanger
Digitalis
How does digitalis work?
Blocks Na/K pump to increase intracellular Na, which decreases Na/Ca exchange to increase intracellular Ca
What decreases contractility of heart?
beta1 blockade –> decreases cAMP
systolic HF
Acidosis
Hypoxia and hypercapnia
Non-dihydropyridine Ca channel blockers
What factors increase myocardial oxygen demand?
Increased afterload
Increased contractility
Increased HR
Increased diameter of ventricle (increased wall tension)
Sympathetic activity
What is the equation for EF?
EF = SV/EDV = (EDV-ESV)/EDV
What is a normal EF?
> 55%
What is the equation for cardiac index?
cardiac index = CO/body surface area
What is the purpose of the cardiac index?
Relates heart performance to the size of the individual
Changes in what result in changes in EDV?
Venous return
What factors increase CO?
Anxiety and excitement
Eating
Exercise
High environment temp
Pregnancy
Epinephrine
Hyperthyroidism
What factors decrease CO?
Sitting/standing from supine
Rapid arrhythmias
Heart disease
How does sleep effect CO?
There is no change in CO during sleep
What is the normal value for CO?
5 L/mmHg
At what value does CO normally plateau?
13 L/mmHg
What can increase CO plateau to 25 L/mmHg?
Combination of SyNS stimulation and PsNS inhibition that increases HR and contractility
What is cardiac reserve?
The maximum percentage that the CO can increase above normal
What is the normal cardiac reserve in healthy adult?
300-400%
What is the cardiac reserve in trained athletes?
500-600%
What is the cardiac reserve in pts with HF?
None
What factors influence venous return?
Posture
Ventilation - thoracic pump
Skeletal muscle activity
Vena cavae compression
Hyperviscosity
Venous compliance
What factors increase venous return?
Increased venous pressure
Decreases R atrial pressure
Decreases venous resistance
What are two causes of hyperviscosity?
Polycythemia
Hyperproteinaemia
At what R atrial pressure does venous return become zero normally?
7 mmHg
At what atrial pressure will venous return reach a plateau normally?
Less than or equal to -2 mmHg
