The Coronary Circulation in the Dog1

In a recent review Gregg (16) has pointed out that although coronary blood flow has been measured by a variety of methods and under many different conditions, practically all of the studies made in the past are open to criticism. More specifically, if the instrument used to measure coronary blood flow was capable of dependable accuracy, the measurements were made under abnormal conditions such as artificial respiration with the chest open, or a heart-lung or coronary perfusion preparation. If the experimental conditions approximated the normal, the measuring device was usually the thermostromuhr, the dependability of which is questionable (15). Gregg concluded (16) that most of the work already done in this field should be repeated with the more accurate instruments now available and under conditions more closely approximating the normal. The experiments described below were undertaken with this purpose in mind. The instrument used to measure coronary blood flow was the bubble flowmeter previously used in studies of the cerebral circulation (5, 23). This device is reasonably accurate and dependable. It has the great advantage over instruments such as the rotameter, venturimeter, and orifice meter of being practically unaffected by changes in blood viscosity or deposition of flecks of fibrin during the course of an experiment. Another advantage is that the bubble meter does not introduce any appreciable resistance to the flow of blood through the coronary system. This was proved by observations both on mercury manometers connected by T tubes proximally and distally to the flowmeter, and on the volume of blood passing through the meter when it was connected to a femoral vein instead of a coronary artery. Experiments of the former type showed no measurable difference in the two lateral pressures; the latter gave flovys exceeding 150 cc. per minute at ordinary arterial pressures-a value almost double the most rapid coronary blood flow encountered in these experiments. The experiments were necessarily made on anesthetized heparinized dogs but in most cases the conditions were superior to those under which measurements of comparable accuracy have been made in that the chest was closed and the animal was breathing spontaneously. METHOD. Dogs, unselected as to breed and sex. and averaging 13 kgm., were initially anesthetized with nembutal (32 mgm. per kilo intravenously) or chloralose (50 mgm. per kilo intravenously) following morphine (2 mgm. per