College Answer

This area remains controversial with, in reality, no single correct answer. Examiners expected and were prepared to accept a range of approaches (if reasonable)

a)  The type of fluid – crystalloid/colloid . No ideal fluid in all clinical settings.

In general no differences in mortality in critically ill patients, between crystalloids and colloids (SAFE study). However in subgroups, albumin may be useful (sepsis) whilst in neurotrauma, crystalloids may be preferable.

b)  Rate of fluid administration (250-500 ml of colloid /500-1000 ml of crystalloid or
20 ml/kg of crystalloid over 30 min.)
Again, no hard data exist to support either regime, but these are rules of thumb and recommended in the Surviving Sepsis Campaign Guidelines.

c)  A clear defined goal such as a MAP/Urine output or resolution of tachycardia –
commonly used goals in clinical practice.

d)  Defining safety limits – such as an upper limit or an increment of CVP/ PAWP Although no criteria for the above end points exist, an increment in CVP 2-5 mm Hg and PCWP 3-7 mm Hg in 30 min or earlier should be used as an indication to cease fluid challenge. In the absence of invasive monitoring, measurement of JVP and signs of pulmonary oedema should be looked for.

Parameters predicting fluid responsiveness

1)  Clinical endpoints such as collapsed veins and state of peripheral circulation not sensitive.
2)  CVP / PCWP changes poor predictors
Other end points have been proposed:
a)  Systolic pressure variation with respiration b)  Pulse pressure variation with respiration
c)  Stroke volume variation with respiration
d)  Aortic blood velocity variation with respiration e)  Intra-thoracic blood volume
f)   Respiratory variation in SVC / IVC diameter
g)  Haemodynamic responses to passive leg raising.

None of the above has been shown to be a reliable predictor, although the haemodynamic response to passive leg raising is thought to be more sensitive than the rest.  The reliability of some of these end points are also influenced by the presence of positive pressure ventilation

Discussion

There are no set guidelines for fluid administration. It sounds like the examiners were prepared to tolerate a range of wacky responses to this. A 2006 article by JL Vincent attempts to bring some sort of order into the lawless Mad Max wasteland of fluid resuscitation practice; another attempt was made in 2011 by Cecconi et al. I will use his suggestions in this answer.

In summary:

• Which fluid?
  • Colloid and crystalloid are equivalent in terms of mortality (SAFE study)
  • Of the colloids, there is insufficient evidence to recommend one over another in terms of mortality (though albumin may have non-oncotic ancillary effects which may be beneficial in sepsis)
  • Of the crystalloids, "balanced" fluids eg. Plasmalyte-148 are associated with improved mortality (at least in sepsis) when compared to isotonic saline.
• How much fluid?
  • At least in sepsis, perhaps less fluid is better (FEAST study).
  • Conventional teaching recommends 500-1000ml of crystalloid, or (in other sources) 10-20ml/kg. This convention may be closely related to the usual fluid bag content.
• How fast?
  • Rate of administration may be more important than the amount and type of fluid.
  • There is no scientific consensus as to how fast is fas enough.
  • Surviving Sepsis people recommend the fluids be given over 30 minutes.
  • Another technique is SV maximisation - a process where 250ml boluses are given over 5-10 minutes until stroke volume (as measured by invasive hemodynamic monitoring) stops increasing by 10-15% with each bolus
• When to stop?
  • Though not based in any firm evidence, resuscitation endpoints have historically included the following parameter theresholds:
    • MAP > 65mmHg
    • CVP >8mmHg, or a change of over 7mmHg in response to the bolus
    • PAOP change of over 5mmHg in response to the bolus
    • Normal lactate (<2.0mmol/L)
    • Urine output >0.5ml/kg/hr
    • ScvO₂~ 75mmHg
    • Resolution of clinical features of hypovolemia which had given rise to the decision to administer the fluid bolus.

As for the assessment of fluid responsiveness - it is a vast topic, and is dealt with in a chapter dedicated to its bewildering detail.

In brief summary:

• Dynamic manoeuvres are better than static measurements.
• Among dynamic manoeuvres:
  • Expiratory hold manoeuvre (the change in pulse pressure with a 15 second expiratory hold) is the most sensitive and specific manoeuvre, but is limited to mechanically ventilated patients in sinus rhythm.
  • Stroke Volume Variation (SVV) is a good predictor of fluid responsiveness under perfect circumstances, when the hypotensive patient has virtually nothing else wrong with them.
  • Passive leg raise autotransfusion (over 1 minute) is the next most sensitive and specific manoeuvre, and it can be perfomed on patients with arrhythmia and breathing spontaneously
  • Inferior Vena Cava ultrasonography is not an accurate method of assessment.
• Among static measurements,
  • Clinical signs are poor at predicting fluid responsiveness
  • Central venous pressure (CVP) and Pulmonary Artery Wedge Pressure (PAWP)have no correlation whatsoever with preload and cannot be used to assess fluid responsiveness
  • GEDVI or GEDI derived from PiCCO correlate well with preload but do not predict fluid responsiveness.