With colloids being miscredited by Cochrane and synthetic colloids being hammered by RCTs, it might seem we’re left with crystalloids. So how good is our standard crystalloid, Ringer’s Lactate, at volume replacing a volume depleted patient? This study withdrew blood and replaced it with 3 times the volume of Ringer’s Lactate. Very little of it stayed in the blood stream.
From the study: “Recent data were interpreted to indicate a comparable intravascular volume effect for crystalloids and colloids, challenging the occasionally suggested advantage of using colloids to treat hypovolemia. General physiological knowledge and clinical experience, however, suggest otherwise”.
We also know most of the crystalloid volume from solutions like Ringer’s Lactate/Hartmann’s and Normal Saline will go interstitial. But how much? And can colloids bring it back inside the vessels?
In this study, in healthy test subjects, less than 20% of the RL stayed in the blood stream. 80% ends up as interstitial fluid – and eventually edema. So for every litre of Ringer’s Lactate, the patient only receives around 200ml of intravascular fluid! Now, what to do with all that interstitial fluid? luckily, colloids can bring much of that back into the blood stream, as shown in this trial.
The volume effect was measured with a fancy method involving dye ICG and measuring its concentration to reflect plasma volume, and some fluorescent labeling to red blood cells to measure these. Plasma and red blood cells, this is the total intravascular volume. So it seems this project had pretty good track of both the red blood cells and total intravascular volume, and could say something meaningful about the patient’s volume changes during this trial.
Infusion of Ringer’s Lactate gave a modest 17% increase in intravascular volume, while infusion of albumin 200mg/ml gave a much more impressive 184% increase in volume per volume infused – providing you have interstitial fluid to draw into the circulation. I guess that’s why they call colloids volume expanders…
Of course, this is all in healthy subjects, with an intact vascular wall, where we assume large colloids stay in the blood stream to keep the oncotic pressure up. Other real life studies with sick patients have observed a much smaller difference between the volume effects of crystalloids and colloids, possibly reflecting an altered vascular barrier that also leaks colloids.
It’s an interesting study. It points out that general physiology still holds true – but it also probably shows how really sick patients will mess up your basic physiology.