Oesophageal Doppler

OVERVIEW

• Oesophageal Doppler Cardiac Output Monitor

USES

• non-invasive cardiac output monitor

Contra-indications

• IABP
• severe coarctation
• known pharyngo-oesophageal pathology or injury
• oesphagectomy
• severe bleeding
• awake patient (relative)

Indications

• major surgery with large fluid/blood shifts
• high risk patients
• haemodynamic instability

DESCRIPTION

• measures blood flow velocity in the descending aorta using a flexible ultrasound probe
• measurement is based on the Doppler Principle: a doppler shift in frequency occurs when the ultrasound wave is reflected back from moving RBCs, with the shift in frequency proportional to the velocity of the blood flow
• measurement is combined with the cross-sectional area of the descending aorta (using a nomogram based on age, weight and height  — some models measure aortic root diameter using TOE)
• probe contains crystals that produce a continuous ultrasound wave of 4 MHz
• single use, latex-free, silicone-based

METHOD OF INSERTION AND/OR USE

• lubricate probe well
• inserted via the nasal or oral route
• advanced and rotated in the oesophagus (typically sits at level of the 5th and 6th thoracic vertebrae where the aorta is adjacent and parallel to the oesophagus)
• some devices display the aortic pressure waveform allowing confirmation of optimal probe position
• turn volume up initially (sharpest sound = best position)
• input age, height and weight
• insert probe to 35-40cm from teeth (T5/6)
• manipulate probe until characteristic Doppler signal found (well defined triangle with black centre surrounded by red with white in trailing edge) = distribution of RBC velocities at a given point in time
• adjust cycle length (5 = good starting point), gain (to decrease ambient noise, start at 5 also)
• the brighter (whiter) the colour the greater the number of RBCs travelling at the given velocity
• it is a dynamic monitor so needs refocusing prior to each reading

INFORMATION OBTAINED

Haemodynamic parameters:

• cardiac output (Q = SV x HR)
  — Q is estimated by minute distance
• stroke volume (SV is dependent on preload, afterload and contractility)
  — SV is  measured by stroke distance x aortic root diameter
  — stroke distance (AUC) x HR, linear cardiac output parameter, distance moved by a column of blood through aorta in 1 minute (1200cm = high flow state)
• corrected (systolic) flow time (FTc)
  — indicates preload
  — normal = 0.33-0.36s
  — interpretation of FTc and PV indicates afterload
• peak velocity (PV)
  — indicates contractility
  — normal range is age related (20yrs 90-120 cm/sec, 90 yrs 30-60 cm/sec)
  — interpretation of FTc and PV indicates afterload
• heart rate

Actions from readings (need to be interpreted in clinical context)

• low SV -> fluid
• low FTc -> fluid
• low PV -> inotrope
• low PV + low FTc -> decrease afterload

PROS AND CONS

Advantages

• minimally invasive
• provides real time measurements

Disadvantages

• learning curve (significant inter and intra-observer variability)
• probe displacement requiring repositioning
• difficult to use in an awake patient

Complications

• esophageal bleeding, ulceration, or perforation
• stricture formation
• inaccurate measurements from probe displacement

OTHER INFORMATION

• Doppler equation: Fd= 2FtVCos θ / C
  • where Ft is the transmitted Doppler frequency, V is the speed of blood flow, Cos θ is the Cosine of the blood flow to beam angle and C is the speed of sound in tissue
• perioperative use in the UK is supported by NICE guidelines and is embedded in ‘enhanced recovery after surgery’ (ERAS) programmes with financial incentives – this based largely on the surrogate outcome of reduced hospital length of stay

References and Links

• Morris C. Oesophageal Doppler monitoring, doubt and equipoise: evidence based medicine means change. Anaesthesia. 2013 Jul;68(7):684-8. PMID: 23672246.