Issues Of Flow Dynamics

Infusion rate is crucial in resuscitation for severe hypovolemia or hemorrhage. Fluid in a medical catheter behaves for practical purposes according to the Poiseuille law:

Rate offlflw- * * (cat^ireT x prtiiitf* <f Mfiefit

The fact that flow is a function of the fourth power of the radius of the tube lumen means that the internal catheter diameter is a major limiting factor. 2 A fluid delivery system is only as effective as its slowest component, whether this is intravenous tubing, in-line filters, or the catheter itself. Flow rates may also be affected by pressure and viscosity, the latter being an especially important consideration in relation to red blood cell transfusion. Rate of infusion is also directly proportional to catheter length, which is why a long central catheter will have a slower infusion rate than a shorter catheter of the same caliber in a peripheral vein. 3

Placement of two large-bore 16-gauge or greater catheters is indicated in stable adult trauma patients whose injuries could cause potentially life-threatening hemorrhage or for initial therapy of medical patients with hypovolemic shock. In the management of exsanguination, an 8.5-Fr catheter with a manually operated pressure bag or a wall-mounted external pneumatic device delivers crystalloid at the rate of almost 1 L/min. A second catheter may be needed for drug infusion. Rapid infusion of larger volumes of fluid should be accompanied by careful monitoring for volume overload, especially in older patients and those with cardiovascular disease.

Volume repletion and measurement of central venous pressure can be accomplished by a Y-arm catheter sheath passed percutaneously into the femoral vein. An 8.5-Fr catheter can then be used for volume repletion, while a smaller catheter can simultaneously be inserted through the other arm of the Y and directed into the right atrium for the measurement of central venous pressure. Femoral catheters should generally be left in place no longer than 48 h, since iliofemoral thrombophlebitis can result. However, with sterile technique and the use of Silastic catheters, the deep femoral system may safely be employed for a longer duration.

Pressure infusion increases flow two to three times above that achieved by gravity alone and is superior to the use of on-line hand-pumped bulbs. Pressure devices are available for the administration of packed red blood cells. Use of a standard urologic Y irrigation set augments flow rates by reducing resistance in the tubing leading to the catheter site. For maximum infusion rates of either blood or crystalloid, use of blood administration tubing eliminates on-line micropore filters, stopcocks, and one-way valves, which increase resistance. Addition of saline to packed red blood cell infusions decreases viscosity, increasing the speed of transfusion. 2

Volume repletion is effective through IV catheters placed distal to an inflated military antishock trouser (MAST) suit. In patients with abdominal hemorrhage, lines in the legs as well as those in the arms augment volume.

Warming of crystalloid and blood before infusion is essential when volume resuscitation is massive. Crystalloid may be stored in a heating bath or oven, safely microwaved, or warmed with a heating coil or heat packs. Blood warming coils that allow transfusion rates of up to 500 mL/min are now available. Alternatively, cold-packed red blood cells may be warmed by diluting them with an equal amount of warmed saline (up to 60°C); this will also decrease viscosity and thus enhance flow.2 An in-line microwave blood warmer may be used to heat blood safely to 49°C without any significant increase in hemolysis.24

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