Minimally invasive cardiac output monitors Flashcards
What is the most invasive method of monitoring cardiac output?
Pulmonary artery catheter, however this is considered the gold standard for cardiac output measurement
Why is cardiac output monitoring useful in anaesthesia?
- marker of O2 delivery to tissues
- can identify patients at high risk of significant morbidity, mortality or both
- guides treatment for fluid resuscitation
- guides use of vasoactive/inotropic drugs
What are the disadvantages of a pulmonary artery catheter?
- infection
- pulmonary artery rupture
- arrhythmias on insertion
- thrombosis
- embolism
What is the pulse contour analysis PiCCO and what is it’s limitations?
Thermistor-tipped arterial line in a central vessel (proximal artery) to measure the aortic trace waveform morphology.
An algorithm is used to determine CO by integrating the area under the curve of the arterial pressure vs time trace. Has to be calibrated using transpulmonary thermodilution.
Requires intermittent recalibration and a central arterial catheter. Less accurate with significant aortic regurgitation, arrhythmia or intraaortic balloon pump.
What does pulse contour analysis LiDCO need? What are it’s disadvantages?
Requires an arterial line +/- CVC.
Uses pulse power analysis rather than pulse contour. Algorithm is based on law of conservation of mass for continuous CO calculation
Requires intermittent recalibration with lithium dilution.
What are the limitations of pulse contour analysis FloTrac/Vigileo?
It requires an arterial line with a blood flow sensor attached. Uses age, height, gender and weight to measure the patient’s vascular compliance.
CO is calculated every 20s using an algorithm. Multiplies arterial pulsatility and a constant (K - from vascular compliance) resulting in a SV to multiply by HR for calculation of CO.
It’s less accurate for absolute measurement than calibrated pulse wave devices or with significant aortic regurg, arrhythmia or intra-aortic balloon pump.
Does not require calibration.
What are the disadvantages of the oesophageal doppler?
Poorly tolerated unless tracheal tube present.
Relies on assumed proportion of blood flow through the descending aorta
What does USCOM need and what are it’s limitations?
Transthoracic doppler probe. Probe is place suprasternally to measure flow through the aorta or left chest to measure transpulmonary flow.
Is completely non-invasive. Uses nomogram for valve area estimation, not accurate with significant valve stenosis.
What does gas rebreathing require?
Tracheal intubation and stable tidal volumes during measurements. Rebreathing circuit.
What are the limitations of transpulmonary thermodilution?
It’s invasive, it requires a thermistor tipped A-line.
It’s less accurate with pulmonary congestion and in the presence of shunting.
What does lithium dilution require?
It’s invasive- needs an arterial line +/- CVC.
Inaccurate with intercurrent lithium use.
Can’t be used in patients <40kg or 1st trimester of pregnancy
What are the limitations of thoracic bioimpedance?
Cutaneous electrodes only so not invasive.
Accuracy with haemodynamic instability is not well tested. Limited usefulness in awake patients.
What is stroke volume variation? (SVV)
It’s the difference between the maximum and minimum stroke volumes over the respiratory cycle and is caused by changes in preload with alterations in intrathoracic pressure.
What is SVV an indicator of?
Fluid responsiveness. If the patient has a SVV <10% they’re unlikely to be fluid responsive, but if they’re >15% they’re likely to benefit from fluid resuscitation.
In general, what can cause inaccuracies in pulse contour analysis?
- over/under damped traces
- arrhythmias
- aortic regurg
- use of intra-aortic balloon pump
- changes in SVR may also lead to inaccuracies
What does the oesophageal aortic doppler meausure? How does it calculate CO?
Blood flow through the descending aorta.
CO is calculated from the VTI (AUC), HR and CSA of the aorta
What is the doppler equation?
V - velocity of blood
Fd - the doppler shift in frequency
c - the speed of ultrasound in tissue (1540 m/s)
Fo - the initial US frequency
Ø = the angle of US beam in relation to blood flow
What is the VTI in the oesophageal doppler?
Velocity-time-integral - it’s calculated from the area under the velocity time curve and used as the stroke distance
What is CSA in the oesophageal doppler?
Aortic cross sectional area. It’s taken from a nomogram using height, age and weight.
What is the equation for calculating CO using the oesophageal doppler?
CO = CSA x VTI x HR
(although a correction factor must be used as the descending aorta only carries 70% of the cardiac output)
What can cause inaccuracies in the oesophageal doppler?
- estimation of CSA because this can change based on volume status or vascular tone
- probe position as even small misalignments can cause underestimation of flow
- any turbulent flow, as the equation assumes laminar flow
How does gas rebreathing determine cardiac output?
It uses the indirect Fick equation.
A rebreathing apparatus is attached to the patient’s tracheal tube and serial measurements are taken every 3 mins. At steady state, the CO2 entering the lungs via the pulmonary artery is proportional to the cardiac output and equals the amount exiting the lungs via expiration and pulmonary veins.
It requires intubation and fixed ventilator settings.
What limits the accuracy of gas rebreathing as a measure of CO?
- severe chest trauma
- significant intra pulmonary shunt
- low minute ventilation
- high cardiac output
- assisted spontaneous breathing patients
All reduce accuracy
How does transpulmonary thermodilution work?
Cold injectate is introduced into the SVC via a CVC.
An arterial line with a thermistor is placed in a major artery (femoral, axillary, brachial) and the change in temperature of the blood is measured after the injectate has traversed the right heart, pulmonary circulation and left heart.
The change in temperature over time curve begins later and has a lower peak compared to the PAC curve.
Thermodilution equation is then used to calculate CO.
