Respiratory toxicity of direct lytic factor in the venom of the southern Chinese cobra (N. naja atra) in dogs.

The effects of direct lytic factor (DLF) on respiratory ventilation, gas exchange as well as hemodynamics were studied in anesthetized dogs. After an intravenous DLF dose of 1 mg/kg, the initial manifestation of intoxication was observed as follows: (1) Increase in airway impedance characterized by slowed air flow rate and increased negative transpulmonary pressure. (2) Decrease in dynamic compliance. (3) Progressive increase in venoarterial shunt (Qs/Qt) and decrease in PaO2, (4) Elevation of pulmonary artery blood pressure and fall of mean systemic blood pressure and maximal left ventricular pressure. Above actions reached the peak values at 15 min and thereafter all respiratory functional parameters, except Qs/Qt and hypoxemia, returned gradually to approach the normal levels at 50 min. The tidal volume, PaCO2 and LVEDP remained unchanged until another DLF dose of 1.5 mg/kg was given. After a second dose of DLF (total 2.5 mg/kg), the respiratory functions and the cardiac performance deteriorated as follows: (1) Further increase in Qs/Qt and hypoxemia. (2) Appearance of hypercapnea and acidosis. (3) Fall of dP/dtmax and elevation of LVEDP, widening of QRS complex of ECG. (4) Blood pressure run a downhill course. From above experimental evidence, we came to the conclusion that as well as the basic cardiotoxicity, respiratory toxicity of DLF must be considered as the primary, because of broad spectrum of action of DLF and early effect on respiratory function.