Cardiac output co and systemic vascular resistance svr the next vital signs
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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” =.

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Cardiac output co and systemic vascular resistance svr the next vital signs

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)

Low SVR x High CO

(e.g. Sepsis)

“Normal BP” =

“Normal BP” =

Normal SVR x Normal CO

(e.g. Healthy person)


What if we could easily measure co and svr
What if we could easily story about health of the circulation. CO and SVR are important too.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
Conditions decreasing SVR directly: story about health of the circulation. CO and SVR are important too.

  • Anemia (viscosity is component of resistance)

  • Fever, hyperthyroidism (increased O2 demand)

  • Sepsis

  • Anaphylaxis

  • Neuraxial and other anesthetics


Conditions increasing svr directly
Conditions increasing SVR directly: story about health of the circulation. CO and SVR are important too.

  • Severe pre-eclampsia

  • Essential hypertension?

  • Diabetes?

  • Smoking?

  • Obesity?


Conditions decreasing co directly
Conditions decreasing CO directly: story about health of the circulation. CO and SVR are important too.

  • 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
Conditions increasing CO directly: story about health of the circulation. CO and SVR are important too.

  • 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
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
Resistance arterioles about COalso 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
Are the resistance arterioles “misbehaving”? about CO

  • Excessive tone: pre-eclampsia, essential hypertension?

  • Deficient tone: sepsis, anaphylaxis, neuraxial block.


Blood vessels about CO

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 about 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
Methods for estimating CO and SVR about CO

  • 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 non-intubated patients. Requires training, labor-intensive, 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 svr1
Methods for estimating CO and SVR in one MRI slice. Same idea applies to echocardiography for calculation of stroke volume.

  • VTI variants (Cardio-Q, USCOM).

  • Ultrasound measures blood flow duration and velocity in abdominal (Cardio-Q) or thoracic aorta (USCOM). Labor intensive, non-continuous and operator dependent.


Velocity-time integral (VTI) = “stroke distance” (SD). SD x aortic diameter = stroke volume

(USCOM advertisement).


Methods for estimating co and svr2
Methods for estimating CO and SVR SD x aortic diameter = stroke volume

  • 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
Impedance cardiography (IC) delivery

  • 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 15 30

Minutes

Archer TL and Shapiro A, UCSD, unpublished


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
What about CVP? and increase CO

  • (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
Summary and increase CO

  • 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.


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