Cardiac output (CO) and systemic
vascular resistance (SVR)–
the next vital signs?
Tom Archer, MD, MBA
UCSD Anesthesia
October 4, 2010
Blood pressure, while important, does not tell the whole story
about health of the circulation. CO and SVR are important too.
“Normal BP” =
High SVR x Low CO
(e.g. Hemorrhagic or
cardiogenic shock)
“Normal BP” =
Low SVR x High CO
(e.g. Sepsis)
“Normal BP” =
Normal SVR x Normal CO
(e.g. Healthy person)
What if we could easily
measure CO and SVR?
• Assist both intensive and general medical care?
• Fine tune medications (e.g. antihypertensives)?
• Detect and monitor disease (e.g. pre-eclampsia,
heart failure, sepsis, hemorrhage)?
• Encourage healthy life style (diet, weight loss,
exercise)?
Conditions decreasing SVR directly:
• Anemia (viscosity is component of resistance)
• Fever, hyperthyroidism (increased O2 demand)
• Sepsis
• Anaphylaxis
• Neuraxial and other anesthetics
Conditions increasing SVR directly:
• Severe pre-eclampsia
• Essential hypertension?
• Diabetes?
• Smoking?
• Obesity?
Conditions decreasing CO directly:
• Heart failure or cardiogenic shock (MI,
tamponade, cardiomyopathy, bradyarrythmia)
• SVR increases in compensation for decreased
CO in attempt to maintain BP.
Conditions increasing CO directly:
• Pain, fear?
• However– increased CO requires increased
venous return.
• Healthy heart pumps out what it receives
(Frank-Starling mechanism).
• Heart can be seen as “passive” servant of
periphery!
In anesthesia we are often
“cardiocentric” in thinking about CO
• Emphasis is on stroke volume and heart rate.
• Preload, contractile state and afterload.
• Is the heart appropriately contractile and full?
• Do we sometimes forget SVR?
Resistance arterioles
also merit attention!
• How much blood flow are the tissues
demanding?
• Is the tone of the resistance arterioles and
capacitance veins appropriate for health?
• What is the state of the endothelium of the
resistance arterioles?
Are the resistance arterioles
“misbehaving”?
• Excessive tone: pre-eclampsia, essential
hypertension?
• Deficient tone: sepsis, anaphylaxis, neuraxial
block.
Blood
vessels
Heart
In health and disease, heart and blood vessels work
together– the function of one affects the function of the
other.
Independent assessment of CO and SVR might be
helpful in clarifying the relationship of heart, resistance
arterioles and capacitance veins.
Blood
vessels
affect SVR
and CO
Heart affects
CO and SVR
Sick heart (cardiogenic shock)
produces low CO and compensatory
high SVR.
Sick arterioles (sepsis) produces low
SVR and compensatory high CO.
Methods for estimating CO and SVR
• PA catheter (thermodilution or Fick)—highly
invasive, but a gold standard. Can be continuous.
• Echocardiography (TEE or TTE)—a minimally or
non-invasive “gold standard”. TEE difficult on nonintubated patients. Requires training, laborintensive, not continuous. Uses velocity time
integral (VTI) to calculate “stroke distance” or
compares diastolic and systolic LV areas to
calculate “stroke area”.
End-diastolic area - End-systolic area = “stroke area” in one MRI slice. Same idea
applies to echocardiography for calculation of stroke volume.
David K. Shelton, Fundamentals of Diagnostic Radiology 3rd edition, 2007
Methods for estimating CO and SVR
• VTI variants (Cardio-Q, USCOM).
• Ultrasound measures blood flow duration and
velocity in abdominal (Cardio-Q) or thoracic
aorta (USCOM). Labor intensive, noncontinuous and operator dependent.
Velocity-time integral (VTI) = “stroke distance” (SD). SD x aortic diameter = stroke volume
(USCOM advertisement).
Methods for estimating CO and SVR
• Pulse contour analysis (Vigileo, LiDCO).
• Stroke volume from contour of the pulse.
Requires arterial line, “minimally invasive”.
Continuous, operator independent, makes
many assumptions. Best for “trend
following”?
Oxytocin bolus decreases SVR and increases CO at cesarean delivery
(data from LiDCO pulse contour analysis)
Archer TL et al. International Journal of Obstetric Anesthesia (2008) 17, 247–254
Impedance cardiography (IC)
• Non-invasive and continuous. Little training
required. “Hands-free”.
•
• Long history (NASA, 1960’s) and multiple
iterations and algorithms.
• Bo-Med, Cardiodynamics, Cheetah, Cardiotronic).
• All look at same signal but interpret it in different
ways.
All IC systems work with the same signal– but process it differently. Processing
algorithms are patented “intellectual property”.
Bo-Med and
Cardiodynamics work
with impedance change
during systole (-dZ(t).
Cheetah and
Cardiotronic work with
rate of impedance
change during systole
dZ(t)/dt.
C. Schmidt et al British Journal of
Anaesthesia 95 (5): 603–10 (2005)
Cardiac and stroke indices increase with uterine contractions
8
CI
3
90
SI
40
100
HR
80
0
Archer TL and Shapiro A, UCSD, unpublished
15
Minutes
30
In severe pre-eclampsia, MgSO4 and labetalol decrease SVR and increase CO
(data from Aesculon electrical velocimetry)
Archer TL, Conrad B. International Journal of Obstetric Anesthesia, In Press
What about CVP?
• (MAP – CVP) = CO x SVR
• Since CVP is usually much less than MAP, we
may be able to ignore or estimate the CVP
value and still get clinically useful estimates
for SVR.
• So, MAP (approximately) = CO x SVR.
Summary
• Currently, measurement of CO and SVR can be
labor-intensive, invasive, risky, uncomfortable
and non-continuous.
• Easy, painless, non-invasive and continuous
estimation of CO and SVR might improve care
of multiple conditions affecting the heart,
resistance arterioles and capacitance veins.
• CO and SVR might be the next vital signs.