Using an isolated, blood-perfused atrium preparation, the effects of temperature on SA nodal pacemaker activity were investigated in 9 preparations. The PP interval decreased as temperature was raised. Regular sinus rhythm and atrial contraction were maintained above approximately 26°C. Below 26°C, sinus depolarization still showed a regular rate, although atrial contractions had ceased. At about 24°C, atrial rhythm became irregular. Below 20°C, atrial depolarization disappeared.
Chronotropic responses to norepinephrine were suppressed at decreased temperatures, not only with respect to maximum PP shortening but also to the threshold dose for inducing sinus acceleration.
Overdrive suppression was not influenced significantly by decreasing temperature.
These results indicate that a temperature decrease causes suppression of SA nodal pacemaker activity, although the SA node continues to function regularly until about 25°C.