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AMBULATORY ANESTHESIA

Carbon Dioxide Absorption During Extraperitoneal and Transperitoneal Endoscopic Hernioplasty

Eberhard Sumpf, MD^*, Thomas Allen Crozier, MD, PhD^*, Dirk Ahrens, MD^*, Amselm Bräuer, MD^*, Thomas Neufang, MD, and Ulrich Braun, MD

Departments of *Anesthesiology, Emergency and Intensive Care Medicine, and Surgery, University of Göttingen Medical School, Göttingen, Germany

Address correspondence and reprint requests to Thomas A. Crozier, MD, PhD, Zentrum Anaesthesiologie, Rettungs- und Intensivmedizin, Robert-Koch-Str. 40, 37075 Göttingen, Germany. Address e-mail to tcrozie{at}gwdg.de

	Abstract

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Transabdominal preperitoneal (TAPP) or total extraperitoneal (TEP) hernioplasty are probably associated with differing degrees of CO₂ absorption which can influence anesthetic management and perioperative morbidity. We studied 20 patients with either TAPP or TEP for perioperative CO₂ absorption (calculated from CO₂ elimination and metabolic CO₂ production) and ventilatory changes required to maintain normocapnia (blood gas analyses). CO₂ absorption reached plateau values in the TAPP group, but increased over time in the TEP group. Median CO₂ absorption during insufflation was 61 mL/min (range 43–78) for TAPP and 114 mL/min (range 75–178) for TEP, with a maximum of 114 mL/min (range 75–178) for TAPP and 258 mL/min (range 112–585) for TEP. Median minute ventilation (_E) required for maintaining normocapnia was 9.5 L/min (range 7.7–11.5) for TAPP and 12.9 L/min (range 9.0–22.6) for TEP (P < 0.01). Seven patients in the TEP group required over 18 L/min _E, although no patient in the TAPP group required more than 14 L/min _E. All patients in the TEP group had significant subcutaneous emphysema resulting in one case of delayed tracheal extubation. We conclude that CO₂ absorption is consistently less with TAPP.

Implications: The greater magnitude of carbon dioxide absorption during total extraperitoneal hernioplasty puts an additional load on the lungs and could pose a risk for patients with chronic lung disease who might be unable to eliminate excess carbon dixoide.

	Introduction

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Endoscopic surgery is replacing conventional surgical techniques for many common operations. Laparoscopic-endoscopic hernioplasty is associated with a faster convalescence, as well as a less frequent rate of complications and of recurrent hernia (1,2). There are two different endoscopic techniques: the transabdominal preperitoneal (TAPP) hernioplasty, in which the surgeon approaches the hernial orifice through a pneumoperitoneum, and the total extraperitoneal (TEP) hernioplasty, in which a cavity is formed in the preperitoneal tissue with a balloon which is then insufflated with carbon dioxide (CO₂) without breaching the peritoneum. The latter technique has fewer surgical complications and could possibly be performed under regional anesthesia (3,4).

CO₂ absorption is a problem associated with laparoscopic surgery, which can cause significant morbidity if alveolar ventilation is not increased sufficiently to avoid hypercarbia and acidosis (5,6). The uptake of exogenous CO₂ in laparoscopic cholecystectomy or pelvic surgery is well documented (7–11); however, data from endoscopic hernioplasty are scarce. Most studies tend to underestimate the true amount of CO₂ absorption, because the protocols permitted significant loading of endogenous CO₂ stores. There is evidence that CO₂ absorption is increased if insufflation is not confined to the peritoneal cavity (12,13). In both techniques of endoscopic hernioplasty, there is significant insufflation of CO₂ into extraperitoneal tissues, with the probability of substantial CO₂ absorption. Many adult patients requiring hernia repair have either chronic obstructive or restrictive lung disease which can impair the increase in alveolar ventilation required to eliminate an additional CO₂ load (14), and could put them at risk of significant hypercapnia and acidosis, with their associated morbidity, during endoscopic hernioplasty.

We quantified the magnitude of CO₂ uptake during laparoscopic hernioplasty and the increase in ventilation required to maintain normocapnia and compared the two endoscopic techniques with respect to these variables. This information should be valuable when deciding which surgical approach to hernia repair is most appropriate for each patient and when assessing the intraoperative risk.

	Methods

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We recruited patients of ASA physical status I–III, older than 18 yr, and scheduled for endoscopic hernioplasty, for this study conducted with the approval of our institutional ethics review committee. Exclusion criteria were a ASA physical status of IV or more, body weight more than 50% above normal according to Broca, allergies to the protocol drugs, or impaired arterial circulation of the hands. After giving written consent, the patients were randomly allocated to have the hernioplasty performed either by the TAPP or the TEP method. If the peritoneum was perforated during TEP, the patients were withdrawn from that group and evaluated in a separate group (extra-intra peritoneal EIP). Additional patients replaced those removed from TEP group to keep the group at 10 patients.

Anesthesia was achieved with sufentanil and propofol by continuous infusion and cisatracurium was used for neuromuscular relaxation. After orotracheal intubation, the lungs were ventilated at an inspiration-expiration time ratio of 1:1 with no inspiratory hold and no rebreathing. Positive end-expiratory pressure was 5 cm H₂O and FIO₂ was 40% in nitrogen. Unwarmed CO₂ was used for insufflation at a maximum flow of 9 L/min; the intracavitary pressure was kept at 14 mm Hg. After insufflation was complete, the patients were positioned in a 10° head down tilt. The initial respiratory rate was 10 breaths/min with a tidal volume of 8 mL/kg body weight. Minute ventilation (_E) was adjusted during insufflation to keep PaCO₂ within ±2 mm Hg of the initial value. An indwelling catheter for blood sampling was inserted into the radial artery of the nondominant arm under local anesthesia before the induction. Arterial blood gases were determined before the induction and then, every 20 min; end-tidal CO₂ was used for adjustments during the intervals. _E was altered by increasing the tidal volume up to 1000 mL and thereafter, by increasing the respiratory rate. Airway pressures and dynamic lung compliance were measured with a Datex Capnomac Ultima^® (Hoyer-Engström, Bremen, Germany).

Oxygen uptake (O₂), total CO₂ elimination (CO_2tot), _E, and respiratory exchange ratio (RER) were measured with the Datex Deltatrac^® metabolic monitor (Hoyer Engström) which can be used either in a respirator mode for mechanically ventilated patients or in a canopy mode for spontaneously breathing, nonintubated patients. This device has been described in detail elsewhere (15). The O₂ and CO₂ analyzers were calibrated with 95% O₂ and 5% CO₂. The measured gas volumes were corrected to standard temperature and pressure dry. The error in respirator mode at an FIO₂ of 0.4 and a mixed expiratory CO₂ concentration of 4%, is 3% for CO₂ and 3.8% for O₂. In canopy mode using room air, the error is 2.5% for CO₂ and 3.5% for O₂. The measurements were started after an equilibration period of 15 min after intubation. The data from this point until the start of insufflation were used to calculate the RER_basal. Metabolic CO₂ production (CO_2metab) was calculated from the measured oxygen uptake assuming RER_basal remained constant during the duration of the study.

Because CO₂ stores were kept constant, the amount of CO₂ absorbed was:

There were four measuring periods for each patient: i) baseline: after equilibration until the start of insufflation; ii) insufflation period: start of insufflation to removal of the trocar; iii) desufflation period: removal of the trocar until extubation; and iv) recovery room: spontaneously breathing patient. Data were collected online at a sampling rate of once per minute.

The extent and severity of subcutaneous emphysema was assessed on a four point scale: 0 = no emphysema, 1 = mild emphysema with crepitations around the puncture sites or in the groin, 2 = marked emphysema with crepitations extending to the abdomen and the thighs, 3 = massive emphysema extending to the chest or affecting the neck and face.

Maximum and median CO₂ elimination and _E were averaged for every patient in each study period. The groups were compared with the Kruskal-Wallis test supplemented with the Mann-Whitney U-test, as necessary. Analysis of change over time in each group was performed with the Wilcoxon’s signed rank test. P < 0.05 was considered significant.

	Results

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A total of 25 patients participated in this study. There were 10 patients in each of the two study groups (TAPP 10 men, 0 women; TEP 10 men, 0 women). Five patients initially in the TEP group who sustained accidental peritoneal perforation were reassigned post hoc to a EIP group (4 men, 1 woman).

The groups were comparable in age and weight (TAPP 51 ± 11 yrs, 85 ± 11 kg; TEP 47 ± 14 yrs, 81 ± 5.6 kg; EIP 58 ± 21 yrs, 76 ± kg). The mean insufflation time was significantly shorter in the group with TEP compared with TAPP (61 [range 25–110] vs 85 [range 58–137] min). The duration of insufflation in the five patients of group EIP was longer than in the two other study groups (110 [range 88–119] min). Insufflation in the supine position reduced dynamic lung compliance in groups TAPP and EIP with a greater reduction in the TAPP group (Table 1). Head-down tilt caused a further decrease in compliance with the greatest reductions again in groups TAPP and EIP.

View this table: [in this window] [in a new window]   Table 2. Carbon Dioxide Elimination (CO2 tot) and Uptake (CO2 absorb), Arterial Carbon Dioxide Tension, and Ventilatory Minute Volumes

View larger version (28K): [in this window] [in a new window]   Figure 1. Time course of intraoperative carbon dioxide absorption for each patient. TAPP = transabdominal preperitoneal, TEP = totally extraperitoneal, EIP = extra-intra peritoneal.

View larger version (30K): [in this window] [in a new window]   Figure 2. Time course of carbon dioxide absorption after termination of insufflation. The break in each curve is the period of transport from the operating room to the postanesthesia care unit. TAPP = transabdominal preperitoneal, TEP = totally extraperitoneal, EIP = extra-intra peritoneal.

All patients in the TEP group, four of five in the EIP group, but only two in the TAPP group had subcutaneous emphysema. The distribution by severity was 8/2/0/0 in the TAPP group, 0/4/3/3 in the TEP group, and 1/0/3/1 in the EIP group (grades 0, 1, 2 and 3, respectively). One TEP patient suffered from severe emphysema involving the entire trunk, as well as the neck and face up to the eyebrows. Pharyngeal emphysema was present and the endotracheal tube was left in position postoperatively for 50 min until the emphysema had subsided and extubation was considered safe. There was a marked association between the severity of subcutaneous emphysema and the magnitude of maximum CO₂ absorption (Figure 3).

View larger version (17K): [in this window] [in a new window]   Figure 3. Relationship between degree of subcutaneous emphysema and magnitude of maximum carbon dioxide (CO2) absorption. TEP = totally extraperitoneal, TAPP = transabdominal preperitoneal, EIP = extra-intra peritoneal.

	Discussion

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One cannot measure CO₂ absorption directly, and precautions must be taken to ensure that indirect measurements accurately reflect CO₂ uptake. The indirect method is based on measuring pulmonary CO₂ elimination and subtracting from it the amount of endogenous CO₂. Accurate calculation of the latter from oxygen uptake and RER_basal requires that the RER_basal remains constant during the study period. Our patients were in a fasting state in which energy is derived mainly from fat metabolism. They were not given carbohydrates, and we assumed that the lipid emulsion of propofol would have no effect on RER_basal. Loading or depleting endogenous CO₂ stores during the study period will lead to an under- or overestimation of absorbed CO₂. Arterial PCO₂ was kept within narrow boundaries because changes of the rapid CO₂ stores are reflected by arterial PCO₂ so that the indirectly calculated values reflect fairly accurately the true extent of CO₂ absorption.

We observed a significant increase of CO₂ elimination in all groups during insufflation. Endogenous CO₂ production remained constant during the study period, so that the increase of CO₂ elimination could be attributed to absorption of exogenous CO₂. In patients undergoing TAPP hernioplasty, total CO₂ elimination increased during the first 20 minutes after beginning insufflation by a mean of 46% more than baseline and then, remained constant. _E had to be increased by an average of 35% to maintain normocapnia. This rate of CO₂ absorbtion is higher than the values of 20 to 40 mL/min described earlier for diagnostic pelviscopy or laparoscopic cholecystectomy (7,9,11,12,16). In these studies, either no attempt was undertaken to keep CO₂ stores constant, or else capnometry, and not arterial blood gas analysis, was used to monitor arterial CO₂ and guide ventilatory adjustments. However, end-tidal CO₂ often underestimates arterial CO₂ during laparoscopic surgery and the inaccuracy increases with the duration of insufflation (17,18).

In patients undergoing TEP hernioplasty, CO₂ absorption increased steadily during insufflation and usually did not level off. The median absorption rate was much higher in this group than in the TAPP group and there was a large interindividual variability. This is similar to the results of previous studies involving TEP CO₂ insufflation for renal surgery or pelvic lymphadenectomy (12,19,20). The magnitude of CO₂ absorption found in our study was higher than that described by others; however, this again can be attributed to the systematic underestimation inherent in the study design of these investigations. Wolf et al. (19) described a mean increase of CO₂ uptake of 44% during TEP pelvic operations; 127% if subcutaneous emphysema was present. That study calculated data retrospectively from ventilator settings and capnometry data, ventilatory adjustments having been made on the basis of end-tidal CO₂. Mullet et al. (12) observed a mean peak total CO₂ elimination of 210 mL/min during TEP pelvic surgery. This is much lower than the corresponding value of 445 mL/min in our patients undergoing TEP hernioplasty. This discrepancy is probably because of the fact that the authors did not adjust ventilation, but kept it constant and allowed end-tidal CO₂ to increase from 24 to 43 mm Hg in the course of 45 minutes. The data of both Mullet et al. (12) and Wolf et al. (19) confirm our observation that CO₂ absorption does not reach a plateau during TEP procedures, but increases steadily as long as insufflation continues.

There are probably two mechanisms responsible for the difference in the rate of CO₂ absorption during intra- or extraperitoneal insufflation. On one hand, absorption from the peritoneum decreases after insufflation pressure increases above a certain level; it is less at the commonly used pressure of 14–18 mm Hg than at 10–12 mm Hg, probably because of a reduction of capillary blood flow (11,21). On the other hand, one can assume that the resorptive surface area is much larger when gas is insufflated directly into extraperitoneal tissue. This assumption is supported by our observation that CO₂ absorption increases with the magnitude of subcutaneous emphysema. Glascock et al. (20) also suggests that direct intravascular uptake of CO₂ might be facilitated by the disruption of microvascular and lymphatic channels during the development of the TEP working space. The resorptive surface can increase with time as subcutaneous emphysema develops and spreads, which would explain why one observes a steady increase in CO₂ absorption during the insufflation period in the TEP procedures. The relevance of subcutaneous emphysema as an independent predictor of increased CO₂ absorption was described in earlier publications (13,22,23) and was confirmed by our data. Blobner et al. (23) used a method similar to that we used to measure CO₂ absorption in patients undergoing laparoscopic cholecystectomy who developed subcutaneous emphysema. They found a maximum median total CO₂ elimination of 441 mL/min with individual maximum values of 600 mL/min. These data resemble ours very closely, although the authors did allow a certain amount of CO₂ retention which is revealed by the increases of arterial PCO₂.

Our data show that CO₂ absorption can vary widely both intra- and interindividually in patients undergoing TEP surgical hernioplasty. In two instances, it was so rapid that it was not possible to maintain normocapnia without incurring the risk of barotrauma. In these patients, airway pressures reached 50 mm Hg at a _E of 30 L/min, at which point we did not increase ventilation any further but accepted hypercapnia instead. In patients with restrictive or obstructive lung disease, such an upper limit could be reached even earlier.

During the period from the end of insufflation to tracheal extubation there is a rapid decrease in CO₂ elimination. A similar time course was described by Wurst and Finsterer (24). The apparent CO₂ absorption was only approximately 30 mL/min 30–50 minutes after terminating insufflation even in patients with extensive subcutaneous emphysema. Of course, the rapid reduction of CO₂ elimination to such low levels was probably not caused solely by a decrease in absorption or terminal elimination of residual CO₂ and it probably underestimates the true amount of CO₂ absorption, because we observed CO₂ retention with an increase of arterial PCO₂ because of residual sufentanil effects. This increase was not sufficient to pose a clinical risk.

The data presented in this study may be a relevant factor in the decision whether the TEP or TAPP approach should be used for hernioplasty. The TEP approach to inguinal hernias offers the advantages that the peritoneum is not opened and the procedure is often faster and can be performed with regional anesthesia. However, our data show that CO₂ absorption is much higher with this approach and that patients with pulmonary pathology can be at risk of significant hypercapnia and acidosis. This would pose an additional risk for patients with cardiopulmonary disease (5,6). The increase in _E necessary to prevent hypercapnia, particularly in the presence of subcutaneous emphysema, would be unpleasant for the spontaneously breathing patient, and might be impossible to achieve or sustain even with mechanical ventilation. CO₂ absorption is consistently lower in the TAPP approach. However, this technique has the disadvantage that it is an intraabdominal operation requiring general anesthesia and that the reduction of pulmonary compliance is greater than in the TEP technique.

	References

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