The platelet has recently undergone an expansion of interest, as understanding of their role in hemostasis, inflammation and healing is being further elucidated. Platelets are clearly important in primary hemostasis and a lack of platelets or platelet dysfunction is commonly clinically significant. The focus of this talk is two-fold- The first is to discuss the preparation and uses of platelet-rich plasma for management of thrombocytopenia or thrombocytopathia, while the second is to discuss the emerging role of platelet-rich plasma, platelet-rich gels, and platelet concentrates in supporting wound healing.
Platelet rich plasma, platelet concentrates and platelet transfusions
Almost all people working in busy 24-hour hospitals with access to transfusions have periodically gotten telephone calls from primary care practitioners asking about availability of platelets for transfusion. When the answer is "no, we don't have any available" there is often frustration on the part of the clinician in practice, and disappointment on the part of the referral hospital, who likes to provide service to their colleagues. The most common cause of the desire to transfuse platelets is the presence of severe thrombocytopenia (eg. IMT or ITP dogs). The survival rate of dogs with IMT is usually good, with > 80-90% survival; however, during the time while waiting for platelet rebound, there can be significant hemorrhage. Gastrointestinal and other site hemorrhages can usually be adequately addressed with transfusions as needed. Bleeding into "important" places like the lungs and brain is often poorly tolerated and may be the proximate cause of death or euthanasia. Transfused platelets in ITP have a very short half life, on the order of minutes, so practically they are of limited use unless a specific invasive procedure is planned or if Sources of platelets: Platelets are perhaps the trickiest of the blood components to transfuse, as platelets tend to aggregate and subsequently become non-functional when outside the body. Fresh whole blood (FWB) contains the same number of platelets that the donor has (eg. 200,000-500,000/ul), although it is possible a few are consumed via blood collection techniques. FWB was historically very popular for canine transfusions, although in the last 25 years or so, the use of component therapy has emerged. FWB is convenient as no particular techniques are required. However, blood collection can take 15-20 minutes, and requires the presence of an on-site donor or the willingness of a team member to go home and get a donor. Most larger hospitals have phased out in-house donors for ethical issues; however, FWB does remain a viable option. Of interest, in human medicine there is an increasing push toward the use of FWB during catastrophic/trauma surgeries and other massive transfusions. In dogs with severe thrombocytopenia with ongoing hemorrhage, if FWB is a possibility, it may be beneficial. The client (and clinician!) should be advised of the limited survival time of transfused platelets in immune-mediated disease. In dogs with other causes of thrombocytopenia (eg. Bone marrow disease) it may be more beneficial to give FWB. Platelet-rich plasma (PRP) may be prepared by a soft spin (2000 g for 5 minutes) of a unit of FWB. Each ml of PRP contains 2-2.5 x/ ml of platelets as the unit of FWB. For example if the unit contained 250,000 platelet/ul, you could realistically expect the unit of PRP to contain 500,000/ul. However, a unit of PRP has LESS platelets in it that the unit of FWB- this is because each platelet is not reliably found in the plasma, some stay back with the red blood cells. Platelet concentrate (PC) may be further prepared by following the soft spin with a hard spin (to remove the plasma) and leave only the platelets. PC has the most platelets per ul, but is in the smallest volume. Recently, Callan et al at Penn documented the possibility of platelet apheresis, using a COBE Spectra. In this study, platelets were removed from healthy dogs with minimal sequelae other than the predicated hypocalcemia. ALL of these techniques rely upon having healthy donors in-house and readily available for blood collection. FWB should be transfused within 6-8 hours, although ideally it is given promptly, as should PRP. Platelet concentrates may be stored at room temperature for up to 5 days, but due to risk of contamination, are not widely available. Current investigations include attempts to preserve the platelets with either Thrombosol or DMSO Very recently, Animal Blood Resources, International had launched a commercial platelet concentrate, which has a shelf-life of one year if frozen at -20. On-going efficacy trials may be beneficial in determining the role of this product, but it offers a potentially exciting development in the world of transfusion medicine. Potential indications for platelet transfusion include aplastic anemia with hemorrhage, post-chemotherapy, bone marrow transplant (rare in dogs currently), and planned surgical intervention with < 20,000 platelets/ul. ITP has been and REMAINS a rare indication for platelet transfusion.
What about platelet rich concentrate as a source of healing factors? For many years, the platelet had been considered important only in the actual formation of the platelet plug and its role in primary hemostasis. However, recent advances have documented that as platelets are activated, they release agents that promote tissue repair, inflammation and angiogenesis. The three major storage compartment in platelets are the alpha and dense granules and the lysozomes, Platelet contain many substances including:
- Basic Fibroblast growth factor
- Platelet-derived growth factor
- Epidermal growth factor
- Vascular endothelial growth factor
- Transforming growth factor
- Insulin derived growth factor
- Connective tissue growth factor
There are particularly appealing as they are made from the individual patient, which is appealing from a cost containment standpoint as well as a safety (eg. No risk of infectious agents). A sample of blood (~ 50 ml) is collected from a patient. The platelet rich component is extracted. There are commercial "bed-side" extractors or it can be performed by centrifuging. Of importance, there is little to no concern if the platelets are lysed or otherwise activated, as the goal is to collect the growth factors, not the hemostatic aspects. The concentrate is then mixed with calcium and thrombin and directly applied to the site of the injury (either topically or injected into the affected ligament or tendon.).
What evidence exists that this is beneficial?
Rats with long bone fractures, treated with PRP had an earlier return to function than those treated with saline. Similarly, experimental rats with tympanic membrane perforations had a more rapid return of function that untreated rats. Finally, of more potential interest to criticalists, the use of PRP appeared to improve the colonic strength following a colonic anastomosis in rats.
In 20 patients with Jumper's knee (chronic patellar tendonosis), there was significant improvement in 6 months. In bones, a meta analysis concluded that "Early clinical results suggest that the use of platelet-rich plasma is safe and feasible, but that at present there is no clinical evidence of benefit in either acute or delayed fracture healing" (Griffin) In people with diabetic foot ulcers, delayed healing of such ulcers is ultimately associated with amputation. One small study of cost-effectiveness (Dougherty) suggested that the use of PRP was associated with an increased quality of life and lower cost. However, this clinically has not yet been performed.
In wounds, one study found that traumatic wounds healed more quickly when treated with PRP, while in another study, the application of PRP to the puncture site reduced complication (infection) at the site of cardiac catheterizations.
Two horse studies have been performed. One study concluded that in small wounds on the limbs, PRP was of no benefit, while in another horse study, injection of PRP into the midbody suspensory ligament desmitis in Standardbreds, that there was improvement. A single study of a non-healing tail wound in a dog showed improved healing after the introduction of PRP therapy.