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FRCA Cardiac Output (Measurements (Non-invasive (Bioimpedence, MRI, TTE),…

- - - - Contractility: Intrinsic ability of myocytes to generate force at given preload and afterload
        
        Surrogate: rate of change of pressure (gradient) during isovolumetric phase of cardiac cycle
        
        Factors affecting contractility
        
        Tachycardia
        
        Drugs
        
        Sympathetic nervous system
        
        Disease states
      - Afterload:
        LV wall stress during ejection
        Related to force opposing sarcomere shortening
        
        SVR and MAP: LV afterload
        PVR: RV afterload
        
        Factors affecting afterload
        
        Vessel tone: constriction or dilation
        
        Blood viscosity: increased viscosity increases afterload
      - Preload:
        End diastolic myocardial wall tension
        Related to diastolic length of myocytes
        
        Surrogate:
        EDVol or EDPres
        
        CVP: indication of RV preload as it affects RVEDV
        
        PCWP: indication of LV preload as it affects LVEDV
        
        Factors affecting venous return
        
        Skeletal pump
        
        Body position
        
        Cardiac pump
        
        Respiratory (abdo-thoracic) pump
        
        Venous pressure
        
        Valves
        
        Anaesthesia/Surgery
    - - Denervated heart 100-120bpm
        At rest, tonic vagal nerve activity via ACh to reduce resting HR: 60-70bpm
      - Tachycardia increases CO:
        With exercise, PNS withdrawn and SNS activated.
        Over 150 bpm, diastolic time ~0.15s, causes reduced filling so decreased SV
      - Bowditch effect:
        Intrinsic autoregulatory phenomenon where tachycardia leads to increased contractility.
        Increased Ca2+ influx during systole, with increased influx of Na+. Na/Ca exchanger is overwhelmed so accumulation of intracellular Ca2+ and thus positive inotropy
- - - - SV, HR, CO
      - Stroke distance (SD): AUC velocity-time. Distance a column of blood moves along aorta per beat
      - Peak Velocity (PV): during systole. Reflects LV contractility. Alters with age. 90-120cm/s at 20yo, 50-80cm/s at 70yo
      - Flow-time corrected (FTc): duration of blood flow corrected for HR. normally 330-360 ms.
        Low FTc suggests hypovolaemia and increased afterload.
        High FTc suggests vasodilatation
    - - Simpson's biplane method:
        EDV and ESV based on planes in 4 chamber and 2 chamber views
      - Doppler and measured cross sectional area
      - Mechanical and structural heart defects
  - - - Direct Fick: CO passes through lungs FIck principle applies.
        CaO2 = Arterial O2 (proportional to partial pressure)
        CvO2 = mixed venous sample from PAC (SvO2)
        VO2 = patient breathes from spirometer with 100% O2 and CO2 absorber. After 1 min, volume of O2 remaining allows of calculation of uptake
      - Thermodilution: 10-15ml cold saline injected through proximal lumen of PAC (RA). Temperature change at distal tip in PA is measured.
        Use modified Stewart-Hamilton equation
      - Dye dilution: Indicator dye (indocyanine green or lithium) injected into PAC then sampled at peripheral arterial line.
    - - Calibrated
        
        Pulse contour cardiac output (PiCCO):
        Standard CVC and thermistor-tipped arterial line.
        Calibrated by thermodilution. Some heat dissipated through lungs.
        e.g. Edwards VolumeView
        
        Lithium dilution CO (LiDCO):
        Requires only standard arterial line.
        Lithium dilution using peripheral or central vein. Avoids heat dissipation of thermodilution.
        NMBs may cross-react with lithium electrode. avoid in patients taking lithium
      - Uncalibrated
        
        FloTrac/Vigileo:
        Specialised pressure sensor on standard arterial line. Estimates aortic vascular compliance based on demographics
        
        LiDCORapid:
        Same algorithm as LiDCO, but using normograms based on demographics