Hyperbaric Oxygen Effects On Angiogenesis

Issues of Concern

Neovascularization occurs during wound healing and encompasses both angiogenesis and vasculogenesis, and the 2 should be distinguished. Vasculogenesis occurs when bone-marrow-derived endothelial precursor cells (EPCs) migrate to damaged tissue to differentiate and grow into new vessels. Though not the focus of this article, it is worth briefly noting that HBOT increases the rate of vasculogenesis via its upregulation of nitric oxide, causing the bone marrow to produce greater numbers of EPCs. Angiogenesis, in contrast to vasculogenesis, is blood vessel growth that occurs as new vessels bud off from existing blood vessels.[4][5][6]

Clinical Significance

Angiogenesis is a complex process that occurs in response to hypoxia, redox stress, and lactate concentration. These stimuli trigger the release of growth factors, principally vascular endothelial growth factor (VEGF), from macrophages. VEGF induces capillary endothelial cells to migrate into the wound, form tubules off post-capillary venules, and connect to existing blood supplies. Various studies, for example, with macrophages in culture, in wound fluid, using rat models, and in human volunteers, have shown that HBOT increases active VEGF production. And there are various proposed mechanisms for how HBOT does so.[7][8][9][10]

One mechanism simply involves the correction of wound hypoxia. As part of the hemostatic process of wound healing, capillaries vasoconstrict, which increases the diffusion distance that oxygen must traverse to reach the endothelial cells. Given that VEGF requires oxygen, the greater diffusion distance decreases the amount of oxygen available to VEGF. HBOT becomes efficacious because it increases oxygen partial pressure gradients between healthy and hypoxic tissues.

Larger oxygen tension gradients improve oxygen diffusion and augment the rate of wound healing. Areas of the body with lower oxygen tension, such as the extremities and trunk, heal more slowly than facial wounds owing to lower oxygen tension. Moreover, it is these tissues (subcutaneous, fascial, tendon, and bone) that are at the most risk of poor wound healing. HOBT mitigates the risk by widening the oxygen partial pressure gradient to increase diffusion distance. One study demonstrated a dramatic diffusion distance increase of 286%, from 64 mcm at an oxygen partial pressure of 100 mm Hg to 247 mcm with a partial pressure of 2000 mm Hg. It requires noting that the wound site must still possess some arterial inflow.

A second proposed mechanism involves oxygen’s induction of VEGF. In a study of human umbilical vein endothelial cells undergoing HBOT, researchers found that VEGF was upregulated in those cells at both mRNA and protein levels. The study hypothesized that HBOT induced binding of AP-1, a transcription factor, to the VEGF promoter to upregulate expression. Two pathways were identified: AP-1 via stress-activated protein kinase/c-June N-terminal kinase (SAPK/JNK) pathway and the extracellular signal-regulated kinase (ERK) pathway. To confirm, both pathways were blocked individually using specific inhibitors and luciferases, which prevented the upregulation of VEGF.

A third proposed mechanism of VEGF induction involves another transcription factor: hypoxia-inducible factor-1-alpha (HIF-1alpha).  During normoxic periods, HIF-1alpha is hydroxylated and then destroyed by the ubiquitin-proteasome pathway. During hypoxic periods, when reactive oxygen species concentration is elevated, HIF-1alpha avoids hydroxylation and thereby acts to increase the concentration of hypoxia-inducible factor (HIF). In turn, HIF induces VEGF expression and angiogenesis.  HBOT is thought to promote HIF-1alpha hydroxylation by spurring the formation of ROS via the cycling between hyperoxic and hypoxic states that occurs after HBOT treatments.

As HBOT increases the rate of angiogenesis, there exist myriad indications for its utilization. One such indication is the prevention of graft compromise. The very nature of grafting lends itself to HBOT. A graft is a section of tissue completely separated from the donor tissue bed and all its vascular connections. When placed on the recipient bed, the graft requires the ingrowth of new blood vessels to take. Until angiogenesis occurs, the oxygen tension in the graft is low, and sustained low oxygen tension caused by ischemia is a common etiology of graft compromise. HBOT promotes new vessel growth, and it merits underscoring that the most effective means for treating a compromised graft is preventing compromise altogether by preparing the recipient bed with HBOT before placement.

Preparation of the wound bed is paramount for successful graft take.  Initial measures include surgical debridement of necrotic tissue and infection control. Once complete, the wound should be evaluated to determine if HBOT can raise the oxygen tension to therapeutic levels.  One method is to use an oxygen challenge test with the patient breathing 100% normobaric oxygen:  if transcutaneous oxygen partial pressure “tcPO2) increases at least 10mmHg, HBOT should be efficacious.  However, studies have shown that patients with a minimal increase after the oxygen challenge may still have significant increases in tcPO2 after HBOT. One study demonstrated an increase in tcPO2 to greater than 200 mm Hg during HBOT at 2.5 ATA, which resulted in more than 80% of problem wounds healing. This was notwithstanding poor oxygen challenge results before HBOT.

Another method is to consider hypoxic any wound that lacks a reconstructible vascular lesion and that possesses a tcPO2 of less than 40 mm Hg. The wound is then treated with HBOT. If the tcPO2 increases greater than 200 mm Hg, the wound has a high probability of accepting the graft. HBOT should be continued with daily treatments of 100% oxygen at 2.0 to 2.4 ATA for 90 minutes 5 times per week. Monitor the tcPO2 every 1 to 2 weeks at least 12 hours post-HBOT with the patient on room air; when the tcPO2 levels crest 40 mm Hg, HBOT can be discontinued and the graft placed.

HBOT is an effective means of promoting angiogenesis. Via its ability to increase the oxygen partial pressure gradient and its induction of VEGF, HBOT is an invaluable tool in a physician’s armamentarium when confronted with compromised skin grafts and problem wounds. In conclusion, consider HBOT for osteoradionecrosis and soft-tissue radionecrosis as well. Both of these conditions are produced by radiation-induced small vessel loss and would benefit from angiogenesis aided by HBOT.[11]