Lecture 14: CV Control through Regulation of Cardiac Output Flashcards
What is the role of CO and the importance of its regulation?
- Transport gases and substrates to and from organs and tissues at a rate commensurate with their requirements
- match blood flow to requirements for total body, local organ and tissue
- excessive CO is nonproductive work!
- oxygen delivery is frequently sued as the cardiac output-defining parameter
- not always the sole determinant of blood flow requirement (kidney, skin, GI tract)
What is the primary parameter that defines cardiac output?
Oxygen delivery
What is the primary parameter that matters for kidney in terms of cardiac output?
Volume rather than oxygen since it is used for filtration
What is the normal value range of CO?
Units = L/min
300 L/hr
7200L/day
200 x 10^6 liters in a lifetime
What is the cardiac index?
Cardiac output normalized to body size
L/min/m^2
Normal cardiac index (normalizes to body size): 2.5-3.5 L/min/m^2
What are the determinants of cardiac output?
No known sensors
Determined as a resultant of other inputs and reflexes such as
i. vascular resistance local metabolically mediated
ii. autonomic and neural inputs to the heart and vasculature
iii. local and central factors interact to determine global systemic vasculature resistance
Cardiac output is adjusted to maintain satisfactory arterial pressure
How do you measure cardiac output?
- By using the indicator dilution technique
- fundamentally a conservation of mass expression
- Addition of an indicator to a flowing stream produces a change in the indicator concentration downstream (therefore your measuring change in liters/min, ala units of CO)
- The magnitude of the concentration change is related
i. directly to the indicator addition rate
ii. inversely to the stream flow rate - Fick principle (oxygen as the indicator)
- Thermal dilution (heat as the indicator)
What is the fick cardiac output technique?
Oxygen is the indicator
May be considered as added to blood through lungs or removed as it passes through systemic circulation
CO = VO2/AVO2D
VO2 = total body oxygen consumption
AVO2D = change in arterial venous oxygen (amount of oxygen bound in Hb)
What are the characteristics of the thermal dilution technique?
Catheter with tip thermistor positioned in the main pulmonary artery
Injection of bolus of room T or iced saline in right atrium
Thermistor is cooled by injectate bolus as it is passed
CO may be calculated by integrating area under the time-temperature curve
-small area = high CO
-large area = low CO
What are the systemic venous O2 saturations?
Normal mixvenous O2 saturation is 75%
¼ of O2 transport capacity is actually utilized
Coronary sinus = 40%
Renal = 85%
What is the significance of mixed venous O2 saturation measures?
Mixed venous O2 measures the adequacy of cardiac output to meet metabolic requirements
- useful clinical parameter easily measured with monitoring catheters
- three-fold increase in metabolic requirements can be met without a change in CO
- conditions with pathologically low cardiac output will have low mixed venous O2 saturation
- *if it’s low, your patient is in trouble**
What is mixed venous oxygen saturation (SvO2)?
Percentage of oxygen bound to hemoglobin in blood returning to the right side of the heart
What are the components of the cardiac output?
CO = HR x SV
Heart rate can fluctuate at a 3-fold range (60-180)
SV is determined by preload, afterload, inotropic state, autonomic NS
What are the physiologic interactions that affect CO?
Increased heart rate = abbreviates diatolic filling tme so may reduce preload
Arterial pressure increases afterload and reduces ventricular ejection and stroke volume
What are the compensatory mechanisms for consequences of increased HR?
Most filling occurs during 1/3 of diastole so heart rates within the physiologic range do not impair diastolic filling
