Part 1. Laboratory investigations: dose-related biological response of neural tissue
Robert E. Wharen Jr., Robert E. Anderson, Bernd Scheithauer and Thoralf M. Sundt Jr.
✓ The biological response of normal cat brain to Nd:YAG laser light was studied both in vitro and in vivo to evaluate the potential safety of this laser for coagulation in brain tissue. Transmission studies revealed a blood:brain absorption ratio of 100:1 indicating the selective absorption of Nd:YAG light by hemoglobin and enabling Nd:YAG light to selectively heat blood vessels compared to brain tissue. In vivo temperature recordings and pathological evaluation demonstrated a remarkable ability of the brain to dissipate the thermal energy produced by Nd:YAG light with only a small amount of structural damage. Powers of 10 W applied for 8 seconds using a 1.2-mm focused probe resulted in a penetration depth in normal brain of only 2 mm. Thermal recordings also revealed that blood is heated to 90% of its maximum temperature within 3 seconds, while the brain temperature increases linearly as the duration of the laser pulse is increased. In addition, the localized heating of brain tissue was cooled rapidly within seconds following cessation of the laser pulse. These findings indicate that by using short, intermittent pulses of light focused upon blood vessels, damage to the surrounding tissue can be minimized, and the Nd:YAG laser can be used safely as an adjunctive measure for hemostasis in many neurosurgical procedures.
Part 2. Clinical studies: an adjunctive measure for hemostasis in resection of arteriovenous malformations
Robert E. Wharen Jr., Robert E. Anderson and Thoralf M. Sundt Jr.
✓ The Nd:YAG laser has been used safely to aid in the resection of 10 cases of parenchymal arteriovenous malformations (AVM's). The laser was, found helpful for: 1) defining the plane between the AVM and the brain; 2) coagulating any dural component of the AVM; and 3) achieving hemostasis of the bed following resection of the lesion. However, its overall benefit in the resection of AVM's remains to be determined, as it could not arrest active high-flow bleeding from the thin-walled vessels feeding the deep portion of the AVM. This was attributed to the inherent characteristics of these vessels, since the instrument has been effective in non-AVM arteries of similar dimensions containing contractile elements in the vessel walls. Future refinements in focusing instrumentation and operative technique should enhance its capabilities and usefulness. When used within the recommended power range, the Nd:YAG laser is safe and its penetration predictable. The fiberoptic cable light delivery system allows excellent mobility of the handpiece, but the protective eyewear laser-light filters reduce the available light to the surgeon. The instrument appears promising but more work is required.