G6iii: Describe the principles & limitations of the measurement of cardiac output using PICCO

Definition

  • PICCO = pulse contour cardiac output (acronym)
  • PICCO = a CO monitor that combines pulse contour analysis & transpulmonary thermodilution technique

Use

  1. Haemodynamic monitoring of shock
  2. PAC unavailable/contraindicated

Contraindicated (inaccurate measurements)

  • Intracardiac shunts
  • Aortic aneurysm
  • Aortic stenosis
  • Pneumonectomy
  • PE
  • Balloon pump
  • Unstable arrhythmias

Set Up/Insertion

  • An arterial line with a Thermistor
  • A pressure transducer
  • A PICCO monitor
  • Sterile, Seldinger technique
    • Central line insertion
    • Proximally placed PICCO cannula (femoral, brachial, axillary) to give a good dicrotic notch & arterial waveform
  • PICCO then calibrated by thermodilution (like PAC) to calculate CO
  • 15mL cold fluid bolus rapidly injected into CVC
  • Thermistor in arterial line detects the ∆ in temperature
  • ∴ in contrast with PAC, the fluid bolus passes through entire R) side circulation →ejected from LV →detected
  • CO is then measured with the modified S-H equation:
Modified S-H Equation

Q = CO

V = volume of injectate

TB = temp blood

T1 = temp injectate

K = constant, which corrects for specific heat & density of inectate

dt = ∆ time

  • This initial calibration sets parameters & then an algorithm computes each SV

SV ~ Area under Curve in systole – diastolic area

CO Then Derived = SV X HR

Transpulmonary Thermodilution

  • All volume parameters are obtained from the thermodilution calibration above
Transpulmonary Thermodilution
  • CO is calculated from AuC substitution into SH Equation
AuC substitution into SH Equation
  • CO x MTt = total volume traversed by indicator (from site of injection →site of detection)
  • CO x DSt = volume of the largest mixing volume in a series
CO x DSt=Volume
  • Intrathoracic compartments are like a ‘series of mixing chambers’
  •  The largest mixing chamber is the lungs because the indicator (cold) has the largest VD here (largest thermal vol.)
  • So the PTV can be calculated using the gradient of the washout curve

TTV = CO x MTt

Pulse Control Analysis

  • Arterial pressure curve provides beat-by-beat SV & CO estimates
  • SV ~ area under diastolic curve (minus diastolic area)
  • SV x HR → CO

Parameters Derived

Thermodilution

Arterial Pulse Contour Analysis

CO 4 – 8L/min

CI →CO which is 3 – 5L/min corrected for BSA

 GEDV = RACDV + RVEDV + LAEDV + LVEDV

  • 600 – 800mL/M2
  • Essentially PRELOAD
  • Does not relate well to fluid responsiveness

 Global ejection fraction (25 – 35%)

  • Radio of 4 stroke volumes divided by GEDV/detects ventricular dysfunction

Intrathoracic Blood Volume (ITBV)

  • = GEDV + pulm. blood vol
  • Volume in heart + volume in lungs
  • ITBV1 = 850 – 1000mL/M2

Extravascular lung water (EVLW)

  • Water content in lungs indicates permeability of pulm. vasculature
  • ELWI 3 – 7mL/kg

→EVLWI = corrected for body weight =

3 – 7mL/kg

Pulse continuous CO

 SV = 50 – 110mL

  • Area under arterial pressure curve

SVI 40 – 60mL/M2

  • SV adjusted for BSA

SVV (<10%)

  • SVV = SVmax –
  • Measures ‘swing in a line over 30 secs’

Any variation >10% warrants a fluid bolus

Advantages

  • Good correlation with PAC thermodilution
  • Does not require PAC
  • Many variables derived

Disadvantages

  • Requires CVC & Art. Line
  • Requires regular calibration
  • Regular + accurate BP transduction →system v. sensitive to Damping