1. Technologic Principles for CO Monitoring
May 8, 2017
Robert H. Thiele, M.D.
Assistant Professor, Department of Anesthesiology
Co-Director, UVA Enhanced Recovery After Surgery Program

2. Disclosures Thiele: Teleflex: Airway Advisory Committee (2012)
Masimo: Real-Time Hemoglobin (SpHb [2014])

3. True (laboratory) Gold Standards
Fick Cardiac Output
Conservation of Mass
Q = VO2/(CO2,arterial-CO2,venous)
Electromagnetic Flowmeters
Apply a magnetic field to fluid
Resultant voltage differential is related to Q
Transit Time Flow Probes
Related to Doppler (but angle independent)
Higher flow rates reduce the transit time of an ultrasound beam travelling in the direction of flow

4. Statistical Considerations
Regression
Utilized in almost all early studies
Disproportionately affected by outliers
Output: probability that the slope of the best fit line is not equal to zero
Limits of Agreement
Relatively new technique
Can be artificially lowered using poor experimental technique (small sample range)
Output: confidence intervals around the difference between two methods
This complicates our ability to compare devices

5. Narrow range of data (bad study, e.g. healthy volunteers)
“Clinically acceptable” agreement!!!
Wide range of data (good study, e.g. sepsis)

7. Classification
Most clinically-utilized devices are based on one (or more) of six techniques
First Generation
Indicator-Dilution
Doppler
Bioimpedance/Bioreactance
Second Generation
Arterial Waveform Analysis
Partial-rebreathing
Pulse wave transit time

8. Indicator-Dilution Physical Basis Conservation of Mass (or Energy)
Inject something (the “indicator”) into the bloodstream
Measure the downstream “change” induced by the indicator
Indicators
Mass (e.g. dye)
Energy (e.g. temperature)

10. Indicator-Dilution Stewart-Hamilton equations: m = Q 0∫∞ c(t)dt
Q = m/[0∫∞ c(t)dt]
Q = V1(TB-TI)K1K2/[0∫∞ ∆TB(t)dt]
V1 represents the volume of injectate
T represents temperature (of blood or the indicator)
∆TB(t) describes the change in blood temperature over time (this is the curve you see on your monitor)
K1 is related to the density of both blood and the indicator
K2 is takes into account catheter dead space, heat transfer during injection, and the rate of injection

11. Indicator-Dilution Implications Pulmonary Artery Catheter
Actually measure right ventricular output
Not always the same as left ventricular output
Misleading in the setting of temperature instability, tricuspid regurgitation
LiDCO and PiCCO
Assesses indicator changes in the peripheral arteries
This is a major limitation of these devices
Early studies looking at [Li] or temperature changes in central arteries exhibited remarkable accuracy
8 studies include 546 subjects with a 2.2 day reduction in LOS

12. Doppler Physical Basis Doppler Principle Problems
Sound (and ultrasound) changes frequency when reflected off of a moving object
The change in pitch is proportionate to the velocity
Problems
Two unknowns which must be measured or estimated
Cross sectional area
Angle of incidence

13. Doppler

14. Doppler

15. Doppler

16. Bioimpedance Physical Basis Ohm’s Law (I = V/R) Application
The thorax resists the flow of electrical current
Bone, skin, etc. are highly resistant to flow
Blood acts as a conductor
The amount of resistance (R) to electrical current is dependent on the volume of blood in the chest
Impedance (Z) is resistance to an oscillating potential
Application
Apply an oscillating voltage differential across the chest
Measure the change in current
Calculate impedance
Transform this into flow using SV =  x (L2/Z02) [VETx(dZ/dtmax)]

17. Bioimpedance Problems Newer Versions Highly susceptible to
Electrical artifact
Electrode positioning, body size, temperature, and humidity
The mathematics are difficult to understand
Understanding how your device works is important
Newer Versions
Electrical Velocimetry (AESCULON®)
Impossible for me to understand (utilizes a “square root transformation of dZ/dtmax/Z0” as well as a “variable-magnitude, mass-based volume conductor” approach )
Bioreactance (NICOM)
Examines the phase difference between voltage and current

18. Bioimpedance
Kubicek WG. Biomedical Engineering September 1974

19. The Windkessel Physical Model Assumptions Important Concepts
Infinitely long tubing
Assumptions
No wave reflections
No backward flow
Important Concepts
Conservation of Mass
Vessel compliance affects flow significantly
Thiele and Durieux. Anesth Analg 113: 776, 2011

20. The Windkessel The Math Qtotal = QS + QD QD = k x P
k is a constant related to vascular resistance, vessel compliance, and in some cases, aortic input impedance
P is pressure
Different authors use different definitions of pressure
Different definitions of pressure necessarily affect k

21. The Windkessel The Math Continued…
If vascular tone (k) is stable over one beat:
QS /AS = QD/AD
QS = QD (AS/AD)
Adding back QD yields:
Qtotal = QS + QD
Qtotal = QD (AS/AD) + QD
Qtotal = QD (1 + AS/AD)
Qtotal = kP(1 + AS/AD)

22. The Windkessel
The Implication: the shape of the peripheral pressure waveform must reveal meaningful information about systemic vascular resistance
Thiele and Durieux. Anesth Analg 113: 776, 2011

23. Windkessels in the Jungle
How to Solve The Afterload Problem?
Calibrate using another technique
Lithium Dilution (LiDCO Plus)
Transpulmonary thermodilution (PiCCO)

24. Windkessels in the Jungle

25. Windkessels in the Jungle
Calibrated vs. Uncalibrated?
Calibrated is probably more accurate
The majority of data seem to suggest that calibrated devices perform better than uncalibrated devices in the setting of hemodynamic instability or changes in afterload
The benefits of calibration have not been universally demonstrated
Requires either increased invasiveness, increased complexity, or both

26. Coming Soon… NICOM (Bioreactance) Completely non-invasive
Analyzes “phase shift” of current applied to thorax
Purportedly less interference than traditional bioimpedance
Completely non-invasive
Very few validation studies or GDT/ERAS literature (POEMAS)

27. Coming Soon… Finger-Cuff Devices
Application of Windkessel to finger cuff-derived ABP waveform
ClearSight (Edwards)
CNAP (CNSystems)

28. Coming Soon… New Arterial Waveform Algorithms MostCare (VyTech)
Long Time Interval Analysis (Michigan State / Retia Medical)

29. Further Reading
Thiele RH, Bartels K, Gan TJ. Cardiac output monitoring: a contemporary assessment and review. Crit Care Med Jan;43(1): Thiele RH, Bartels K, Gan TJ. Inter-device differences in monitoring for goal-directed fluid therapy. Can J Anaesth Feb;62(2): Thiele RH, Durieux ME. Arterial waveform analysis for the anesthesiologist: past, present, and future concepts. Anesth Analg Oct;113(4):766-76