Cardiac output (CO) monitoring using oesophageal Doppler monitoring (ODM):

A.Uses frequency shifts in reflected sound waves to estimate flow velocity

B.Will overestimate values of stroke volume (SV) and CO if the probe is poorly positioned

C.Can be used in children

D.Does not measure blood flow to the head or arms

E. Requires measurement or estimation of aortic cross-sectional area.

All are true except B.

An animation illustrating how the Doppler effect causes a car engine or siren to sound higher in pitch when it is approaching than when it is receding. The pink circles represent sound waves.
The Doppler effect (or the Doppler shift) is the change in frequency of a wave (or other periodic event) for an observer moving relative to its source. It is named after the Austrian physicist Christian Doppler, who proposed it in 1842 in Prague.

To understand what happens, consider the following analogy. Someone throws one ball every second at a man. Assume that balls travel with constant velocity. If the thrower is stationary, the man will receive one ball every second. However, if the thrower is moving towards the man, he will receive balls more frequently because the balls will be less spaced out. The inverse is true if the thrower is moving away from the man. So it is actually the wavelength which is affected; as a consequence, the received frequency is also affected. It may also be said that the velocity of the wave remains constant whereas wavelength changes; hence frequency also changes.

When the probe has been placed and focused , blood f low velocity can be calculated based on the Doppler principle. Emitted ultrasound is scattered by erythrocytes travelling in the descending aorta and partially reflected back to the probe.When ultrasound is reflected by a moving object such as erythrocytes, the frequency of reflected ultrasound is higher than the emitted frequency when erythrocytes move towards the probe and lower when they move away from the probe . Erythrocyte velocity is directly proportional to the Doppler frequency shift (Df), i.e. the discrepancy between transmitted frequency (fT) and received frequency. This relationship is described by the Doppler equation : Blood flow velocity (v) can now be calculated from the frequency shift, because other factors that determine velocity are basically known and constant (c is the velocity of ultrasound waves in body tissue, fT the transmitted frequency and θ is the angle between ultrasound beam and blood flow). The angle θ is not precisely known, however, it closely approximates the angle with which the ultrasound transducer is mounted to the central axis of the probe (i.e. 45° with the CardioQ device), because the oesophagus and aorta run nearly parallel at mid-thoracic level.

Technical limitations derive from the blood flow velocity measurement itself as well as the assumptions needed to translate velocity into CO. The intensivist needs to be aware of these limitations, as there are conditions in which TED derived measurements may not necessarily be reliable. The measurement of blood flow velocity assumes that all erythrocytes travel in the same direction and at the same speed. Indeed, in healthy subjects, descending aortic blood flow is usually laminar with a relative uniform velocity profile over the aortic cross section . However, skewed velocity profiles and rotational blood flow in the descending aorta has also been described. Non-laminar f low is likely to occur in patients with pathology of the aorta or aortic valve. Another important aspect in velocity measurement is the angle with which ultrasound is insonated into the aorta, because the cosine of this angle makes part of the Doppler equation. As described earlier this angle is not precisely known .but is assumed to equal the angle with which the ultrasound transducer is mounted to the central axis of the probe. However, this assumption may not hold true in those patients who have an altered relationship between the aorta and oesophagus, e.g. because of previous surgery, tumour mass or severe scoliosis. Due to the non-linear character of the cosine function, deviations in the actual angle from the assumed angle will result in relatively small errors at small degrees but become unacceptably high when the angle of insonation exceeds 60°. With the angle of 45°, which the CardioQ uses, a deviation of 1°, 5° and 10° in the actual from the assumed angle results in measurement errors of approximately 2%, 8% and 16%, respectively. Translation of blood flow velocity into CO requires estimations of the aortic cross sectional area as well as the distribution of blood flow between the descending aorta and supraaortic arteries. However, neither of the above measurements is actually taken. Rather, the CardioQ uses a nomogram and provides a calibration factor to translate descending aortic blood flow velocity to total left ventricular cardiac output over a wide range of patient conditions. However, a nomogram derived from average values in a population does not always accurately predict individual values. Moreover, the nomogram neither accounts for changes in aortic diameter nor in changes of blood f low distribution. Aortic diameter may change due to changes in blood volume and blood pressure.Furthermore, blood f low distribution may also change, e.g. due to changes in vascular tone caused by pharmacologic action, sympathetic blockade, blood loss, sepsis or anaphylaxis.Despite these limitations, trend monitoring should generally be possible if the basic conditions remain constant. However, it is important to realize that changes in the basic condition, e.g. sudden changes of aortic diameter and blood flow distribution due to acute haemorrhage, may lead to inconsistent or even misleading CO readings . Thus, the intensivist needs to be aware of the technical limitations of TED to avoid misinterpretation of displayed data.. If θ is 0, cos θ is 1, but as θ gets progressively closer to 90° the velocity of blood flow becomes more and more underestimated. At 90° cos θ = 0 (velocity is not measured).

Oesophageal Doppler:
a.Utilises the Doppler shift to measure blood velocity.
B.Velocity of blood (m/s) in the descending aorta can be calculated provided the aortic cross-sectional area is known.
C.It is assumed 70% of cardiac output is distributed caudally to the descending aorta.
D. Doppler probes must be removed after 1 week.
E. Is accurate when used with a working epidural.

A & C are True only.

Oesophageal Doppler:
A.Peak velocity is a good estimate of myocardial contractility.
B.By age 70 peak velocity falls to less than 50cm/second.
C. Stroke distance (SD) is the area under the velocity-time curve and provides an estimate of stroke volume.
D .An increase in stroke volume of less than 5-10% with a fluid bolus suggests hypovolaemia.
E. A corrected flow time (FTc) greater than 400ms is due to hypovolaemia.

A & C are true only
Peak velocity declines with age, with normal values of 90-120cm/s at the age of 20, falling to 50-80cm/s by the age of 70.

Concerning corrected flow time (FTc) measured by oesophageal Doppler:

A.An FTc less than 330ms may be due to excessive metaraminol use.
B.An FTc of up to 400ms may be normal in anaesthetised patients.
C.A normal FTc is 230 to 260ms.
D.A low peak velocity and FTc less than 330ms may be due to increased preload.
E.Goal-directed therapy using oesophageal Doppler protocols may improve outcomes for surgical patients.

A,B and E are true
FTc is prolonged by vasodilatation, and therefore an FTc of up to 400ms may be considered normal in anaesthetised patients, in particular, in those with a working epidural in situ, due to the vasodilatation that is encountered.
Increased afterload may result in a low peak velocity and low FTc.

A 74-year-old lady with a history of ischaemic heart disease and severe congestive cardiac failure is admitted to the ICU with hypotension and presumed sepsis. She is sedated and ventilated in pressure support mode. On examination she is confused, BP is 85/35mmHg, HR is 115bpm (sinus tachycardia), Sp02 is 95% on 60% oxygen. Arterial blood gas analysis shows a lactate of 4.3mmol/L (39mg/dL). Which is the BEST guide to the need for further intravenous fluid replacement?

A.Response of oesophageal Doppler to passive leg raising. B.Insertion of a pulmonary artery catheter and pulmonary artery occlusion pressure measurement.
C.Titrate fluid resuscitation against repeated blood lactate measurements.
D.Assess pulse pressure variation.
E.Urine output measurement.

The following assumptions are made when determining stroke volume using an oesophageal Doppler probe:
a. 70% of total cardiac output passes the probe.
B. The ascending aorta runs parallel to the oesophagus.
C.The diameter of the aorta is constant throughout systole.
D.Haematocrit is unchanged between measurements.

A 58-year-old man is ventilated on the ICU following a Whipple’s pancreatoduodenectomy. He is hypotensive (BP 85/65mmHg) and tachycardic (HR 120bpm). Initial oesophageal Doppler measurements include a flow time corrected (FTc) of 290ms, which rises to 310ms following a 250ml fluid challenge. Which statement is TRUE?
a. A low FTc invariably means more fluid is required.
B.This patient has normal cardiac contractility.
C. FTc is a marker of afterload. D. Fluid should be titrated to an FTc of 340ms.
E. Dobutamine should be started at this point.

A 42-year-old woman is admitted to the ICU generally unwell 1 week following abdominal surgery. She is oliguric, hypotensive (BP 80/35mmHg) and tachycardic (HR 130bpm). An oesophageal Doppler probe is sited, showing: flow time (corrected) 380ms, peak velocity 110cm/s, stroke volume 90m1. Which of the following statements is TRUE? a. A fluid challenge should be administered.
B.Cardiac contractility is impaired.
C.A cautious dose of frusemide should be given.
D.The measured (uncorrected) flow time will be greater than 380ms.
E.The data are consistent with massive pulmonary embolism