The effect of increasing the intraluminal pressure in the right heart on the activity of isolated perfused dog hearts and the effect of increasing the intraluminal pressure on the activity of isolated right atria of dogs, rabbits, guinea pigs and albino rats have been studied. It has been observed that there is a direct relationship between the magnitude of intraluminal right atrial pressure and the inotropic and chronotropic responses of the whole heart and right atrium up to certain critical pressures (5–15 mm Hg). Some modifications in this behavior of the hearts due to repeated changes in the intraluminal pressure have also been described. It has been suggested that stretch of the cardiac tissue due to increased intraluminal pressure may cause alterations in membrane metabolism at the pacemaker.
As the appropriate usage (59.65%) of blood components was low in the present study, regular auditing of transfusion practices from time to time is indicated. This not only helps guide their judicious use but also serves to evaluate and decrease their inappropriate usage.
Intravenous infusions on intact dog hearts both in closed- and open-chest experiments indicate that a rise in the intra-atrial pressure is accompanied by either a positive, a negative or a no chronotropic effect, irrespective of the integrity of vagi. Positive chronotropic effects of similar intensity were also obtained even after bilateral vagotomy, topical application of cocaine or parenteral administration of atropine, tetraethylammonium or hexamethonium compounds. Moreover, the rise in the intra-atrial pressure or the degree of visible distention of the right atrium was not necessarily associated with a parallel rise in the heart rate. Thus, the infusion acceleration response obtained with some infusions only, fails to fulfill the requirements of a reflex action. There is, therefore, no justification for labeling it as a "reflex" effect, as advocated by Bainbridge. The chronotropic effects of the infusions seem to be due to stretch-induced alterations in impulse generation as a result of changes in the intra-atrial pressure and possibly depend upon the initial conditions at the pacemaker.
Mechanical stretch is a fundamental biological stimulus for pacemaker activity. Stretch-sensitiveness enables the heart to autoregulate its rate in relation to venous return. Phylogenetically the importance of this primitive mechanism has been diminished by the emergence of neurohumoral influences which permit a quicker and more effective adaptation of cardiac performance. Nevertheless, the intrinsic autoregulatory mechanism is capable of independent operation and may support extrinsic neurohumoral influences under various conditions.
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