[The effect of clonidine on the intrapulmonary right-to-left shunt in one-lung ventilation in the dog].

Many vasoactive drugs--vasoconstrictors as well as vasodilators--impair the efficiency of hypoxic pulmonary vasoconstriction (HPV) and may cause dangerous hypoxemia during one-lung ventilation (1-LV). The effect of clonidine, a widely used alpha 2-agonistic antihypertensive agent, on intrapulmonary right-to-left shunt (Qs/Qt) during 1-LV has not yet been studied. METHODS. Twenty anesthetized beagle dogs were instrumented with a 7F Swan-Ganz catheter and an arterial line. A left thoracotomy in the lateral position was performed and the left main bronchus was prepared for clamping. Intravenous clonidine (5 micrograms/kg) was administered to 10 dogs. Anesthesia was maintained with either thiopental and fentanyl or halothane (0.8 insp. vol%) and fentanyl. The ventilatory frequency (100% O2, tidal volume 150 ml/kg) was adjusted to obtain an end-expiratory CO2 of 4.5%. Each dog underwent the following experimental sequence: (1) baseline measurements with 2-LV; (2) 20 min 1-LV (left lung atelectasis was achieved by clamping the left main bronchus); and (3) 20 min 2-LV. After allowing 30 min for conversion from i.v. to halothane anesthesia or vice versa, the procedure was repeated (5 dogs in each group received first i.v. and 5 dogs first halothane anesthesia). RESULTS. Qs/Qt was slightly but not significantly higher under halothane anesthesia in both groups during 1-LV as well as 2-LV (Table 1). No differences existed between the halothane and i.v. groups with regard to arterial pO2 and cardiac index. In the control group (i.v. or halothane, n = 20) Qs/Qt increased during 1-LV from 10.8 +/- 2.8% (mean +/- S.D.) to 26.6 +/- 5.5%, and in the clonidine group (i.v. or halothane, n = 20) from 7.2 +/- 2.3% to 18.3 +/- 5.4%. These differences between the control and clonidine groups are significant (P less than 0.001) for 1-LV as well as 2-LV. Arterial pO2 during 1-LV was reduced less in the clonidine group (573 +/- 48 to 390 +/- 99 mmHg) than in the control group (550 +/- 44 to 323 +/- 94 mmHg) (P less than 0.05). During 1-LV no significant differences between the two groups could be detected with regard to pulmonary artery pressure, mixed-venous pO2, pCO2, and pH. Heart rate, cardiac index, and oxygen uptake were significantly lower and arteriovenous O2 difference was significantly greater in the clonidine group, indicating better control of sympathoadrenergic activity under clonidine. CONCLUSIONS. We conclude that pretreatment with clonidine improves HPV during 1-LV. This effect of clonidine is probably due to reduced sympathetic nervous system activity.