Continuous oesophageal Doppler for estimation of cardiac output

Depicted is a pulse-wave doppler of the descending aorta:

Flow time corrected for heart rate (FTc) correlates with LV afterload AND with LV preload

i.e. this is your SVRI.
A loss of ventricular preload will also usually result in an increase in afterload (systemic resistance must rise to maintain a reasonable mean arterial pressure) – and so this is also a surrogate measure of left ventricular preload.

A shortened flow time will also occur with hypothermia and heart failure.

Mean acceleration of the aortic flow:

Like flow time, correlates with left ventricular afterload and preload. Not specific, and so not used very often.

Respiratory variation in Vpeak correlates with responsiveness to fluid challenge

... in the same way as stroke volume variation does

Vpeak is an index of contractility

Obviously the left ventricle contractility needs to be good to generate a normal peak flow velocity. A diseased ventricle will generate crappy slow Vpeaks.

Method of continuous trans-oesophageal measurement

• • A 6mm TOE probe  is sent down your oesophagus, orogastric-style
  • The tip is furnished with a pulsed-wave of continuous-wave Doppler
  • The probe tip is positioned 30-40cm from the teeth
  • The idea is, this is where the aorta cross-sectional area of the aorta varies least
  • The probe is rotated until the characteristic waveform is achieved.
  • The VTI is measured from this waveform (remember the VTI, it is the Velocity-Time Integral, the area under the velocity-time curve)
  • The cross-sectional area (CSA) for the stroke volume equation is established either from direct measurement or from body size nomograms (turns out your aortic crossection is vaguely related to your age, weight and height)
  • Then, you adjust your cardiac output result, taking into account that only 70% of the cardiac output travels down the descending aorta.
  •  This probe is left in situ and provides continuous cardiac output monitoring information.

Contraindications

• • ANYTHING WEIRD IN THE DESCENDING AORTA:
    • Aortic balloon counterpulsation
    • Aortic dissection
    • Severe aortic coarctation
    • Thoracic aortic aneurysm

Assumptions upon which the validity rests

• The descending aortic flow is 70% of the cardiac output (this may not be so in many patients)
• The aortic crossection is estimated accurately by nomograms
• The aortic flow is laminar – it may not be in tachycardia, valve disease or in anaemia which alters the viscosity of the blood.
• The aortic crossection is cylindrical
• The aortic crossectional area remains fixed -  this is never so, the aorta is a compliant vessel with a dynamically changing crossection; in children it even pulsates.
• The probe position is unchanged

Advantages

• • So simple to insert that a shaved ape could do it.
  • The probe is a minimally invasive 6mm object, not much bigger than a nasogastric tube

Disadvantages

• The probe position can change
• The nasogastric tube can get in the way
• The patient needs to be sedated to tolerate this