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Just learned some really fascinating things about shock and volume resuscitation and the importance of crystalloid solutions. I found this lecture by Dr Jeffery Guy while going through the PHTLS class and found it to be a real eye opener (and a great lecture). (More great PHTLS lectures here)
Third Space Resuscitation
Consider the following; in a study a number of dogs were allowed to bleed into Level III shock and then they were all transfused with all their own blood that was lost. They all died shortly thereafter. They tried this again, this time giving them an infusion of an isotonic crystalloid solution before transfusing their blood back and many more of them lived and recovered. What they discovered was that in a state of hypovolemic shock, the body initially compensates by shifting fluid from the interstitial spaces and intracellular fluid into the vascular space to try and maintain plasma volume. Therefore for a favorable outcome, it is imperative to resuscitate the third spaces as well. This is achieved by infusing hypertonic solutions of which only one third remains in the vascular space with the remaining two thirds moving out into the interstitial and intracellular spaces within an hour. If only blood or colloids were used this third space resuscitation would not happen and the patient would die.
Permissive Hypotension
He speaks about how much is the right amount to infuse in the field, the short answer is just enough to maintain a systolic BP of around 90, or even just enough to maintain a palpable radial pulse. The point of permissive hypotension can be illustrated by filling a balloon with water and making a small hole in it. The more the balloon is squezeed (increased pressure) the faster water will spurt from the “wound” also clots that may have formed can be blown out by higher pressures. Therefore until bleeding can be definitely stopped (i.e. surgery) permissive hypotension will go a long way to enhance patient outcome.
ARDS, Systemic Edema
Another complication of overhydrating shock patients is the development of ARDS and other systemic edema. These situations typically arise days and weeks after the initial trauma and can then cause death. Many organs are susceptible such as the lungs, kidneys, liver and brain. Many protocols therefore speak of switching to a colloid or hypertonic solution once a defined amount of isotonic crystalloids have been infused.
Increased Respiratory Rate in Shock
I always thought that the primary reason for increased respirations in a shock patient was due the patient’s need to hyperoxygenate their remaining blood supply to maintain tissue perfusion. While this may be true, Dr. Guy mentions another reason for the tachypnea. He mentions the “bellows effect” of ventilation whereby the negative pressure created in the chest during inspiration also draws blood up through the vena cava into the heart. He states that this is the reason for tachypnea, to help increase preload and cardiac output by creating more negative pressure in the thorax.
Normal Saline Vs. Lactated Ringers
PHTLS and many other sources mention that Lactated Ringers is the fluid of choice in hypovolemic shock yet many locales (including NYC and Hudson Valley) generally use normal saline. The reason that LR is the fluid of choice is because prolonged infusion of NS will cause the the patient to become acidotic, impairing many important metabolic functions. The reason given for use of NS is because LR is not compatible with blood products and since the patient will most likely need a transfusion this might be problematic. Dr. Guy mentions that many blood banks use blood that is compatible with LR and also that LR itself is being refined to be more compatible so hopefully many more of us will start using the preferred fluids. (I find this whole thing a bit bizarre, can’t we coordinate with the hospitals and blood banks in order to benefit the patients who need this most?)
