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

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

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

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

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

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

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)

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

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

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

Doppler

Doppler

Doppler

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)]

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

Bioimpedance Kubicek WG. Biomedical Engineering September 1974

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

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

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)

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

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

Windkessels in the Jungle

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

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)

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

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

Further Reading Thiele RH, Bartels K, Gan TJ. Cardiac output monitoring: a contemporary assessment and review. Crit Care Med. 2015 Jan;43(1):177-85 Thiele RH, Bartels K, Gan TJ. Inter-device differences in monitoring for goal-directed fluid therapy. Can J Anaesth. 2015 Feb;62(2):169-81. Thiele RH, Durieux ME. Arterial waveform analysis for the anesthesiologist: past, present, and future concepts. Anesth Analg. 2011 Oct;113(4):766-76