Accumulation of carbon dioxide under ophthalmic drapes during eye surgery: a comparison of three different drapes
Carbon dioxide accumulation under ophthalmic drapes is caused by their impaired permeability to exhaled carbon dioxide in spontaneously breathing patients. Three different ophthalmic drapes were examined under clinical conditions. Sixty unpremedicated patients of each gender, aged over 60 years and with an ASA status of I–III undergoing cataract surgery under retrobulbar anaesthesia were included in the study. Patients with known pulmonary diseases were excluded. The patients were divided into three groups of 20 patients each. In all groups, oxygen was insufflated under the drapes at a constant flow of 2 l.min−1. Carbon dioxide concentration in the inspired air, transcutaneous carbon dioxide pressures, respiratory rate and oxygen saturation by pulse oximetry were measured. Accumulation of carbon dioxide under the drapes, increase of partial pressure of transcutaneous carbon dioxide and hyperventilation were observed in all three groups. An oxygen supply of 2 l.min−1 prevented hypoxaemia but not hypercapnia. Therefore, producers of ophthalmic drapes are encouraged to look for further ways to increase the carbon dioxide permeability of their drapes with the aim of reducing carbon dioxide accumulation and hyperventilation in spontaneously breathing patients undergoing eye surgery.
The accumulation of carbon dioxide (CO2) under ophthalmic drapes has been reported in spontaneously breathing patients undergoing ophthalmic surgery under retrobulbar anaesthesia [1–5]. The CO2 exhaled by the patient escapes incompletely through ophthalmic drapes and hence results in an increase of CO2 in the ambient air surrounding the patient's head. This causes an increase in arterial partial CO2 pressure and thus hyperventilation [1, 4].
In recent years, several different types of ophthalmic drapes have been produced. The present study was designed to determine possible differences in CO2 accumulation under different types of ophthalmic drapes used under clinical conditions in spontaneously breathing patients undergoing cataract surgery under retrobulbar anaesthesia.
The University Ethics Committee approved the study protocol. Three different types of ophthalmic drapes were examined in patients undergoing cataract surgery under retrobulbar anaesthesia in a randomised single-blind manner. After receiving written informed consent, 60 unpremedicated patients (ASA I–III) scheduled for cataract surgery under retrobulbar anaesthesia were included in this study. Patients with pre-existing pulmonary disease, psychological or neurological disorders were not studied. Patients needing intra-operative sedation or additional opioid analgesia and patients who developed psychic problems during the operation were not included in the evaluation of data. All patients were allowed solid food or clear fluids until 8 h before anaesthesia. On the morning of surgery, patients were allowed to take all previously prescribed oral medications except diuretics and sedatives.
The study patients were allocated randomly to one of three study groups comprising 20 persons each. An anaesthetist monitored all patients throughout surgery. Electrocardiogram, pulse oximetry and noninvasive blood pressure were established using a standardised monitor (Cardiocap II®, Datex®, Helsinki, Finland).
Draping of the patients
A large drape was placed over each patient, fully covering them except for the head. Then each patient's head was covered with one of the three ophthalmic drapes to be investigated. In group A, the BarrierTM Ophthalmic Drape by Johnson and Johnson® (140 × 140 mm) and in group B, the SteriDrapeTM 1062 by 3M® (135 × 135 cm) was used to drape the patient's head. In group C, each patient's head was covered with a sterile cotton drape (100 × 100 cm) that had a central, circular aperture (80 mm in diameter) over the eye on which the operation was performed. An ophthalmic plastic drape (SteriDrapeTM 1024, 3M®) measuring 46 × 39 cm was put over this cotton drape.
Immediately after draping the patient's head, in all groups, oxygen was insufflated under the drape at a constant flow of 2 l.min−1 throughout the procedure.
CO2 concentration in the ambient air surrounding the patient's head under the drapes (Pco2)
Carbon dioxide was measured using an anaesthesia monitor (Cardiocap® Datex®). To prevent variations in the CO2 curves caused by the patient's breathing, a collection reservoir (150 ml) was placed in the line of the gas sampling tube.
Transcutaneous partial carbon dioxide pressure (Ptcco2) measurement
Calibration of the measuring electrode was performed at 43 °C using a standardised CO2/O2/N2 gas mixture (5% CO2, 20.9% O2, N2 balanced). The electrode for Ptcco2 measurements (TCM 3®-Monitor, Radiometer®, Copenhagen, Denmark) was placed on the left lateral thorax at the level of the fourth intercostal space.
Thoracic excursions were counted for a period of 1 min at each measurement point.
Peripheral oxygen saturation by pulse oximetry (Sp o 2)
Peripheral oxygen saturation was measured using an anaesthesia monitor (Cardiocap®, Datex®).
Baseline values were obtained immediately before the patient's head was draped. Additional measurements were taken at 3, 6, 9, 12 and 15 min after draping the head, and then at 5-min intervals until the end of surgery (drape removed from head) as well as 5 min after complete removal of the remaining drapes.
For statistical analysis, SPSS® 8.0 (SPSS, Inc., Chicago, IL, USA) was used. Demographic data and baseline values were compared among the groups using one-way anova. Because of various individual values at the baseline, delta values (value at measurement point — baseline value) of the respiratory rate, Pco2 and Ptcco2 values were calculated for data analysis and illustration. Data analysis was performed using anova for repeated measurements followed by the two-tailed unpaired Student's t-test for post hoc comparison. Comparisons within the groups were analysed by the paired two-tailed Student's t-test. Differences were considered significant if p ≤ 0.05 after Bonferroni correction for multiple comparison.
Fifty-seven patients were enrolled in the study. Three patients were excluded from data analysis, because in two cases surgery was discontinued for ophthalmic reasons and in one case there were technical problems with the CO2 analyser.
Demographic data were similar in all groups (Table 1). During the investigative period no significant differences in MAP and heart rate were seen either within or between the groups. The total time for which the drapes were applied did not differ significantly between the three groups (Table 1).
Partial pressure of carbon dioxide under the drapes
Covering the patient's head increased Pco2 values significantly in the ambient air in all three types of ophthalmic drapes. (Fig. 1). The lowest Pco2 values were found under the SteriDrape 1064 followed by the Barrier Drape and SteriDrape 1024. Under the SteriDrape 1024, Pco2 values were significantly higher at the third and sixth minute after draping. Beyond that no significant differences were found between the groups.
Transcutaneously measured partial pressure of carbon dioxide
As shown in Fig. 2, in all three study groups Ptcco2 values increased significantly immediately after covering the patient's head. After removing the drapes, Ptcco2 reverted to levels close to baseline.
Comparison between the groups showed that in patients draped with the SteriDrape 1064, Ptcco2 values were lower than in patients draped with the other two types of drape. The observed differences, however, were not significant.
In all three groups, respiratory rate (RR) increased continuously after covering the patient's head. Starting at the third minute after covering, the RR differed significantly in all groups. No significant differences were found between the three groups. Five minutes after removal of the drapes, in all groups, RR decreased significantly compared with the values measured 25 min after draping, but was significantly higher when compared with the baseline values (Fig. 3).
Peripheral oxygen saturation
Three minutes after starting oxygen insufflation, the Spo2 values increased significantly in all groups without differences between the groups (SteriDrape 1024: 95.7 ± 0.7% to 98.1 ± 0.8%, p = 0.003; SteriDrape 1062: 95.6 ± 1.3% to 97.7 ± 1.1%, p = 0.002, Barrier: 95.4 ± 1.2% to 97.9 ± 0.9%, p = 0.001).
Several studies have reported an increase in CO2 concentration under ophthalmic drapes during eye surgery in spontaneously breathing patients using impermeable plastic drapes over the patient's head [2, 4–6]. After covering the head, Pco2 values under the drapes differed from 5 to 14 mmHg [2, 4–6].
In the present study, three different drapes, namely one plastic drape and two paper drapes, were investigated under clinical conditions in spontaneously breathing patients undergoing cataract surgery under retrobulbar anaesthesia. The results of the present study showed a significant accumulation of CO2 under all three ophthalmic drapes investigated. The CO2 increased by values between 8 and 10 mmHg. This was caused by an impaired permeability to CO2 in these drapes. Exhaled CO2 in spontaneously breathing patients undergoing eye surgery thus escapes only partially through the drapes. As a result of the observed accumulation of CO2 under the drapes, the patients inhaled air with a higher CO2 concentration. Therefore CO2 concentration in the patients' blood increased immediately after covering the head, as shown by an increasing transcutaneously measured CO2 partial pressure (Ptcco2) . To reduce this rising CO2 concentration in the blood, patients increased their respiratory rate [4, 7]. As soon as 3 min after covering the patient's head, the respiratory rate had increased significantly in all groups. Despite progressive hyperventilation, the patients in all three groups failed to reduce their transcutaneous measured partial pressure of CO2 during the entire course of the surgery. The highest respiratory rates were measured 25 min after draping the head. One must assume that rebreathing of CO2 intensifies with the duration of eye surgery, because after removal of the drapes both respiratory rate and Ptcco2 returned to values near baseline.
Comparison between the drapes showed significantly higher CO2 concentrations under the plastic drape (SteriDrape 1024) within the first 6 min after draping than under the SteriDrape 1062. As further measurements were made, the CO2 values under the plastic drape were higher than under the paper drapes but this difference was not significant. Transcutaneous partial pressure of CO2 and respiratory rate were lower using the SteriDrape 1062 than for the other two drapes. The lower parameters in the SteriDrape 1062 group may be explained by the stiffness of this type of drape; because of its stiffness it does not fit as tightly as the other two types of drape. Therefore, it may be possible that a higher degree of air circulation occurred under the SteriDrape 1064 than under the other two types and this may have caused lower Pco2 values under the drapes with no significantly lower Ptcco2 values or respiratory rate.
Furthermore, the results of the present study show that an insufflated oxygen flow of 2 l.min−1 was high enough to provide sufficient oxygenation in patients who had no pre-existing pulmonary disorders. Previous studies used fresh gas flows of 5–10 l.min−1 under the drapes [2, 5, 6]. In contrast to these studies, Risdall & Geraghty  suggested that low flow (2 l.min−1) of oxygen supplementation should be provided for all patients having ophthalmic surgery under local anaesthesia. Therefore, in the present study, oxygen insufflation was performed with a constant flow of 2 l.min−1. The SpO2 values increased significantly after covering the patient's head and starting oxygen insufflation in all study groups. In all three drapes investigated, however, the oxygen flow of 2 l.min−1 was too low to prevent CO2 accumulation under the drapes.
In previous studies, a combination of suction and fresh gas supply was used to prevent accumulation of CO2 under ophthalmic drapes [3, 4]. However, this is not a routine procedure. Using plastic drapes, or the paper drapes examined here, together with a combination of oxygen supply and suction under ophthalmic drapes seems to be the most effective means of preventing CO2 accumulation under the drapes in spontaneously breathing patients.
In summary, CO2 accumulation in the ambient air beneath the drapes, increase of the partial pressure of transcutaneous CO2 and hyperventilation were seen for all three ophthalmic drapes investigated during eye surgery in spontaneously breathing patients. An oxygen supply of 2 l.min−1 seems adequate to prevent hypoxaemia, but it is not high enough to prevent hypercapnia. The present study included only patients with no known pulmonary disorders and who received no sedation. Therefore one cannot make any statements about Ptcco2 values and respiratory rate in patients with impaired pulmonary function or in sedated patients. However, it may be assumed that CO2 retention is more pronounced in patients with pre-existing pulmonary disorders and in patients undergoing surgery under retrobulbar anaesthesia with additional sedation. Higher arterial Pco2 values result in raised choroidal blood flow and intraocular pressure [9, 10]. This in turn may complicate the operation and aggravate outcome .
The present data suggest that when no ophthalmic drapes with high CO2 permeability are available, the use of suction equipment will be necessary to prevent hypercapnia and hyperventilation in spontaneously breathing patients undergoing eye surgery. Producers of ophthalmic drapes are therefore encouraged to look for further ways to increase the CO2 permeability of their drapes in order to reduce CO2 accumulation under the drapes and hyperventilation in patients undergoing eye surgery under retrobulbar anaesthesia.
I acknowledge Johnson and Johnson® Austria and 3M® Austria for supplying their ophthalmic drapes for this study. In addition, I gratefully acknowledge the expert advice and assistance of Dr H. Ulmer (Institute for Biostatistics and Documentation at the Medical Faculty of the University of Innsbruck) in carrying out the statistical analysis of data.
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