Question 1 : Assessment of fluid status with ECHO

Question 1

A haemodynamically decompensating patient requires assessment of the intravascular fluid status. Describe the Doppler echo cardiographic methods that can assist outlining the supportive evidence for their use, in addition to any obvious limitations in their application

Assessment of fluid status can be achieved by using the LVOT VTI as a surrogate for stroke volume

• SV (cm3) = Area of LVOT (cm2) X VTI (cm)
• VTI measurement of 18 – 22 cm is normal if HR between 55 and 95 /min
• VTI of 18 cm and HR <55/min indicates decreased SV
• HR > 95/min VTI should be < 22 cm otherwise increased SV and cardiac output is assumed
• VTI should always be around 18 cm irrespective of HR

Because LVOT area remains the same for each patient, a surrogate for the SV is measurement of the VTI only. Changes in VTI measurements will enable a “trend” to be followed as the basis for haemodynamic monitoring.
Initial evaluation of SV :
• Have the patient lying supine.
• Obtain a good 5 or 3 chamber apical view.
• Place the PW Doppler through the AV with the sample volume just above the aortic valve.
• Have the Doppler placed as much in line with the left ventricular outflow tract flow as possible by using colour Doppler to evaluate flow direction.
• Trace the VTI and obtain a measurement.
• Then 2 options, one, raise the legs from horizontal to 90 degrees. Repeat the LVOT VTI measurement within 90 seconds. A change in LVOT VTI of > 12% indicates fluid responsiveness.
• The other option is to give a bolus of 500 mls of fluid and then repeat the LVOT VTI measurement.
A consistent increase in the VTI (12 – 20%) after intervention (fluids, ionotropes, vasopressors, pericardiocentesis) or PLR indicates a successful response.

Limitations of the LVOT VTI as a surrogate for SV:
1. Dynamic LVOT obstruction such as in

1. Severe hypovolaemia
2. Asymmetric LV septal hypertrophy esp at low preload and high ionotropic stimulation)
3. Compensatory hyper dynamic basal segments in anterior myocardial infarction

2. Moderate to severe aortic regurgitation ( the regurgitant diastolic volume into the LV is added onto the LVOT VTI in systole – check for AR with colour Doppler

3. LVOT VTI measurements at baseline may be difficult to obtain because of difficulty in obtaining 3 or 5 chamber views

• May use VTI measurements then through RVOT or mitral valve instead to look at trending of VTI measurement and assessment of therapeutic responses.
• May use VTI in descending aorta (should be 70% of LVOT VTI as 30% of flow has gone to coronary arteries)

Problems with this evaluation

1.  PLR can’t be done on an amputee
2.  PLR cannot be done on patients  where mobilisation is not allowed (head injury, trauma)
3. Requires operator skill – (Proper Beam alignment of PW Doppler, good apical window) of requirements for proper evaluation requires around having done about 150 supervised scans with correct VTI measurements
4. Requires operator to correctly interpret data

References:

1. Rapid Ultrasound in Shock (RUSH) Velocity-Time Integral A Proposal to Expand the RUSH Protocol Authors Blanco Pablo MD, Aguiar Francisco Miralles MD, Blaivas Michael
   Assessment of Stroke volume for fluid responsiveness by measuring the increase in stroke volume with passive leg raising or a fluid bolus challenge.