Changes in transcutaneous carbon dioxide, oxygen saturation, and respiratory rate after interscalene block.

Background: We used transcutaneous (TC) carbon dioxide (C[O.sub.2]) monitoring to prospectively evaluate changes in respiratory status after interscalene anesthesia in 45 adults (40 successful and 5 unsuccessful blocks).

Methods: TC-C[O.sub.2] oxygen saturation and respiratory rate were recorded every minute for 5 minutes before block and every 2 minutes for a total of 30 minutes (15 data sets) after injection of the local anesthetic solution.

Results: After successful block, TC-C[O.sub.2] increased from 41 [+ or -] 5 mm Hg to a maximum value of 44 [+ or -] 6 mm Hg (P < 0.0001) and the respiratory rate increased from 14 [+ or -] 2 breaths/min to a maximum of 20 [+ or -] 4 breaths/min (P < 0.001). The increase in TC-C[O.sub.2] was [greater than or equal to]5 mm Hg in 11 patients and [greater than or equal to]10 mm Hg in 4 patients, with a maximum increase of 12 mm Hg. Of the 600 TC-C[O.sub.2] data points recorded (15 each from the 40 patients with a successful block), 62 (10.3%) showed a TC-C[O.sub.2] value of 50 mm Hg or greater, with a maximum value of 57 mm Hg. No significant change in TC-C[O.sub.2] or respiratory rate was seen in the five patients with unsuccessful block.

Conclusion: After interscalene blockade, we found an increase in respiratory rate and hypercarbia that resulted in no clinically significant effect.

Key Words: brachial plexus anesthesia, hypercarbia, interscalene anesthesia, regional anesthesia, transcutaneous carbon dioxide monitoring

**********

The interscalene approach remains a popular method of providing regional anesthesia for surgical procedures of the shoulder and arm. In addition to anesthesia of the brachial plexus, phrenic nerve block and hemidiaphragmatic paralysis invariably accompany successful block. Studies using ultrasonography have shown that there is a 100% incidence of hemidiaphragmatic paralysis after interscalene (IS) block. (1) Urmey and McDonald (2) showed alterations in respiratory function resulting from the hemidiaphragmatic paralysis including a mean decrease in forced vital capacity of 27.2% and forced expiratory volume of 26.4%. They also noted a mean decrease in peak expiratory flow rate of 15.4% and a mean decrease in maximum mid-expiratory flow rate of 17.9%. Similar alterations in respiratory function have been reported by other investigators. (3-5) However, to date, there are no reports evaluating changes in arterial partial pressure of carbon dioxide (PaC[O.sub.2]) related to the altered respiratory function after IS anesthesia. Using transcutaneous (TC) C[O.sub.2] monitoring, we prospectively evaluated changes that followed IS anesthesia in adults.

Patients and Methods

The Institutional Review Board and the Committee for the Protection of Human Subjects of the University of Missouri approved the study. Verbal informed consent was obtained from all patients. Eligible patients were those aged 18 years or older presenting for shoulder or upper arm surgery that was amenable to brachial plexus anesthesia using the IS approach. Demographic data included age, weight, and sex.

The patients were fasted for 6 to 8 hours. IV access was secured in the nonoperative upper extremity. Supplemental oxygen (2 L/min) was administered by nasal cannula. Oxygen saturation was measured by pulse oximetry. Sedation and analgesia were provided by midazolam (0.03-0.05 mg/kg) and fentanyl (0.5-1 [micro]g/kg). TC-C[O.sub.2] was continuously measured using a TCM3 TC-C[O.sub.2] device (Radiometer, Copenhagen, Denmark). The probe was cleaned and a new membrane was applied before placement. The working temperature of the probe was set at 45[degrees]C. The device was then calibrated against a known concentration of C[O.sub.2] from the calibration canister. After successful calibration, the probe was applied to the forearm of the nonoperative side and allowed an equilibration time of 10 to 15 minutes. Midazolam and fentanyl were administered 10 to 15 minutes before the collection of the baseline data during probe calibration and equilibration. No additional medication was administered during collection or after the baseline readings were recorded.

The IS block was performed using an insulated needle with a nerve stimulator to inject either 40 ml of 0.5% ropivacaine or levobupivacaine without epinephrine or 30 ml of 0.5% ropivacaine or levobupivacaine plus 10 ml of 2% lidocaine without epinephrine. During performance of the block, the patient was supine. After the block, the head of the bed was elevated 30 to 45 degrees. A successful block was defined as motor and sensory block in the ulnar, median, musculocutaneous, and radial nerve distribution of the brachial plexus.

Data collection included TC-C[O.sub.2] value, oxygen saturation, and respiratory rate. These values were collected every minute for 5 minutes before injection and were then averaged to serve as the baseline value for each patient. The same data were recorded every 2 minutes for a total of 30 minutes (15 data sets) after injection of the local anesthetic solution. No additional medications were administered, and no surgical manipulation was done from the time of injection until the 30-minute postblock data were completed.

Data analysis included a one-way analysis of variance to evaluate the changes in respiratory rate, oxygen saturation, and TC-C[O.sub.2] at each 2-minute interval after the IS block versus the baseline. Fentanyl and midazolam doses in the successful and unsuccessful blocks were compared using an unpaired t test. Fisher's exact test was used to compare the number of patients who had at any point a [greater than or equal to]5-mm Hg increase in TC-C[O.sub.2], a TC-C[O.sub.2] value of [greater than or equal to]50 mm Hg, or a respiratory rate increase of [greater than or equal to]5 breaths/min after the IS block between the group that had a successful block and the group with unsuccessful block. The data are presented as the mean [+ or -] SD, with P < 0.05 considered significant.

Results

Of the 45 patients enrolled in the study, 27 were male patients and 18 were female patients (Table 1). There was no difference in the amount of midazolam and fentanyl administered to the patients with a successful versus an unsuccessful block. The IS block was successful in 40 of the 45 patients. There was no significant change in oxygen saturation in either group (successful or unsuccessful block). Figure 1 shows the changes in respiratory rate and TC-C[O.sub.2] over time in the 40 patients who had successful IS block. In the 30 minutes after successful blockade, the baseline TC-C[O.sub.2] increased from 41 [+ or -] 5 mm Hg to a maximum value of 44 [+ or -] 6 mm Hg (P < 0.0001), and the respiratory rate increased from a baseline of 14 [+ or -] 2 breaths/min to a maximum of 20 [+ or -]4 breaths/min (P < 0.001). Among the cases of successful IS block, the increase in TC-C[O.sub.2] was [greater than or equal to]5 mm Hg in 11 patients (27.5%) and [greater than or equal to]10 mm Hg in 4 (10%), with a maximum increase of 12 mm Hg. Of the 600 TC-C[O.sub.2] data points recorded (15 each from the 40 patients with a successful block), 62 (10.3%) had a TC-C[O.sub.2] value of 50 mm Hg or greater, with a maximum value of 57 mm Hg. None of the five patients with unsuccessful block had an increase in TC-C[O.sub.2] of [greater than or equal to]5 mm Hg from baseline or a TC-C[O.sub.2] value greater than 50 mm Hg (P < 0.01).

With successful IS blockade, the increase in respiratory rate was [greater than or equal to]5 breaths/min in 24 patients (60%) and 10 breaths/min in 4 patients (10%), with a maximum increase of 15 breaths/min. None of the five patients with a failed block had an increase in respiratory rate of [greater than or equal to]5 breaths/min from baseline (P < 0.02).

Discussion

Although previous studies have evaluated alterations in pulmonary function related to hemidiaphragmatic paralysis after IS block, our study is the first to provide data regarding changes in TC-C[O.sub.2]. No change in respiratory rate or TC-C[O.sub.2] was noted in the patients who did not have successful brachial plexus anesthesia. With successful block, significantly more of the patients had an increase in respiratory rate of [greater than or equal to]5 breaths/min when compared with those who had unsuccessful block, and there was a significant increase in the respiratory rate from baseline. Although the number of patients whose TC-C[O.sub.2] increased [greater than or equal to]5 mm Hg from baseline was not statistically different between groups, patients with successful block had a maximum TC-C[O.sub.2] value that was greater than baseline, whereas no difference was noted in the group with unsuccessful block.

[FIGURE 1 OMITTED]

The peak increase in both TC-C[O.sub.2] and respiratory rate occurred within 8 to 12 minutes after injection of the local anesthetic agent (Fig. 1). Urmey and McDonald (2) found changes in diaphragmatic motion 2 minutes after IS block. Although diaphragmatic paralysis occurs early (2 minutes), we noted peak changes at 8 to 12 minutes after IS block. Two factors may have accounted for this delay. First, it takes time for the PaC[O.sub.2] to increase after a decrease in alveolar ventilation. Second, there can be a delay in the response time of changes in the TC-C[O.sub.2] when the PaC[O.sub.2] changes.

Although several factors may cause an increased respiratory rate, other potential factors such as pain or anxiety were eliminated because the data were recorded at a time when no other stimuli were present. In addition, the increased respiratory rate was concomitant with increased TC-C[O.sub.2] values. If some other stimulus such as pain or anxiety were causing the increased respiratory rate, the TC-C[O.sub.2] value would be expected to decrease. In addition, because no change was noted in patients who did not have successful brachial plexus anesthesia, we would postulate that the changes are directly related to IS anesthesia.

Previous studies have shown that there is a decrease in lung volumes and expiratory flow rates after IS block. (2), (3) It is not surprising to find that the compensatory mechanism for these decreased volumes is an increase in respiratory rate to maintain minute ventilation. Although minor increases in TC-C[O.sub.2] occurred, the increase in respiratory rate was sufficient to compensate for the decreased tidal volume and thereby prevent clinically significant hypercarbia. Although we did not a TC-C[O.sub.2] increase of 5 mm Hg or greater in 11 patients and 10 mm Hg or greater in 4 patients, no clinically significant effect was noted in any of the patients. Although 10% of the 600 TC-C[O.sub.2] data points recorded had a TC-C[O.sub.2] value of 50 mm Hg or greater, 50 of the points occurred in only four of the patients.

By administering fentanyl and midazolam before collecting baseline data, we have attempted to eliminate the effects that these agents may have had on our results. Although these blocks are not excessively painful, we did not think it appropriate to perform them without offering some anxiolytic and analgesic agents to our patients, because this is our standard practice. No change in TC-C[O.sub.2] was noted in the patients who had unsuccessful block even though they also received midazolam and fentanyl, thus showing that these small doses of fentanyl and midazolam had little or no impact on baseline respiratory function in our patients. The baseline data were recorded after administration of fentanyl and midazolam at a time when we would have expected to see their peak respiratory depressant effects (10-15 minutes).

Midazolam and fentanyl do blunt C[O.sub.2] responsiveness and may have accounted for a depressed ventilatory response to the hypercarbia induced by hemidiaphragmatic paralysis from IS block. Although we did note an increase in respiratory rate after successful IS block, we do not know whether the magnitude would have been greater without sedative agents, thereby further diminishing the increase in and eliminating the isolated episodes of hypercarbia after successful IS block.

Because this study was performed in patients without the need for intra-arterial access, a noninvasive method of estimating PaC[O.sub.2] was used. With appropriate technique and experience, we have found that noninvasive estimation of PaC[O.sub.2] can be achieved using TC-C[O.sub.2] monitoring and is more accurate than end-tidal C[O.sub.2] monitoring. (6), (7) The reliability of the technique has been proven in our previous studies and by a mean TC-C[O.sub.2] at baseline of 41 mm Hg. TC-C[O.sub.2] is generally greater than PaC[O.sub.2] (mean of 3-4 mm Hg in our previous study in adults (7)), and therefore the degree of true hypercarbia (increase in PaC[O.sub.2]) may be less than what is represented by changes in TC-C[O.sub.2].

Because the IS blocks were performed by several anesthesiologists, we chose not to control the local anesthetic agent, its volume, or its concentration. Previous studies have shown phrenic nerve blockade and hemidiaphragmatic paralysis with concentrations as low as 0.125%, (5) whereas Urmey and Gloeggler (4) found no decreased effect on diaphragmatic function with the use of only 20 ml.

The current study adds to previous information regarding the respiratory effects of IS block. For the first time, we provide data regarding alterations in respiratory rate and TC-C[O.sub.2] values after IS block. With the onset of IS block, we noted an increase in respiratory rate and occasional periods of hypercarbia. In all of the patients studies, the hypercarbia resulted in no clinically significant effect. However, our patient population did not include a significant number of patients with respiratory problems. At baseline, only two of the patients had an oxygen requirement and only three had an elevated TC-C[O.sub.2] (baseline values of 50, 52 and 52 mm Hg, respectively). Therefore, we cannot generalize as to whether similar changes would be found in patients with underlying respiratory compromise.

Knowledge and Wisdom are not the same, because Knowledge is what you are taught, but Wisdom is what you bring to it.

--Robertson Davies

Table 1. Demographic data of the study cohort (N = 45)

Sex     27 male/18 female
Age     48.8 [+ or -] 15.6 yr
Weight  89.0 [+ or -] 15.6 kg

From the Departments of Anesthesiology, Orthopedic Surgery, and Child Health, University of Missouri, Columbia, MO.

Reprint requests to Joseph D. Tobias, MD, Department of Anesthesiology, University of Missouri, 3W40H, One Hospital Drive, Columbia, MO 65212. Email: [e-mail]

Accepted July 23, 2002.

Copyright [c] 2004 by The Southern Medical Association 0038-4348/04/9701-0021

References

(1.) Urmey WF, Talts KH, Sharrock NE. One hundred percent incidence of hemidiaphragmatic paresis associated with interscalene brachial plexus anesthesia as diagnosed by ultrasonography. Anesth Analg 1991;72:498-503.

(2.) Urmey WF, McDonald M. Hemidiaphragmatic paresis during interscalene brachial plexus block: Effects on pulmonary function and chest wall mechanics. Anesth Analg 1992;74:352-357.

(3.) Pere P, Pitkanen M, Rosenberg PH, et al. Effect of continuous interscalene brachial plexus block on diaphragm motion and on ventilatory function. Acta Anaesthesiol Scand 1992;36:53-57.

(4.) Urmey WF, Gloeggler PJ. Pulmonary function changes during interscalene brachial plexus block: Effects of decreasing local anesthetic volume. Reg Anesth 1993;18:244-249.

(5.) Pere P. The effect of continuous interscalene brachial plexus block with 0.125% bupivacaine plus fentanyl on diaphragmatic motility and ventilatory function. Reg Anesth 1993;18:93-97.

(6.) Berkenbosch JW, Lam J, Burd RS, et al. Noninvasive monitoring of carbon dioxide during mechanical ventilation in older children: End-tidal versus transcutaneous techniques. Anesth Analg 2001;92:1427-1431.

(7.) McBride DS, Johnson JO, Tobias JD. Non-invasive carbon dioxide monitoring during neurosurgical procedures in adults: End-tidal versus transcutaneous techniques. South Med J 2001;95:870-874.

RELATED ARTICLE: Key Points

* The interscalene approach remains a popular method of providing regional anesthesia for surgical procedures of the shoulder and arm.

* Studies using ultrasonography have shown that there is a 100% incidence of hemidiaphragmatic paralysis after interscalene block.

* Although previous studies have evaluated alterations in pulmonary function related to hemidiaphragmatic paralysis after interscalene block, our study is the first to provide data regarding changes in transcutaneous carbon dioxide.

Joseph D. Tobias, MD, Louis Del Campo, MD, Keith Kenter, MD, Kelly Groeper, MD, Bruce Gray, MD, and James Edwards, MD