CORRESPONDENCE 863 received 2 mL of methylprednisolone acetate without The effect of right and left lateral posi- blocking the posterior tibial nerve. To ensure unifor- mity in the deposition of injected material in all tion on PA-aO2 and PA-ETCO2 in tho- patients, the needle was guided fluroscopically. racic surgery Pain during the procedure was assessed using a sim- ple, categorical, verbal rating scale. Except for a mild To the Editor: burning sensation, pain during posterior tibial block A preliminary small-sampled study was conducted to was negligible in all cases. In Group I, nine patients investigate the effect of right and left lateral position on had no pain during methylprednisolone injection, and PA-aO2 and PA-ETCO2. After approval from the hospital one patient suffered from mild pain while in Group II, Ethics Committee, we examined 23 ASA I–III patients seven patients had severe pain and three patients suf- undergoing elective exploratory thoracotomy for lung fered from moderate pain. cancer. Exclusion criteria were dyspnea or angina with The Table shows that patients receiving posterior tib- minimal activity (New York Heart Association classifi- ial nerve block prior to methylprednisolone injection had cation $ III), clinically important radiological findings no complications and were able to complete treatment in the dependent lung and values of forced expiratory when compared with the other group (P < 0.001). volume in the first second and forced vital capacity Two patients were lost to follow-up in Group II. below 60% of predicted values. Induction and mainte- Upon learning about the less painful alternative, they nance of anesthesia were achieved with propofol, returned to the pain centre for subsequent injections remifentanil and 66% N2O in O2 during two-lung ven- under posterior tibial nerve block. tilation (TLV) and 100% O2 during one-lung ventila- Our preliminary results indicate that patient’s per- tion (OLV). Size 39–41 Fr double-lumen tubes were ception of pain from steroid injection and compliance used, according to body weight. Tidal volume was set with treatment were significantly altered by prior at 10 mL·kg–1 and respiratory frequency at 8 administration of tibial nerve block. beats·min–1. After opening of the pleura, arterial blood samples were drawn first during TLV and then during

Ramasamy Govindarajan MD OLV. End-tidal CO2 values were recorded at those Tzonka Bakalova MD moments. PAO2 was calculated according to the equa- Nabil W. Doss MD tion: PAO2 = FIO2*(Patm-PH2O)- PaCO2/0.8. During Shepard H. Splain DO FACS OLV, PA-aO2 did not differ significantly in the left or in Rafik Michael MD PhD the right lateral position, but during TLV it was Adel R. Abadir MD increased in the right lateral compared to the left later-

Brooklyn, New York al position (Table). In the left lateral position, Pa- 2 ETCO was lower than in the right lateral position. References These findings were similar during OLV and TLV.1

1 Atkins D, Crawford F, Edwards J, Lambert M. A sys- In conclusion, it seems that, during OLV, PA-aO2, tematic review of treatments for the painful heel. which reflects intraoperative shunt, remains unaffect- Rheumatology 1999; 38: 968–73. ed by lateral positioning, contradicting earlier studies,2 2 Alastair SE, Younger, Bruce S. Pain in the leg, ankle, while during TLV it is increased in the right lateral

and foot. In: John DL (Ed.). Bonica’s Management of position. Pa-ETCO2 is increased in the right lateral Pain. Philadelphia: LR; 2001: 1614–46. position during both types of ventilation. Therefore in

3 Acevedo JI, Baskin JL. Complications of plantar fascia the left lateral position, PaCO2 is estimated more accu- rupture associated with corticosteroid injection. Foot rately from PETCO2 than in the right lateral position. Ankle Int 1998; 19: 91–7.

TABLE PA-aO2 and Pa-ETCO2 during OLV and TLV

PA-aO2 in mmHg Pa-ETCO2 in mmHg OLV TLV OLV TLV Left lateral position 341 ± 218.3* 63.2 ± 49.4† 1.5 ± 1.6‡ 1.7 ± 1.7§ (n = 12) Right lateral position 418.2 ± 222.9* 122.3 ± 22.2† 4.1 ± 3.3‡ 4.6 ± 3.2§ (n = 11)

Data are mean ± SD. * = NS. †P < 0.02; ‡P < 0.04; §P < 0.02 by Student’s t test. OLV = one-lung ventilation; TLV = two-lung ventilation. 864 CANADIAN JOURNAL OF ANESTHESIA

Michael Vassiliadis MD References Nikolaos Barbetakis MD 1 Slinger P, Suissa S, Triolet W. Predicting arterial oxy- Thessaloniki, Greece genation during one-lung anaesthesia. Can J Anaesth 1992; 39: 1030–5. References 2 Lewis JW Jr, Serwin JP, Gabriel FS, Bastanfar M, 1 Ip Yam PC, Innes PA, Jackson M, Snowdon SL, Jacobsen G. The utility of a double-lumen tube for one-

Russel GN. Variations in the arterial to end-tidal PCO2 lung ventilation in a variety of noncardiac thoracic sur- difference during one-lung thoracic anaesthesia. Br J gical procedures. J Cardiothorac Vasc Anesth 1992; 6: Anesth 1994; 72: 21–4. 705–10. 2 Slinger P, Suissa S, Triolet W. Predicting arterial oxy- 3 Hurford WE, Kolker AC, Strauss HW. The use of ven- genation during one-lung anaesthesia. Can J Anaesth tialtion/perfusion lung scans to predict oxygenation 1992; 39: 1030–5. during one-lung anesthesia. Anesthesiology 1987; 67: 841–4. 4 Lerman J. Study design in clinical research: sample size estimation and power analysis. Can J Anaesth 1996; 43: 184–91. R EPLY: 5 Rothen HU, Sporre B, Engberg G, Wegenius G, I thank Drs. Vassiliadis and Barbetakis for their inter- Hedenstierna G. Airway closure, atelectasis and gas est in the effects of lateral position on one-lung anesthe- exchange during general anaesthesia. Br J Anaesth sia. In their study of 23 patients they were not able to 1998; 81: 681–6. demonstrate any difference in oxygenation between left and right thoracotomies. Our previous study1 of 80 patients and another of 200 patients2 found that the mean PaO2 levels were higher during left thoracotomies. Usually, the left lung is less perfused than the right and Assessment of neuromuscular blockade there is less shunt when the left lung is collapsed. Hurford at the vastus medialis 3 et al. showed that the PaO2 during one-lung anesthesia varies with the proportion of perfusion to the ventilated To the Editor: lung. Given the data of Drs. Vassiliadis and Barbetakis, I would like to comment on the interesting experi- it would require 130 subjects to have adequate power to ment on neuromuscular blockade (NMB) monitoring determine that the side of surgery did not have an effect by Saitoh et al.1 Their article may be more interesting on oxygenation during one-lung anesthesia.4 than they realize. The article alludes to the phenome- These authors found a trend towards poorer oxygena- non of direct muscle stimulation, but fails to observe tion during both two- and one-lung ventilation in the that the innervation of the vastus medialis is via a right-lateral position. This may be due to shunt in the motor nerve entering the muscle from its deep sur- 2 dependent lung. The authors used 66% N2O in oxygen face. There may be an additional small branch enter- during two-lung ventilation. This mixture contributes ing anteriorly. Nonetheless, all of the motor to atelectasis, particularly in the dependent lung in the innervation enters the muscle near its midpoint,3 well lateral position.5 above the placement of the stimulating electrodes. Their observations on positioning and arterial end- This makes specific motor nerve stimulation impos- tidal CO2 (Pa-ETCO2) gradients are interesting. At the sible with the set-up illustrated, and suggests that the onset of one-lung ventilation there is a sudden increase in response is due to muscle stimulation. Thus, the deadspace that is ameliorated over the first several min- changes in acceleration shown, which closely mirror utes as hypoxic pulmonary vasoconstriction redistributes the change in NMB, must be due to stimulation of blood flow back to the ventilated lung. The Pa-ETCO2 small im branches of the vastus medialis nerve. gradient during one-lung anesthesia is dynamic and The implications are important. Traditional NMB measurements will depend on the time they are taken. monitoring uses the adductor pollicis because the Unfortunately the authors do not specify at what time muscle is remote from the site of stimulation and during lung collapse they made their measurements. avoids direct muscle stimulation. The authors may have shown the way to using the accelerometer-based Peter Slinger MD FRCPC instrument on any muscle, anywhere, without regard Toronto, Ontario to the anatomy of innervation. This hypothesis, of course, requires scientific study.