Subcutaneous Carbon Dioxide Insufflation Does Not Cause Hypercarbia During Endoscopic Thyroidectomy

Abstract

Endoscopic neck surgery with subcutaneous carbon dioxide (CO2) insufflation has produced severe hypercarbia, making CO2 unsuitable for use in this procedure (1). We report five cases of endoscopic thyroidectomy using a new subcutaneous CO2 insufflation technique that prevents significant hypercarbia by decreasing insufflation pressure. In two of the cases, indirect calorimetry was used to estimate CO2 absorption from the surgical field. Case Report We performed five cases of endoscopic hemithyroidectomy for benign thyroidal adenoma using subcutaneous CO2 insufflation. Preoperative physical examination and laboratory tests revealed no significant abnormalities. The patients were well informed about the surgical procedure as well as the clinical tests to be performed during anesthesia. The study was approved by the institutional human investigation committee, and written, informed consent was obtained from all subjects. The induction of anesthesia was performed with IV administration of propofol and fentanyl with muscle relaxation by vecuronium. In Cases 1 and 2, anesthesia was maintained by a continuous infusion of propofol and fentanyl. In Cases 3, 4, and 5, anesthesia was maintained by nitrous oxide and sevoflurane in oxygen. In all cases, muscle relaxation was achieved by vecuronium. Mechanical ventilation was performed at a tidal volume of 10 mL/kg and a frequency of 12 breaths/min. The usual monitoring was used. The patient was placed in the supine position with the neck extended by using a shoulder pillow. A 15-mm transverse skin incision was made at the precordium at the level of the nipple. The subcutaneous space in the precordial area and the subplatysmal space in the neck were dissected bluntly by using a long subcutaneous dissector. A 12-mm trocar was inserted and CO2 was insufflated at a pressure of 6 mm Hg. The surgical field was visualized by a 10-mm flexible video endoscope via the trocar, with two additional trocars for surgical instrumentation. The subcutaneous area insufflated with CO2 was approximately 20 cm × 25 cm. After dissecting the strap muscles, the thyroid was fully exposed, and hemithyroidectomy was performed. The skin was reapproximated cosmetically, and compression dressing was applied. After the surgery, chest radiographs were examined to eliminate the possibility of pneumomediastinum and to evaluate the extent of subcutaneous emphysema. The duration of CO2 insufflation varied from 160 to 370 min, depending on the technical difficulties (Table 1). PaCO2 gradually increased during CO2 insufflation, and the highest PaCO2 was 44.5 mm Hg (in Case 2) and the PaCO2-PETCO2 gradient was <7 mm Hg in all cases.Table 1: Demographic DataIntraoperative hemodynamics were stable and without complication. Emergence from anesthesia was prompt, and no postoperative events were noted in any of the cases. In Cases 1 and 2, indirect calorimetry was performed by using a metabolic monitor (CS-3, DATEX, Helsinki, Finland). Also, pHa, PaCO2, and PaO2 were measured by using an ex vivo blood gas monitor (VIA; Baxter, San Diego, CA) every 10 min throughout the administration of anesthesia. Oxygen uptake (V̇O2) and CO2 elimination (VECO2) were measured via the airway by using indirect calorimetry. The measured VECO2 includes CO2 absorption through the surgical field (VSCO2) in addition to CO2 production (V̇CO2) of the patients themselves. Because our preliminary observations revealed that the respiratory quotient (RQ) does not significantly change during standard neck surgery, we assumed that the RQ values before CO2 insufflation did not change throughout the surgery. This allows V̇CO2 to be estimated as the product of V̇O2 and RQ. VSCO2 can be calculated as the difference between VECO2 and V̇CO2. MATH 1MATH 2MATH 3 RQ before CO2 insufflation was 0.835 in Case 1 and 0.748 in Case 2. VSCO2 increased significantly, and its median value was 7.3 and 5.3 mL/min in Cases 1 and 2, respectively. VSCO2 contributed 8.9% and 3.3% of V̇CO2, in Cases 1 and 2, respectively. Discussion Minimizing surgical stress and cosmetic problems and improving the postoperative quality of life are cited as important reasons for using the endoscopic approach via incisions in the shoulder and clavicular region in neck surgery (2,3). However, Gottlieb et al. (1) reported severe increases in PaCO2 during endoscopic transcervical parathyroidectomy with CO2 insufflation. In their report, the introduction of high CO2 pressure (as high as 20 mm Hg) was necessary to create a new cavity, and CO2 insufflation was maintained at pressures of 15 mm Hg or higher throughout surgery resulted in severe hypercarbia even with increased minute ventilation up to 20 L/min. Their conclusion was that CO2 is not suitable for use as an expansion agent in the neck. The occurrence of hypercarbia and its severity have been reported as dependent on the CO2 insufflation pressure, and that to avoid complications, the pressure should not exceed 16 mm Hg (4,5). Our solution to this problem was to use a long subcutaneous dissector to create a subcutaneous space. After the dissection, it was clear that an insufflation pressure of 6 mm Hg was high enough to maintain the surgical space and, at the same time, was low enough to avoid significant CO2 absorption through the subcutaneous tissue. Our observations in five cases suggest that CO2 insufflation pressure is more responsible for hypercarbia, because normal minute ventilation of 120 mL · kg−1 · min−1 was sufficient to maintain normocarbia during CO2 insufflation. Indeed, the estimated CO2 absorption by indirect calorimetry during CO2 insufflation was <10% of the estimated CO2 production. Two issues remain regarding the estimation of CO2 production by using indirect calorimetry during CO2 insufflation. Surgical stress increases the secretion of stress hormones (6,7), which usually enhance glucose metabolism, thus increasing RQ (8,9). Such sympathetic stimulation might alter RQ, resulting in higher estimated values for CO2 absorption. However, propofol modifies the metabolic response by increasing fat load. A solvent of propofol, 10% fat emulsion, reduces RQ and CO2 production during propofol infusion (10), resulting in the underestimation of CO2 absorption during CO2 insufflation. It should be noted that CO2 absorption through the subcutaneous tissue was still <10% of CO2 production, even if the actual RQ was lower than our estimates because of the propofol infusion. Further study will be required to confirm our observations. Several clinical trials evaluated the use of various gases, including nitrogen, nitrous oxide, helium or argon, for insufflation to avoid hypercarbia (11–13). Of those gases, helium seems ideal for such purpose, but its low water solubility leads to a lower safety margin than CO2 in the event of gas embolism. If CO2 is chosen as an agent to maintain the surgical field during surgery, its absorption and the resultant hypercarbia must be monitored closely to avoid perioperative complication (14). Our observations suggest that CO2 can be used without hypercarbia in endoscopic surgery as long as insufflation pressure is maintained within a safe range. We also suggest that CO2 absorption can be estimated with reasonable accuracy during surgery by using indirect calorimetry.