Oesophageal doppler and cardiac output monitoring Flashcards
What is an oesophageal doppler
- Non-invasive method for measurement of cardiac output
- Directly measures the velocity and duration of blood flow in the descending aorta. From this, stroke volume and hence cardiac output may be calculated
Doppler probe: how is velocity of blood flow measured
- Doppler effect exhibited by ultrasound waves is used to measure velocity of blood flow in blood vessels.
- Ultrasound beam of known frequency is directed at a known ange, to intersect the path of the blood flow, and is reflected back by the red blood cells to an ultrasound detector
- Change in frequency of the reflected ultrasound waves is directly proportional to the velocity of the blood towards or away from the Doppler probe
Velocity of blood flow is calculated from the Doppler equation:
Flow velocity (V) in cm/s = [ speed of sound in body tissue * Doppler frequency shift (Hz) ] / [2 * frequency of transmitted ultrasound * cosine of angle between sound beam axis and velocity vector ]
What is the Doppler equation
Note the 2 is to account for the use of reflected sound
How is cardiac output calculated from velocity of blood flow measured using a doppler probe
Measuring blood flow in descending aorta
* Oesophageal Doppler measures velocity of blood in descending aorta in cm/s
* Aortic blood flow (cm3/s) = velocity (cm/s) * aortic cross-sectional area (cm2)
* Note 1 cm3 = 1ml so aortic blood flow can also be expressed as ml/s
Estimating cross-sectional area of aorta
* Certain models of oesophageal Doppler monitor use ultrasound to measure the diameter of the descending aorta, then calculate the cross-sectional area
* Others use ‘calibration factor’ nomogram based on age, height and weight
Calculating cardiac output from blood flow in descending aorta
* In healthy person at rest, ~70% of stroke volume enters descending aorta. Therefore if flow of blood in descending aorta is known, this can be used to estimate stroke volume
* Cardiac output = SV * HR
Assumptions made when calculating cardiac output by oesophageal Doppler (5)
- Distribution of blood to the descending aorta is a constant 70% of the stroke volume
- Diameter of the aorta does not alter during systole
- All blood within the aorta is moving at the same velocity
- There is negligible blood flow in diastole
- The velocity of the blood in the aorta is being measured accurately.
Positioning of the oesophageal doppler probe
Depth, angle to descending aorta, insertion
- Depth 35-40cm from the teeth: at this depth the descending aorta runs parallel and immediately adjacent ot the oesophagus
- When probe is at appropraite depth, rotated so that the bevel of the probe is facing posteriorly
- Tip of probe is angled at 45 degrees so that with the probe correctly positioned, the angle of insonation (theta) of the blood flowing in the descending aorta is a constant 45 degrees
- Note most commonly inserted orally in sedated or anaesthetized patients, but may be introduced nasally in awake patients with sedation and topical anaesthesia.
Oesophageal doppler monitor waveform
Direction of blood flow
Blood flow away from probe is displayed on monitor as wave above the baseline
Blood flow towards probe is displayed as wave beneath the baseline
(NB opposite convention to that when using Doppler to measure blood flow in peripheral vessels or across heart valves)
Recognising the correct doppler waveforms
Optimal waveform will have sharp definition, higher peak velocity - see image
Determining correct position towards aorta:
* No flow or minimal flow in diastole (in other vessels e.g. celiac artery, may see continuing flow in diastole)
* Waveform above baseline i.e. flow away from probe (in e.g. pulmonary artery, waveform will be below baseline as blood flowing towards probe)
Safety of and contraindications to oesophageal doppler monitoring
- Acceptable degree of competence can be achieved after insertion of 12 probes
- Once inserted, have been used in unconscious patients for up to 14 days without complication
- Contraindications: clotting abnormalities, oesophageal varices, oesophageal surgery
Conditions that may cause inaccurate readings (3)
- Coarctation of the aorta
- Thoracic aortic aneurysm
- Working epidural/ intrathecal anaesthetic with subsequent lower limb vasodilation
Absoloute figures may be inaccurate, trends can still be used.
Commonly used parameters calculated from oesophageal doppler (4)
- Peak velocity
- Stroke volume
- Cardiac output
- Flow time corrected (FTc)
Peak velocity of blood in aorta
Clinical implications, normal values, influence of preload/afterload
- Gives a good estimate of myocardial contractility
- Normal value varies with age: approx 90-120cm/s at 20 years, falls to 50-80 by 70 years
- Influenced by preload and afterload. Low preload and/or high afterload -> decrease in peak velocity, and vice versa
Stroke volume
Calculation from oesophageal doppler waveform
Stroke distance = area under velocity-time waveform
Stroke distance * aortic diameter ~= stroke volume
Due to beat-to-beat variability, reading usually averaged over several beats.
Cycle length = number of beats used for calculation. 5 is usual, more may be required if marked beat to beat variability e.g. arrhythmias
Cardiac output, cardiac index
Normal values
Cardiac output = SV * HR
Cardiac index = cardiac output / body surface area
Normal cardiac index: 2.5-3.6 L/min/m2 regardless of body size
Flow time corrected
Calculation, normal range, causes of increase or decrease
Flow time = duration of forward flow of blood in aorta i.e. the width of the base of the velocity-time waveform
* Varies with heart rate
Flow time corrected (FTc) is flow time corrected to a heart rate of 60bpm
* FTc = flow time / square root of cardiac cycle time
* i.e. FTc is independent of heart rate
Normal FTc range: 330-360ms
Causes of prolonged FTc: vasodilation
* Sepsis
* Anaesthesia (up to 400ms may be normal, especially in presence of working epidural)
Causes of reduction: anything that impedes filling or emptying of LV
* Hypovolaemia
* Mitral stenosis
* PE
* Excessive vasopressors
Assessing response to a fluid challenge using oesophageal doppler
- Note SV and FTc
- Give fluid bolus 200-250mls over <10 minutes
- Increase in SV of at least 10% indicates ‘fluid responsiveness’ i.e. hypovolaemia
Graph shows effect of serial fluid challanges on stroke volume, FTc and peak velocity, as patient approaches normovloaemia
Oesophageal doppler waveform in
* Hypovolaemia
* Sepsis
Hypovolaemia - see picture
* Small SV
* Reduced FTc (below 330)
* Needs to be confirmed by response to fluid challenge
Sepsis (i.e. vasodilation, high cardiac output)
* Large stroke volume
* High peak velocity
* Prolonged FTc (above 360)
Interpret this oesophageal doppler waveform
Typical of that seen in hypovolaemia
Interpret this oesphageal doppler waveform
Aortic regurgitation.
Note the flow beneath the baseline (towards the probe) during diastole.
Interpret this waveform
Typical of LV failure
* Small stroke volume
* Low peak velocity
* Normal FTc
Interpret this waveform
Typical of sepsis
* Large stroke volume
* High peak velocity
* Prolonged FTc
