Insufflation pressures

FIGURE 16.3 Pulmonary surfactant plays a vital stabilizing role in pulmonary mechanics, (a) In the absence of pulmonary surfactant, airflow is directed into the larger alveolus where pressures are lowest (Pj < P ). resulting in collapse and overdistension of the smaller and larger alveoli, respectively, (b) In the healthy lung with functional pulmonary surfactant, the increased interfacial density of pulmonary surfactant molecules in the compressed smaller alveolus reduces the surface tension. This reduces the insufflation pressure and stabilizes the lung. 

After a barium enema, perforation occurs rarely in children and debilitated adults or when the colon is already weakened by inflammatory, malignant, or parasitic diseases. Perforation can be triggered by manipulations involved in giving the barium enema or can result from hydrostatic pressure. In one case, perforation followed air contrast insufflation for barium enema in a patient in whom the sigmoid colon became trapped in an inguinal hernia. 

Allen, W.F., Effect on respiration, blood pressure, and carotid pulse of various inhaled and insufflated vapors when stimulating one cranial nerve and various combinations of cranial nerves. Am. J. Physiol., 87, 319-325, 1928-1929. 

The basic equipment required for the CT colonography examination is little more than a red rubber catheter with a hand held insufflation bulb similar to that used for barium enema examinations. There are a variety of rectal catheters available of varying size, typically 5-15 mm in diameter. Although we routinely use a balloon-tipped enema catheter, many researchers now avoid balloon insufflation. Traditionally room air has been the gas of choice for colonic insufflation at CT colonography due to its availability and lack of additional expense. However, there is a growing body of evidence advocating the use of carbon dioxide (COj) which is associated with less abdominal cramps and is more rapidly reabsorbed (Yee and Galindo 2002). COj is supplied from a refillable cylinder via a disposable administration set which allows constant gas pressure influx with the facility to record both gas pressures and the volume of COj administered. 

Carbon dioxide may also be insufflated directly from a gas cylinder via a tube with side hole for digitally controlling volume and pressure (Rogalla et al. 2004a). Clearly the pressure of insufflated gas must be carefully controlled using this method. 

Automated insufflation devices are now widely utilised across Europe and the US, despite the additional equipment costs. Advocates suggest that insufflating carbon dioxide at controlled flow rates and pressures is convenient for the operator, and improves distension and patient compliance. 

At the time of writing, the authors are aware of only one commercially available device specifically designed for colonic insufflation (Fig. 5.5, Protocol colon insufflation system, E-Z-EM Inc, Westbury, NY, USA). This system electronically controls the flow rate of carbon dioxide increasing over time in a step wise fashion from 1 to 3 L/min to prevent spasm (1 L/min for the first 0.5 L, 2 L/min from 0.5 to 1.0 L, and then 3.0 L/min thereafter). The total volume of gas administered is displayed continuously and, if intracolonic pressure (measured at the rectal catheter tip) increases beyond the limit set by the user (up to a maximum of 25 mm Hg), the system automatically shuts down to prevent further insufflation and so reduces the risk of colonic perforation. In the latest version, insufflation automatically ceases when a total of 4 L of gas have been administered and then for every 2 L administered beyond this. To recommence insufflation, the operator needs to manually override this additional safety feature by pressing the start button. 

The company s recommended technique is to insufflate the patient in the supine position. The pressure limit is set at 15 mm Hg initially, increasing to 25 mm Hg depending upon patient tolerance. Three litres of carbon dioxide are instilled (again dependent on patient tolerance), at which point the... 

Fig. 5.5. Automated colonic insufflator, connected to a thin rectal catheter, displaying the intralmninal rectal pressure and total volmne of carbon dioxide administered...

Once the supine study has been acquired, the rectal catheter is left in situ and the patient asked to turn prone. A second scout is performed and if distension is deemed suboptimal, the pressure limit will be increased to 25 mm Hg to encourage further gas insufflation. A further scout is performed and when this demonstrates optimal insufflation, the second study is acquired. The examination is then complete and the rectal catheter removed. The patient is reassured that much of the insufflated gas will be absorbed (rather than expelled), and that any abdominal cramping should ease within a few minutes. 

Gomez-Merino E, Sancho J, Marin J, et al. Mechanical insufflation-exsufflation pressure, volume, and flow relationships and the adequacy of the manufacturer s guidelines. Am J Phys... 

Mechanical insufflator-exsufflators (Cough-Assist, J. H. Emerson Co., Cambridge, Massachusettes, U.S.A.) deliver deep insufflations (at positive pressures of 30 to 50 cmH20) followed immediately by deep exsufflations (at negative pressures of -30 to -50 cmH20). The insufflation and exsufflation pressures and delivery times are independently adjustable (71). With an inspiratory time of two seconds and an expiratory time of three seconds, there exists a very good correlation between the pressures used and the flows obtained (72). 

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