DAN Medical Research
Preliminary studies beginning as early as 1985, the incidence of nitrogen bubbles in divers has been a continuing source of study for dive researchers at DAN and the Center for Hyperbaric Medicine and Environmental Physiology at Duke University Medical Center.
In this study, doppler monitoring at the precordial site (around the heart) were used. This area contains all the bubbles passing from the tissues to the lungs, so it is considered to be the best point to estimate bubble production in the body.
For recreational divers DAN has observed, bubble levels tend to peak at one hour after surfacing and to fall off in two to three hours, so most Doppler testing is done within an hour time frame. Likewise, because this time frame presents a poor clinical tool for physicians, who usually do not receive patients suffering from DCS within an hour after diving, Doppler technology works better as a tool for decompression sickness risk assessment rather than as a diagnostic tool.
DAN collected data and combined these trips with Project Dive Exploration (PDE) studies for 10 years over three phases. It incorporated the methodology of Project Dive Exploration during Phase III. The Doppler field studies took place on liveaboard dive boats in the Caribbean and the Pacific. Subjects were monitored with Doppler ultrasound 20–40 minutes post dive at a precordial site. Knee bends and hand squeezes were used as the movement phase. Doppler scoring Kisman-Masurel scoring was collapsed to Spencer scale and dichotomized to grades 0.1 vs. 2.3.
Doppler Use in Research
Dr. Merrill Spencer of Virginia Mason Medical Center in Seattle, Washington, found a new way to use Doppler ultrasound waves for diving research in the mid-1960s. This led to ultrasound devices used now which send waves through body tissues and catch waves reflected back towards a receiving crystal. Any tissue moving toward or away from the sending crystal will reflect a frequency shift by the velocity of the moving object. Doppler systems analyze the shift and emit an audible chirp or pop when they occur.
As it happens, gas bubbles reflect much more of the sending signal than the blood flow or body tissues, and therefore these bubbles result in a chirp or pop on the Doppler device. Researchers use these detected bubbles to calculate the levels of bubbling, from “no bubbling” to “grade four,” which represents well in excess of 1,000 or more bubbles per minute detected in the veins.