CO and SVR Next Vital Signs 0640

8/11/2019 CO and SVR Next Vital Signs 0640

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Cardiac output (CO) and systemic

vascular resistance (SVR) –

the next vital signs?

Tom Archer, MD, MBAUCSD Anesthesia

October 4, 2010



“Normal BP” = High SVR x Low CO

(e.g. Hemorrhagic or

cardiogenic shock)

“Normal BP” =

“Normal BP” =

Normal SVR x Normal CO

(e.g. Healthy person)

Low SVR x High CO

(e.g. Sepsis)

Blood pressure, while important, does not tell the whole story

about health of the circulation. CO and SVR are important too.



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?





Heart

Blood

vessels

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.



Heart affects

CO and SVR

Blood

vessels

affect SVRand CO

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)—highlyinvasive, but a gold standard. Can be continuous.

• Echocardiography (TEE or TTE)—a minimally ornon-invasive “gold standard”. TEE difficult on non-intubated patients. Requires training, labor-intensive, not continuous. Uses velocity time

integral (VTI) to calculate “stroke distance” orcompares diastolic and systolic LV areas tocalculate “stroke area”.






• 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).




• 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 trainingrequired. “Hands-free”.

•

• Long history (NASA, 1960’s) and multipleiterations and algorithms.

• Bo-Med, Cardiodynamics, Cheetah, Cardiotronic).

• All look at same signal but interpret it in differentways.



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 systoledZ(t)/dt.

C. Schmidt et al British Journal of Anaesthesia 95 (5): 603 –10 (2005)



Cardiac and stroke indices increase with uterine contractions

CI

SI

HR

90

40

8

3

100

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





Summary

• Currently, measurement of CO and SVR can belabor-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

Documents

CO and SVR Next Vital Signs 0640