RESPIRATORY ALTERATIONS WITH INTRAPERICARDIAL PROCAINE IN THE CONSCIOUS RABBIT

1. Intrapericardial procaine, used to produce cardiac nerve blockade i n both conscious and anaesthetized animals, has been reported to also produce changes in respiration. This study systematically investigated the effects of two doses of intrapericardial procaine on respiration in the conscious rabbit. 2. Rabbits were pre-instrumented with a chron ic diaphragm electromyogram (dEMG) recording electrode and intraperica rdial catheter. Arterial pressure, heart rate, dEMG and respiratory ex cursions (recorded with a pneumograph) were monitored in the conscious rabbit before and after intrapericardial and intravenous infusion of 2 and 5% procaine. Efficacy of cardiac nerve blockade was tested by in travenous infusion of phenyl biguanide. Arterial blood gases were dete rmined at rest and during changes in respiration. 3. Following a low d ose of intrapericardial procaine (12 mg), dEMG and respiratory excursi ons increased (65 +/- 13 and 76 +/- 32%, respectively) with no change in breathing frequency or arterial blood gases. Following a high dose of intrapericardial procaine (30 mg), four of six animals exhibited a similar response. However, four of the six rabbits also exhibited a se cond type of response pattern characterized by a further increase in r espiratory efforts (430 +/- 336%), abolition of dEMG, and a mild hypox aemia. 4. Intravenous infusion of a low dose of procaine was without e ffect, whereas intravenous infusion of a high dose of procaine produce d minor behavioural responses. 5. In four additional anaesthetized rab bits, it was demonstrated that high doses of intrapericardial procaine anaesthetized the phrenic nerve to produce the observed alterations i n respiration. 6. We conclude that if intrapericardial procaine is use d to block cardiac nerves in conscious rabbits, it should be used in a low concentration and at the lowest possible total dose to avoid comp lications due to changes in respiration.