Fluid Challenge
The fluid challenge is a common test of haemodynamic status used to assess whether a patient is responsive to receiving fluid. Oesophageal Doppler monitoring (ODM) reliably tracks any change in Stroke Volume (SV) in response to a rapidly delivered bolus of 200-250 ml in an adult.
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What does a Fluid Challenge tell us
Preload, afterload and contractility are interdependent.
The Starling Curve demonstrates the increase in contractile force associated with increasing ventricular volume. However, myocardial muscle has functional limits, so the curve reaches a plateau and any downward arm is not viable for sustaining life.(1)
In clinical practice, it is important to know whether your patient’s heart is responsive to additional fluid and this can be measured by oesophageal Doppler, which can precisely identify a 10% change in Stroke Volume (SV).(2)
As such, an increase in SV of 10% or more following a rapid fluid challenge is a reliable indicator that the heart is responsive to volume. This has been observed with boluses of 200-250 mls in clinical trials.(3-5) Other technologies may have to rely on giving 500 mls to look for an increase in SV of 15-20% or more.(6,7)
The 10% change in SV method is only validated:
- with oesophageal Doppler technology
- measuring aortic blood flow directly
- using precise quartz crystal Doppler measurement to detect change.
1 Katz. Circulation 1965;32:871-5.
2 Singer et al. Crit Care Med 1989;17:447-52.
3 Mythen and Webb. Arch Surg 1995;130:423-9.
4 Singer and Bennett. Crit Care Med 1991;19:1132-7.
5 Wakeling et al. Br J Anaesth 2005;95:634-42.
6 Benomar et al. Intensive Care Med 2010;36:1875-81.
7 Cecconi et al. Minerva Anestesiol 2010;76:1010-7.
Doppler Flow Parameters
Heart Rate (HR) and Blood Pressure (BP) are commonly recorded. More direct measurements of blood flow from the heart can be achieved with oesophageal Doppler technology:
- Cardiac Output (CO) = Stroke Volume (SV) x HR
- SV = Stroke Distance (SD) x aortic root diameter (nomogram)
(The latter being mostly influenced by an adult patient’s age as aortic diameter generally increases with age.)
SD, also known as Velocity-Time Integral (VTI), is the direct measurement of blood flow using highly precise and accurate Doppler ultrasound centrally at the descending thoracic aorta. The area under the curve of the Doppler waveform is the VTI spectrum analysis of red blood cells passing through the aorta with each heartbeat. Doppler measurements are made 180 times per second (60 per heart beat) to acquire this information.
Note: Doppler’s precision means any change in flow is important.
Direct measurement with oesophageal Doppler tracks changes in aortic blood flow. Conversely, CO, calculated from arterial pressure waveforms, is adversely affected by pressure change and may be a poor guide for changes in blood volume or contractility. (8-10)
8 Chatti et al. BJA 2009;102:463-9.
9 Nordström et al. BJA 2012;110:374-80.
10 Hamilton-Davies et al. Intensive Care Med 1997;23:276-81.
Peak Velocity (PV) = the fastest speed of red blood cells ejected during the cardiac cycle. Normal PV values are age related and give you an indication of contractility. It is affected by ventricular loading and the resistance or afterload against which the heart is pumping.
The shape of the waveform gives information about left ventricular function. More upright waveforms may indicate better left ventricular (LV) function, whereas a flatter waveform may indicate reduced ventricular function.
Flow Time (FT) = the duration of systolic blood flow and this is corrected for heart rate (FTc). The range is 330-360 ms in healthy resting individuals.
FTc is inversely related to afterload:
- If afterload/resistance increases, FTc is likely to decrease, most commonly caused by hypovolaemia (preload decreases). This is because a reduction in splanchnic flow occurs immediately following a drop in circulating blood volume. This results in an increased afterload and a subsequent drop in FTc. Other causes of an increasing afterload could be the use of vasoconstricting drugs or hypothermia. Then consider LV dysfunction or perhaps right ventricular (RV) issues.
- FTc greater than 360 ms will be seen in low resistance/afterload states such as sepsis or with the use of anaesthesia or other vasodilating drugs. If afterload/resistance increases, FTc is likely to decrease (<330 ms).
Mean Acceleration (MA) can also be used as an indicator of contractility. A more upright stroke will show a higher MA and a more rounded waveform will show a lower MA. Normal values are not available, therefore trends can be used.
Note: It may be prudent to aim for optimal values rather than simply aiming for the normal ranges of healthy resting individuals.
Flow Indicators
While individual patient history needs to be considered, the type of response observed with a fluid challenge can be a basic indicator of current patient status.
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