Does Hypercarbia Develop Faster During Laparoscopic Herniorrhaphy Than During Laparoscopic Cholecystectomy? Assessment with Continuous Blood Gas Monitoring Mike S. L. Liem, MD*, Jan-Willem Kallewaard, MDt, Anne Marie G. A. de Smet, MDt, and Theo J. M. V. van Vroonhoven, MD, rhD*

Departments of *Surgery and tAnesthesiology, University Hospital Utrecht, Utrecht, The Netherlands

The use of CO, to create and maintain a pneumoperito- neum during laparoscopic surgery may lead to hypercarbia and acidosis. CO, is also insufflated into the preperitoneal space to create and maintain a pneu- mopreperitoneum for laparoscopic herniorrhaphy. This study examined the influence of CO, pneumopre- peritoneum on the development of hypercarbia and ac- idosis assessed with continuous intraarterial blood gas monitoring. Changes in blood gas values were meas- ured with both continuous intraarterial and intermit- tent blood gas monitoring. Over a 4-mo period, blood gas values of 14 patients undergoing laparoscopic her- niorrhaphy (pneumopreperitoneum) were compared with those of 13 patients undergoing laparoscopic cho- lecystectomy (pneumoperitoneum) in a tertiary referral

center. Additionally, heart rate and blood pressure were measured during stable ventilation at constant insufflation pressure. Pneumopreperitoneum resulted in a significantly faster development of hypercarbia (P = 0.023) and acidosis (P = 0.027) than pneumoperi- toneum. These results were not explained when cor- rected for changes in hemodynamic and ventilatory variables using analysis of covariance. We conclude that the more rapid development of hypercarbia and acidosis during pneumopreperitoneum can be ex- plained by increased CO, absorption through an in- creasing gas exchange area during the procedure and through a larger wound bed.

(Anesth Analg 1995;81:1243-9)

T he use of laparoscopy in general surgery using CO, to create and maintain a pneumoperito- neum has become widely accepted (1,2). Conse-

quently, not only young, fit patients, but also older patients, are exposed to the effects of intraperitoneal CO2 for longer periods of time (3,4). Hypercarbia and acidosis, ventilatory, i.e., decreased pulmonary com- pliance and vital capacity, and cardiovascular effects, i.e., decreased cardiac index and increased systemic vascular resistance, may be seen during this exposure (4 - 8). In older patients with cardiopulmonary disease, it has been suggested that careful intraoperative arte- rial blood gas monitoring may be required (3).

In previous studies (3-5,7), intermittent arterial blood samples were used to obtain values for pH and Pace,. Therefore, this method may fail to detect sud- den changes in Pace, and pH, and merely provides isolated “snapshots” of the course of these values during the procedure. With the introduction of a new

Accepted for publication August 2, 1995. Address correspondence and reprint requests to Mike S. L. Liem,

MD, Department of Surgery, G04.228, University Hospital Utrecht, PO Box 85.500, 3508 GA Utrecht, The Netherlands.

01995 by the International Anesthesia Research Society 0003-2999/95/$5.00

intraarterial blood gas monitor, blood gas trends can be observed accurately and other disadvantages of blood gas analyzers, such as a time delay and in- creased risk of bleeding and infection, can be over- come (9,lO).

Totally extraperitoneal laparoscopic inguinal hernia repair is a promising technique that is increasingly performed (11,12). During the performance of this technique the preperitoneal space is insufflated with CO, (pneumopreperitoneum). The influence of a pneumopreperitoneum on lung compression and COz absorption may differ from that of a pneumoperito- neum.

This study was undertaken to compare the effect of CO, pneumopreperitoneum with CO, pneumoperito- neum on the development of hypercarbia and acido- sis. Additionally, we evaluated continuous intraarte- rial blood gas monitoring in this setting.

Methods

The study was approved by the institutional ethics committee of the University Hospital Utrecht. Written,

Anesth Analg 1995;81:1243-9 1243

1244 LIEM ET AL. ANESTH ANALG HYPERCARBIA DURING LAPAROSCOPIC HERNIA REPAIR 1995;81:1243-9

informed consent was obtained from all patients par- ticipating in this study, which was conducted in a prospective, nonrandomized fashion. Patients who were operated for inguinal hernia repair using a to- tally extraperitoneal laparoscopic approach involving only pneumopreperitoneum were compared with pa- tients who underwent laparoscopic cholecystectomy involving pneumoperitoneum. Both groups met the following inclusion criteria: 1) patients older than 20 yr; 2) suitable for general anesthesia; 3) possibility of a laparoscopic approach, i.e., no history of extensive abdominal surgery, radiotherapy, or serious abdomi- nal inflammation nor advanced pregnancy.

Patients were excluded if serious atherosclerotic pe- ripheral arterial disease or cardiovascular disease was present (9). Additional exclusion criteria were: 1) in- termittent claudication; 2) impossible radial artery cannulation; 3) coagulopathy; 4) use of vasoconstrictor drugs (13).

Patients who were selected for laparoscopic ingui- nal hernia repair were part of an ongoing prospective randomized study to compare totally extraperitoneal hernia repair with conventional inguinal hernia re- pair. These patients met the following additional in- clusion criteria: 1) primary or first recurrent unilateral inguinal hernia; 2) reducible inguinal hernia.

Preoperatively, forced expiratory volume at 1 s and forced vital capacity, were measured in all patients using a Vitalograph (Y (Lameris, Buckingham, UK).

Patients were given similar general anesthesia using the following anesthetics: propofol (2 mg/kg for induction and 2 mg * kg-’ * h-i for maintenance) or midazolam (0.25 mg/kg for induction and 0.2 mg + kg-* * h-’ for maintenance) and sufentanil (0.5 pg/kg for induction and 0.5 pg. kg-i * h-i for main- tenance) for analgesia, and vecuronium (0.1 mg/kg for induction and 0.06 mg * kg-’ * h-i for mainte- nance) or atracrurium (0.3 mg/kg for induction and 0.3 mg * kg-i . h-’ for maintenance) for muscle relax- ation. Patients were ventilated with a gaseous mixture of O2 (40%) and air.

After tracheal intubation, the lungs were mechani- cally ventilated using a Drager 656 Narkosespiromat (Drager, Liibeck, Germany). Patients were ventilated to obtain a steady state, defined by a stable end-tidal carbon-dioxide partial pressure (PETCO~) between 27 and 31.5 mm Hg during at least 5 min. PETCO~ was measured continuously by a side-stream infrared analysis with the sensor placed between the endotra- cheal tube and the breathing circuit (Datex Capnomac Ultima; Instrumentation Corp., Helsinki, Finland). Minute ventilation was recorded using a flow trans- ducer on the expiratory side of the circuit. Peak airway pressure was recorded by an aneroid pressure gauge in the ventilator. Ventilation frequency, tidal volume, FIO~, and minimum airway pressure were recorded on

digital hard copy every 5 min, starting directly after induction.

After tracheal intubation, the radial artery was can- nulated with a 20-gauge intraarterial cannula and the patients were connected to a continuous intraarterial blood gas system (Paratrend 7, Biomedical Sensors; Pfizer, Highwycombe, United Kingdom) for registra- tion of Paoz, Paco2, and pH. The intraarterial blood gas sensor was calibrated and handled strictly accord- ing to the guidelines of the manufacturer. These val- ues were monitored continuously during the opera- tion period and on the recovery ward and were recorded every 5 min. Furthermore, the maximum value of Pace, and the minimum value of pH were identified from all values seen from the continuous blood gas monitor during the operative procedure.

Two arterial blood samples were taken from each patient to analyze Pao,, Paco2, and pH using an ABL520 blood gas analyzer (Radiometer, Copenha- gen, Danmark), and to validate the measurements obtained by the Paratrend 7. Blood samples were taken during the steady state of ventilation before induction of the pneumo(pre)peritoneum (baseline) and 15 min after induction of a pneumo(pre)perito- neum. If there was any reason to doubt the accuracy of the blood gas values obtained by the sensor, an addi- tional blood sample was taken. This was done when blood pressure, electrocardiography, or capnography suggested the possibility of excessive hypercarbia.

Blood pressure was measured by the same intraar- terial catheter as used for the continuous intraarterial blood gas sensor. If a depressed pressure curve was seen, blood pressure was measured noninvasively with a blood pressure manometer. Heart rate was measured by electrocardiography. Values were re- corded every 5 min on digital hard copy.

Ventilation settings were not adjusted unless a pH of 7.20 or lower was observed by continuous intraar- terial blood gas monitoring, at which time ventilator settings were adjusted by altering ventilatory fre- quency and tidal volume. If cardiac arrhythmias or hemodynamic instability were seen, these corrections were made sooner.

Laparoscopic cholecystectomy and totally extraperi- toneal laparoscopic hernia repair were performed in standardized fashions as have been described previ- ously (11,12,14). Both pneumoperitoneum and pneu- mopreperitoneum were induced with CO2 allowing a maximum intra(pre)peritoneal pressure of 15 mm Hg.

As the main outcome measure, we used Pace,. A SD

of 2.5 mm Hg was estimated from the range of Pace, (normal range, 34.9-45.0 mm Hg). With a sample size of 15 patients in each group, a power of 80% and an a! of 0.05, a difference of 2.55 mm Hg between the two groups could be detected.

ANESTH ANALG LIEM ET AL. 1245 1995;81:1243-9 HYPERCARBIA DURING LAPAROSCOPIC HERNIA REPAIR

Table 1. Patient Characteristics

Herniorrhaphy Cholecystectomy pneumopreperitoneum pneumoperitoneum Mean SD Mean SD P value

gex ratio (M/F) ASA grade (I/II) Age (yr) Length (cm) Weight (kg) Vital lung capacity (L) Forced expiratory volume in 1 s (L)

14 13 12/z 5/8 10/4 11/z

56 20.5 48 8.9 0.21 175 10.7 171 9.9 0.36

76.3 15.3 75.3 11.4 0.86 3.77 1.43 3.78 1.13 0.99 3.40 1.40 3.69 1.20 0.57

Baseline and other differences between groups were tested with unpaired Student’s t-test where appropriate.

The method described by Bland and A&man was used previously to assess agreement between blood gas analyzer and intraarterial blood gas sensor (9,15) and was also used because, in fact, the true value remains unknown as both methods have meas- urement errors. This method plots the difference be- tween two methods against their mean, and uses the mean difference (bias) and the SD of the difference (precision) to describe agreement. To assess agree- ment between both methods in their capacity to detect changes in time within one patient, we performed a similar analysis by calculating the change after 15 min of insufflation (15-min value minus baseline value) for each method within each patient. Accordingly, for pH and Pace, we plotted the differences of the change between the methods against the mean changes. Sim- ilarly, bias and precision were calculated. Addition- ally, the 95% confidence intervals of mean, lower, and upper limit of agreement (mean - 2 SD and mean + 2 SD, respectively) were calculated.

The main analyses compared the pneumopreperito- neum with the pneumoperitoneum group to evaluate time trends (every 5 min) in Pace, and pH using either the multivariate or the split-plot approach anal- ysis of variance for repeated measures. As we were interested in changes, we subtracted baseline values from all following measurements. Additionally, heart rate, systolic and diastolic arterial blood pressure, FIO~, Pao*, tidal and minute volume, peak expiratory pressure and positive end-expiratory pressure, pul- monary function, age, weight, and length were used as covariates in the corresponding analysis of covariance.

Values are expressed as mean ? SD. A P value of co.05 was considered statistically significant. All re- ported P values are two-tailed.

Results

Fourteen consecutive patients undergoing elective to- tally extraperitoneal laparoscopic hernia repair and 13

consecutive patients undergoing elective laparoscopic cholecystectomy were included from September 1994 to January 1995 (Table 1). During this period, three patients undergoing laparoscopic cholecystectomy refused to participate. After informed consent no data could be collected from two patients undergoing cholecystectomy and one patient undergoing hernia repair due to failure of the intraarterial blood gas sensor.

Baseline values of Pace, (37.5 + 2.6 mm Hg vs 37.7 + 3.3 mm Hg, y1 = 27, P = 0.81) and pH (7.43 + 0.04 vs 7.44 + 0.03, y1 = 27, P = 0.50) for pneumopreperi- toneum and pneumoperitoneum groups measured by the sensor did not differ between the two groups.

The agreement between measurements made by the intraarterial sensor and blood gas analyzer are shown in Figure 1 A-D. There was a consistent bias in Pace, values (Figure 1B). Bias of pH values 15 min after insufflation was 0.000 and the precision was 0.024 (Figure 1A). Bias of Pace, values at 15 min after in- sufflation was -2.32 mm Hg and precision of Pace, was 2.07 mm Hg (Figure 1B).

Bias and precision of changes in pH were -0.011 and 0.023 (Figure 10. Bias and precision of changes in Pace, were 0.29 mm Hg and 2.35 mm Hg (Figure 1D).

Table 2 displays hemodynamic variables during both procedures. Blood pressure increased slightly in both groups, but increased more during pneumoperi- toneum than during pneumopreperitoneum.

Mean maximum values of Pace, for both proce- dures did not differ statistically significantly (56.1 + 9.2 mm Hg for pneumopreperitoneum, n = 14, vs 51.8 + 9.2 mm Hg for pneumoperitoneum, n = 13; P = 0.24), nor did mean minimum values of pH (7.31 f 0.7, n = 14, vs 7.32 ? 0.7, n = 13; P = 0.92). If we omitted the results of three patients in whom the true maximum value of Pace, and true minimum value of pH were not reached because we intervened ac- cording to our protocol (i.e., a pH of 7.2) the results did not reach significance (54.6 + 9.1 mm Hg for pneumopreperitoneum, n = 12, vs 49.9 mm Hg + 6.2 for pneumoperitoneum, n = 12; P = 0.15), nor did mean minimum values of pH (7.33 + 0.05 for

1246 LIEM ET AL. HYPERCARBIA DURING LAPAROSCOPIC HERNIA REPAIR

ANESTH ANALG 1995;81:1243-9

B “1

average change Of pti P” sensor am blood gas analyzer aVerage change Of P,Cc12 DY sensor and blood 91 anamer

Figure 1. Agreement between intraarterial blood gas sensor and blood gas analyzer. Differences against means for pH (A) and Pace, (B) at 15 min after commencing insufflation. Differences in pH and Pace, on the y axis are calculated by subtracting the values obtained by the intraarterial blood gas sensor from the values obtained by the blood gas analyzer. Mean + 2 SD and mean ~ 2 so represent upper and lower limit of agreement. Agreement between both methods to detect changes wrthin one patient: differences in change against average changes for pH (C) and Pace, (D). Baseline values are subtracted from 15-min values. Differences in pH and Paco, changes on they axts are calculated as above.

Table 2. Hemodynamic Variables During Pneumopreperitoneum and Pneumoperitoneum

Baseline +5 min $10 min +15 min +20min +25 min +30 min +35 min +40 min

Pneumopreperitoneum

kl7 (bpm) 61 14 5 12 59 14 i 11 60 14 -t 10 58 14 +- 9 59 14 + 12 57? 13 6 SAP (mm Hg) 108? 19 127k 22 131 ? 25 131 2 28 127 i- 24 127 -c 16 DAP (mm Hg) 72 -t 15 86 i- 17 86? 15 87? 15 85 + 18 82 t 12

Pneumoperitoneum

kl7 (bpm) 57 13 -t 9 58 13 i 9 59 13 -c 6 60 13 ? 8 62 13 ? 9 61 13 t 11 SAP (mm Hg) 105 i 17 121 t 24 136 -t 35 142 -t 29 144 527 140 5 24 DAP (mm Hg) 65 +- 11 80 i 15 90? 18 94 t 13 89 t 12 88 -c 11

13 11 10 59 i 7 57 -t 4 57 -c 5

125 i 15 122 2 16 120 i- 19 79-tlO 79?8 79 k 10

13 13 13 61 i 10 62 ? 12 61 i 11

135 i 23 132 -t 23 137 -c 30 85 i 13 83 2 14 86 i 15

HR = heart rate; SAP = systohc arterral blood pressure, DAP = dlastohc arterral blood pressure.

pneumopreperitoneum vs 7.33 -+ 0.06 for pneumo- peritoneum; P = 0.74). The maximum observed value of Pace, during pneumopreperitoneum occurred sooner than the maximum value for pneumoperito- neum (41 +- 10 vs 65 ? 29 min after induction of pneumo(pre)peritoneum). During pneumopreperito- neum no clear plateau could be observed.

The insufflation period for all patients during her- niorrhaphy was much shorter than during cholecys- tectomy (42 + 12 min, range 21-62 min, vs 84 2 34 min, range 45-151 min; P = 0.001).

With analysis of variance for repeated measures there was an expected general increase in Pace, (F = 19.8; P < 0.0005) and decrease in pH (F = 17.1; P < 0.0005) averaged over both groups. Furthermore, we found that patients in the pneumopreperitoneum group developed hypercarbia and acidosis more rap- idly. This result was statistically significant. There

were differences for linear time trends between the two groups: a larger increase, i.e., a steeper slope, in Pace, (F = 3.37; P = 0.023; first order polynomial, F = 15.69; P = 0.0007) and a larger decrease in pH (F = 3.23; P = 0.027; first order polynomial, F = 8.55; P = 0.008) for the pneumopreperitoneum group. These re- sults were unaltered when corrected for with one of the covariates and thus none of the covariates could explain the results (analysis of covariance). Four pa- tients undergoing hernia repair were excluded from this analysis, since the insufflation period of their op- eration lasted less than 40 min.

Mean values of Pace, at 5-min intervals are shown in Figure 2 for all patients.

We were forced to adjust ventilation settings in three patients because unacceptable hypercarbia and acidosis developed. Two of them underwent hernior- rhaphy and pneumopreperitoneum. The first was an

ANESTH ANALG LIEM ET AL 1247 1995.81 1243-9 HYPERCARBIA DURING LAPAROSCOPIC HERNIA REPAIR

70

r

30 I

0 5 10 15 20 25 30 35 40

time after insufflation (min)

-.- pneumo-peritoneum -=- pneumo-pre-peritoneum

Figure 2. Pace, values after induction of pneumo(pre)peritoneum. Mean Paco, values at 5-min intervals after insufflation for all pa- tients undergoing either pneumopreperitoneum (squares) or p&u- moperitoneum (triangles). Error bars indicate SD.

80-yr-old male with marginal pulmonary function (forced vital capacity = 1.83, 64% of predicted value; forced expiratory volume at 1 s = 0.97, 47% of pre- dicted value), the second, however, was a 60-yr-old male with adequate pulmonary function. Ventilation had to be adjusted after 45 and 43 min, respectively. The third patient, a healthy 40-yr-old female, under- went cholecystectomy and pneumoperitoneum. After an accidentally increased intraabdominal pressure up to 20 mm Hg, profound hypercarbia developed within 5 min and ventilation had to be adjusted 56 min after insufflation. Hypercarbia and acidosis in all three pa- tients were corrected without complications.

During laparoscopic herniorrhaphy, in most cases, two slopes were observed during the increase in Pace, and the decrease in pH. During the first part of the operation (preperitoneal dissection) the slope was steeper than during the second part (positioning of the mesh). No sudden changes of Pace, and pH after position changes (Trendelenburg and reverse Tren- delenburg position) were observed in any patient.

Postoperatively, no complications of radial artery cannulation nor the operation occurred; recovery and hospital stay were not prolonged.

Discussion

We found that pneumopreperitoneum for laparo- scopic herniorrhaphy resulted in a more rapid in- crease in Pace, and consequent decrease in pH, than pneumoperitoneum for laparoscopic cholecystectomy. However, the maximum observed value for Pace, and the minimum observed value for pH, during the two procedures were not statistically significantly differ- ent. The observed difference in increase in Pace, be- tween the two procedures can be explained by a more

rapid CO, absorption during pneumopreperitoneum. Several other differences between pneumopreperito- neum and pneumoperitoneum could possibly also ac- count for the difference in Pace, and pH,.

Firstly, the lungs may be differently affected by the two procedures, resulting in different dead space or alveolar ventilation changes. A faster and higher in- crease in Pace, may be explained by increased dead space or less alveolar ventilation. However, one would expect this to be less during pneumopreperito- neum than during pneumoperitoneum. Nevertheless, we used tidal and minute volume and peak expiratory pressure as covariates to exclude a possible influence, but no significant change in outcome was observed. Thus, this explanation is not likely to account for our observations.

Secondly, there may have been differences between the two groups in metabolic carbon dioxide produc- tion. This may have been induced by different me- chanical (compressive) effects on venous return and systemic vascular resistance which would alter car- diac output (5,6,16). In both groups, Pace, increased. If this had been the result of changes in the metabolic rate it would have been indicated by more significant changes in oxygen consumption, heart rate, and blood pressure in the pneumopreperitoneum group. Blood pressure slightly increased in both groups, but blood pressure increased more during pneumoperitoneum (Table 2). FIO, was unaltered and Pao, was stable throughout the procedure. Nevertheless, we used blood pressure, heart rate, FIO,, and Pao, as covariates to exclude a possible influence, but no significant change in outcome was observed. Therefore, the ob- served changes cannot be accounted for by differences in metabolic carbon dioxide production.

Hence, we believe that the observed effect is entirely due to absorption of CO,, which is more extensive in the preperitoneal space than in the intraperitoneal space. This is probably due to a larger total gas ex- change area as a result of the absence of a natural border. CO, can easily diffuse into and between sub- cutaneous tissues or into the scrotum. In contrast, this is prevented during pneumoperitoneum as the perito- neum functions as a natural border. Furthermore, dur- ing laparoscopic herniorrhaphy before commencing insufflation, a preperitoneal dissection is performed. Lateral dissection for mesh placement during the pro- cedure results in an increasing total gas exchange area (12).

This explanation can be considered in terms of the components of the formula for diffusion of gases be- tween two compartments. Diffusion of gases is depen- dent on a pressure gradient between the two compart- ments and not dependent on the infused volume into the compartments (17). Therefore, it was relevant to control only for intra(pre)peritoneal pressure of CO,.

1248 LIEM ET AL. HYPERCARBIA DURING LAPAROSCOPIC HERNIA REPAIR

ANESTH ANALG 1995:81:1243-9

In this setting, especially the larger gas exchange area but probably also a shorter anatomic distance (diffu- sion through the vascular wall instead of diffusion through the vascular wall and peritoneum) may result in an increased influx into the circulation. This is further supported by our observation of a biphasic Pace, and pH curve during herniorrhaphy. Change in the slope of the curve during increase in Pace, and decrease in pH occurred at the time when preperito- neal dissection was finished and thus no active in- crease in total gas exchange area occurred. However, the larger exchange area remained. Additionally, in- creasing gas exchange area and resultant CO, uptake would explain the slight increases in Pace, sometimes observed when the gall bladder was dissected of its liver bed.

Our observations are also in concordance with those of several others. Mullet et al. (18) measured Vco, and PETCO~ but not Pace, and found a continued increase in these values during extraperitoneal pelviscopy. They concluded that it takes longer during extraperi- toneal absorption to reach a steady state and that there is a continued recruitment of gas exchange area dur- ing the continued dissection. Hall et al. (19) detected a case of hypercarbia during laparoscopic cholecystec- tomy with continuous capnometry due to extensive subcutaneous emphysema which increased the gas exchange area.

The maximum difference between pneumopreperi- toneum and pneumoperitoneum of Pace, amounted to 4.7 mm Hg. This difference was not statistically significant. However, the measured difference is ap- preciable (49% vs 38% increase from baseline values) and might have reached significance if pneumopre- peritoneum was prolonged. This might be expected because maximum Pace, and minimum pH, occurred at the end of the procedure and no clear plateau could be observed. At that time preperitoneal CO, had not yet reached an equilibrium with Pace,. In other words the insufflation period during laparoscopic hernior- rhaphy could have been too short to measure signifi- cant differences.

In three cases it was necessary to adjust ventilation to correct hypercarbia and acidosis. The first patient had poor pulmonary function. Wittgen et al. (20) found preoperative pulmonary function tests to be predictive of whether acidosis develops during the procedure. Although we measured only vital and forced expiratory capacity, their assumption is in agreement with our observation in the BO-yr-old pa- tient. However, it cannot explain why ventilation in the other two patients in our study had to be adjusted due to acidosis. One case could be explained by an abnormally high intraperitoneal pressure; in the other case the patient had adequate pulmonary function. The acidosis in this case might be explained by the use

of sharp preperitoneal dissection during the proce- dure and therefore a larger wound bed than usual.

Position changes are likely to be of minimal influ- ence on Pace, changes. The Trendelenburg position may compromise pulmonary function, i.e., decrease of functional residual capacity and decrease in compli- ance (21). The fact that no clinically important effects on Pace, were seen during insufflation directly after position changes either from Trendelenburg or reverse Trendelenburg to supine position or vice versa, sug- gests that different lung compression caused by posi- tion changes is of minimal influence on Pace,.

We used continuous intraarterial blood gas moni- toring to detect maximum values of Pace, and mini- mum values of pH, exactly and not to miss any trends. There was a consistent bias in absolute values of Paco2, but there was good agreement between contin- uous and traditional blood gas monitoring to detect changes in Pace, and pH, within one patient. There- fore, this bias did not influence our main analyses, since we were interested in changes. Although not the main goal of this study, our analysis of ability to detect changes with continuous blood gas monitoring has not been performed previously, as the validation of previous studies was done during blood gas stability (9). During laparoscopy the expected changes after 15 min of CO, insufflation were detected by both intermittent and continuous blood gas monitoring (Figure 1 C and D).

The advantage of continuous intraarterial monitor- ing was seen clearly in only one case. The acute de- velopment of hypercarbia in this patient was detected by intraarterial blood gas monitoring, but would have also been detected if only capnography was used. However, PETCO~ may sometimes remain constant while Pace, further increases (17) or may inade- quately reflect changes in Pace, (3,4). We believe that the clinical value of continuous intraarterial blood gas monitoring must be limited during laparoscopy, but that it can be useful for further investigations.

In conclusion, it may be expected that laparoscopic herniorrhaphy will be used more often, as more sur- geons are trying to master this technique. Conse- quently, operation time may often be prolonged, especially in difficult cases. Given this, and the devel- opment of more rapid hypercarbia and resultant aci- dosis, patients undergoing this procedure must be monitored adequately and intensely.

We are indebted to Yolanda van der Graaf, MD, PhD, Department of Clinical Epidemiology, for statistical advice and for carefully reviewing the manuscript. We acknowledge the helpful comments of Nigel Turner, MD. Finally, we thank Walter Hermans, Baxter BV, The Netherlands, for logistic support and Ronald van der Meer, Datex, medical electronics BV, The Netherlands, for lending us the video printer for the Datex Capnomac Ultima.

ANESTH ANALG LIEM ET AL. 1249 1995;81:1243-9 HYPERCARBIA DURING LAPAROSCOPIC HERNIA REPAIR

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