Of all the sites where bubbles form from supersaturated dissolved gas, most is known about bubbles in the veins. This reason in no small part is that Doppler technology has made it relatively simple to detect bubbles moving in the venous system. Doppler ultrasound systems can detect flow in blood vessels and generate an audible flow signal. The passage of a bubble through the ultrasound beam causes a characteristic ‘chirp’ over and above the background ‘whooshing’ sound of the flowing blood. More recently, high-quality echocardiography has become feasible with the use of small portable devices. This allows direct visualization of bubbles as they arrive in the right side of the heart. Much research about the quality of decompression has been conducted using quantitative estimates of venous bubble ‘grades’ as the outcome measure.
It has become clear that when bubbles form from supersaturated dissolved gas in blood they first appear in the veins (as opposed to the arteries, which are discussed later). This is logical because these bubbles almost certainly develop in the capillary beds of supersaturated tissues, and they can be expected to distribute with the flow of blood into the venous system. Bubbles may be present minutes after a dive, but their detection typically peaks around 30 minutes after surfacing and then may be sustained for several hours. Studies in animals suggest that these bubbles vary in size between 19 and 700 micrometres. This variability is difficult to interpret because once formed, bubbles may coalesce to create larger bubbles. Nevertheless, when it is considered that capillary diameter is somewhere around 5 to 10 micrometres, it is clear that even the smallest of these bubbles are likely to interact physically with any tissue bed that they enter.
Bubbles forming in the veins, often referred to as ‘venous gas emboli’ (VGE), pass to the right side of the heart and thence to the lungs, where they encounter a capillary network for the first time. Given that the pulmonary circulation is a low-pressure system, it is perhaps not surprising that the lungs are an efficient filter for VGE. Removal of most incoming VGE by the lungs has been documented in numerous experiments in both animals and humans. In most cases this does not appear to be associated with obvious harm, although harm can occur (see later) and this is probably dependent on the number of bubbles and their rate of arrival.
The formation of VGE after diving has been extensively studied. These studies have required a standardized means of quantifying the detected bubbles. There are two bubble classification and scoring systems commonly in use: the methods described by Spencer, and by Kisman and Masurel.
Spencer’s grading system monitors the precordium for bubbles with the subject sitting quietly:
Grade 0 – No bubble signals on Doppler.
Grade I – An occasional bubble but with most cardiac periods free.
Grade 2 – With many, but less than half, of the cardiac periods containing Doppler signals.
Grade 3 – Most of the cardiac periods containing showers or single bubble signals, but not dominating or overriding the cardiac motion signals.
Grade 4 – The maximal detectable bubble signals sounding continuously throughout the heart cycle and overriding the amplitude of the normal cardiac signal.
The Kisman-Masurel classification system was designed with the aim of easily incorporating the bubble signal into a computer program. The bubble signal is divided into three separate categories – frequency, percentage of cardiac cycles with bubbles/duration of bubbles and amplitude. Each component is graded separately and then a single-digit bubble grade is awarded. The primary site for monitoring is the precordium over the right side of the heart. Monitoring is conducted at rest and after a specified movement, typically a deep knee bend (squatting up and down in a continuous fashion). Although this system appears more complicated than the Spencer system, it is the preference of many researchers and is easily learned.
Both systems suffer from interobserver error with some difficulties in subjectivity. Although computer-based counting programs are being developed, human observers are still more accurate than the automated models.
Most studies of VGE formation have had one or both of two interrelated goals in mind – first, to establish the relationship between appearance of VGE and DCS; and second, refining decompression strategies. In regard to the first goal, it has become abundantly clear that VGE are commonly detectable after recreational dives that do not result in DCS. Indeed, even the presence of high-grade bubbles is often not associated with the appearance of symptoms, and the positive predictive value of VGE grades for DCS is thus poor. Large studies do report a correlation between VGE grade and risk (Table 10.1), but this is not precise enough for VGE grades to be used in diagnosis of DCS.
The lack of a ‘diagnostic correlation’ between VGE and DCS symptoms is perhaps not surprising when it is considered that some manifestations of DCS are almost certainly not directly caused by VGE (see later). However, even the weak correlation demonstrated in Table 10.1 suggests that numbers of VGE are at least indicative of the probability of significant bubble disorders in other sites, and this has encouraged the use of VGE grading as an admittedly imperfect tool for assessing and refining decompression strategies. The ongoing use of VGE grading in this regard is also influenced by the lack of readily acceptable alternative outcome measures. For example, performing manned dive trials using DCS as the outcome requires extremely large studies and may encounter ethical objections. It should be obvious from this discussion that although VGE counts are a useful tool in decompression research, great care must be taken over the conclusions that are drawn in relation to the risk of DCS.