In Vivo Analysis
Samples were obtained from 10 patients during elective laparoscopic procedures at the Children’s Hospital of Wisconsin in March 2010 (6 in group 1 and 4 in group 2) of similar ages (1.3 ± 2.3 vs 1.4 ± 2.4 years, range 22 days to 6 years), and weights (7.2 ± 6.9 vs 8.7 ± 8.2 kg; range 2.7–21 kg). The volume of gas required for initial insufflation ranged from 0.2 to 1.5 L (0.033 to 0.3 L/kg). The concentrations of gases measured at 3 time points in the procedure are shown in Figure 4. The concentration of air was affected both by the technique (use or nonuse of a preflush of CO2 through the laparoscopic gas delivery tubing, P = 0.0423) and by the time of sampling (P = 0.0015), and these factors explained a large amount of the observed rank variation (ANOVA r2 = 0.79, P = 0.0009). A substantial reduction in air concentration was observed at initial insufflation with preflushing (99.8% ± 0.3% vs 29.9% ± 20.1%, effect −69.9% ± 0.5%, P = 0.0048 by Wilcoxon rank sum test). With both flush techniques, some air was present in the peritoneum through final deflation (17.5% ± 12.5% vs 12.8% ± 3.6%, effect −4.8% ± 13%, P = 0.718).
Air contamination of gas delivered by insufflation and recovered from the intracavitary space was observed in both in vitro and in vivo laparoscopic settings. In contrast to the artificial peritoneum with a known volume of 0.5 L, initial in vivo–measured insufflation volumes ranged from 0.2 to 1.5 L. Pressure-limit settings, degree of muscle relaxation, and the inflation requirements of different surgical procedures contributed to the wide variation in insufflation volumes and air concentrations observed at end-insufflation. Nonetheless, the clinical data indicate that nearly all the delivered insufflation gas may be air rather than CO2 in smaller patients with lower insufflation volumes (weight ≤10 kg and initial insufflation volume 0.03L/kg, as observed in some of our patients). In contrast, even in the absence of preflushing, the concentration of N2 in the adult peritoneum should not exceed 6% to 12 % when the initial insufflation volumes are 1.5 to 3 L, as reported previously.11
The concentrations of N2, O2, and air should be largely predictable, with knowledge of apparatus dead space, peritoneal volume, and the volume of gas used to inflate to the peritoneum. In vitro, the intracavitary concentration of N2 could be predicted by simple dilution as:
where apparatus volume = 0.23 L, fN2roomair = 0.78, in vitro intracavitary volume = 0.5 L, yielding predicted fN2 = 0.40 or 40%. The actual deviations from this idealized predicted fN2 are somewhat flow rate dependent, with a higher air concentration observed at lower flush flow rates (Table 1). This may be explained by deviation from simple bulk flow in the flush tubing, with delayed elimination of air in the slowest moving lateral lamina in the tube, as described by the Hagan-Poiseuille law, and augmented at high flow rates, predicted by application of the Darcy-Weisbach principle of fluid dynamics. The effect of these principles is to increase flush volume required for full air elimination from that predicted by simple volume displacement.
In a patient whose weight is 6 kg, and initial insufflation volume is 0.6 L, the predicted concentration of N2 at end-insufflation is (0.23 L × 0.78)/0.6 L = 0.3 or 30%, similar to the results in the Table for the concentrations observed in our 0.5-L peritoneum simulation. Our clinical results for this patient yielded an air concentration of 35%–38%, as calculated by O2 and N2 concentrations measured, respectively. In the case of 2 infants whose initial insufflation volumes were ≤0.2 L, the air concentration at end-insufflation was >80%. As the initial insufflation volume diminishes and approaches the volume of the insufflation tubing, the concentration of N2 will approximate that of room air in an unflushed system.
Importantly, we found that N2 persisted in the peritoneal cavity during the procedures regardless of initial flush technique. This might be predicted because of its low solubility in blood. In fact, diffusion of intravascular N2 from the circulation into the peritoneum will occur if its partial pressure in blood is more than that within the peritoneum. Using an inspired gas mixture of air and O2 provides a continuous source of N2 to the circulation. Another potentially important source of N2 is entrainment of ambient air during trocar replacements. However, the most significant source of air contamination occurs during initial insufflation from unflushed tubing in small patients.
In adults,12 the greatest risk of vascular injury occurs during initial trocar placement. In neonates, access to the intraperitoneal cavity through the umbilical stump is common. Reports of catastrophic emboli in this population document laceration of the umbilical vein with this approach.7,8 Subsequent risk of vascular disruption during diagnostic and most other laparoscopic procedures is limited. However, dissection of solid organs may cause vessel rupture and bleeding, creating additional opportunity for gas entrainment into the circulation. Kim et al.13 studied the occurrence of venous air embolism during laparoscopic and open abdominal hysterectomy and found that all patients undergoing laparoscopic procedures had gas detectable by transesophageal echocardiography, although none was clinically significant. Additional studies report the presence of intravascular gas during radical prostatectomy,14 hepatic resection,15 cholecystectomy,16 nephrectomy,17 and some rare cases of cardiac arrest and death.18–20
Multiple factors contribute to the risk of clinically significant gas embolus. The size of the vessel tear and the pressure gradient that exists between the gas source and the vasculature will determine the rate of entry of gas into the circulation. Flanagan et al.21 reported that a 14-G needle in the subclavian vein permitted air entry into the circulation at a rate of 100 mL/min when the pressure gradient was 5 cm H2O. Case reports suggest that the lethal volume of IV air for humans is 3 to 5 mL/kg.22,23 Intra-abdominal pressures that exceed venous and right atrial pressures allow gas to enter the venous circulation. Neonatal operations suitable for laparoscopic procedures, such as duodenal atresia and pyloric stenosis, are frequently associated with intravascular volume depletion and low right atrial pressures that create a favorable gradient for gas entrainment. Furthermore, entry of even small volumes of air through residual fetal vessels, including the umbilical vein and ductus venosus, and right-to-left shunting via the ductus arteriosus or patent foramen ovale provide a mechanism for paradoxical emboli and end-organ injury.
Ideally, intracavitary gas during endoscopic procedures is an inert, soluble gas such as CO2 that will minimize morbidity associated with embolus. However, as we have demonstrated in this study, N2 was present in all laparoscopic gases sampled. Efforts to eliminate the air contamination of laparoscopic gases will reduce the fraction of insoluble N2, even if it is not possible to eliminate all of it. This becomes critically significant when the intracavitary volume approaches the dead-space volume of the delivery system. Further study will define optimal flow rates, total flush volumes, and other techniques necessary to reduce the N2 volume to a safe level in the smallest patients.
We recognize several limitations to our study. Although the 0.5-L anesthesia bag was maximally deflated before insufflation, a small volume of room air likely contaminated the samples. However, this error should be relatively constant across all samples and should not affect the results and conclusions of the study. The wide range of weights of our patients contributed to large differences in initial laparoscopic gas volumes and resulted in the large variation of the nonflushed end-insufflation gas concentrations. However, infants, whose initial insufflation volumes were <0.2 L, had 78% to 82% N2 present. Our initial hypothesis proposed that surgical techniques that included preflushing the insufflation tubing would eliminate air contamination (<10%). We did not anticipate the wide variation in flush volumes that resulted in the presence of >20% air in the intraperitoneal gas.
Although the concept of apparatus dead space is familiar to surgeons and anesthesiologists who frequently deal with interfaces between patients and extracorporeal devices (vascular flush systems, breathing circuits and systems, and cardiopulmonary bypass circuits), this concept appears to be underappreciated in the teaching and application of minimally invasive techniques that require gas insufflation for adequate visualization. Because there are no visible distinctions between (undesired) residual air and desired CO2 in the circuit, casual technique allows a significant and potentially lethal volume of pure air to be delivered into a biologic cavity, during which time the insufflation device displays a metered flow of CO2. This may be misleading, as evidenced by our observation of nonuniform air elimination in the preflush group. Simple design and labeling changes could reduce the use of relatively unsafe insufflation techniques. First, the volume of dead space in insufflation tubing should be clearly indicated. Insufflation devices should warn the operator to adequately flush the delivery tubing with at least twice its dead-space volume before insufflation. Insufflation tubing specifically used for neonatal and pediatric surgery should be designed to minimize dead-space volume. Finally, consideration should be given to real-time analysis and display of the composition of gas at the distal insufflation port or from the biologic cavity.
Although we used a Raman spectroscopic device that could directly and accurately measure the concentrations of all relevant gases, the back-calculation of air fraction was nearly the same whether O2 or N2 was used as the analyte. The presence of N2 in insufflation and cavitary gases can be as accurately inferred by using the concentration of O2 as an indicator of air contamination (Figs. 2 and 3). Thus, standard infrared/paramagnetic gas analyzers can be used to test for complete air washout by detecting the concentration of O2 in the samples. Operators and institutions can develop processes to minimize the introduction of N2-containing air into body cavities during laparoscopic surgery, and the efficacy of these processes can be checked using gas analysis equipment readily available in all operating rooms.
In summary, this study demonstrated the presence of N2 and O2 consistent with air concentrations >10% in laparoscopic gas using contemporary equipment and techniques for infant laparoscopy and determined that preflushing techniques requiring <15 seconds can reduce air contamination (concentration <10%) at insufflation flow rates commonly used in clinical practice (3 L/min). Air contamination of insufflation gases can be detected using standard anesthetic gas analyzers.
Name: Susan P. Taylor, MD, MPH.
Contribution: This author helped design the study, conduct the study, and write the manuscript.
Attestation: Susan P. Taylor approved the final manuscript.
Name: Thomas T. Sato, MD.
Contribution: This author helped conduct the study and write the manuscript.
Attestation: Thomas T. Sato approved the final manuscript.
Name: Anthony H. Balcom, MD.
Contribution: This author helped design the study and conduct the study.
Attestation: Anthony H. Balcom approved the final manuscript.
Name: Travis Groth, MD.
Contribution: This author helped conduct the study.
Attestation: Travis Groth approved the final manuscript.
Name: George M. Hoffman, MD.
Contribution: This author helped design the study, conduct the study, analyze the data, and write the manuscript.
Attestation: George M. Hoffman approved the final manuscript.
This manuscript was handled by: Maxime Cannesson, MD, PhD.
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