Q = SV x HR (L/min)
CI = SV x HR/BSA (L/min/m2)
Normal CI = 2.5-4.2
SV = end-diastolic volume – end systolic volume
EF = (SV/EDV) x 100%
Shock = failure of tissue perfusion -> end organ injury
THE FICK PRINCIPLE
- Adoph Fick (1870)
- assumes oxygen consumption is a function of rate of blood flow and rate of oxygen pick pick up by RBC’s.
- involves measurement of oxygen concentration of arterial and venous blood and subsequent calculation of O2 consumption.
- Q can then be derived
- VO2 = oxygen consumption/min (from spirometer with subject rebreathing air through a CO2 absorber)
- Cv = oxygen content of blood taken from pulmonary artery (deoxygenated)
- Ca = oxygen content of blood taken from a peripheral artery (oxygenated)
VO2 = (Q x Ca) – (Q x Cv)
Q = VO2/(Ca-Cv)
- assumes no shunt (pulmonary blood flow = systemic blood flow)
- assumes arterial blood is equal to pulmonary venous blood
- known quantity of tracer substance introduced into a space to be measured
- concentration measured after complete mixing
C1 x V1 = C2 x (V1 + V2)
C1 = initial concentration of indicator
C2 = final concentration of indicator
V1 = volume of indicator
V2 = volume to be measured
- marker injected proximally to right ventricle and concentration measure distally (pulmonary artery or a peripheral artery)
- concentration vs time plotted -> integration allow calculation of area under curve (SV x HR = Q)
- suitable substances: radioiosotope, dye, cold water, temperature of blood.
- Non-invasive BP monitoring
- Central venous monitoring
- Arterial monitoring
- Pulmonary arterial monitoring
- ECHO: TOE and TTE
- Pulse contour analysis (PiCCO)
- Oesophageal Doppler
- Cardiac catheterisation and angiography
- Aortovelography – dopper U/S probe in suprasternal notch to measure blood velocity and acceleration in ascending aorta.
- Ballistocardiography – detection of body motion due to movement of blood within body with each heart beat.
- Electromagnetic flow meters
- Oxygen consumption estimation (Fick)
- Impedance plethymography
TIPS WHEN USING CARDIAC OUTPUT MONITORS
- there is no ‘normal’ CVP or wedge -> follow trend and look at the response to treatment
- abnormal hearts (ischaemic, fibrotic, contused) are less compliant so require higher filling pressures to reach ‘normal’ SV.
- use SV rather than Q as a response to treatment as Q is calculated from HR which may be fast and mask a poorly performing ventricle.
- low SvO2 usually indicates under-resuscitation.
- the first treatment for all shock (including cardiogenic) = volume, volume and more volume.
- a little extravascular lung water is less harmful than vasoactive drugs.
- there is no formula to calculate the effect of PEEP on PCWP and CVP -> if kept constant, trend should be consistent.
- during resuscitation if becomes apparent what CVP the patient ‘likes’ -> aim for this.
- be cautious of all derived variables, particularly SVR.