Up to this time, the Blalock research on shock had been relatively crude, with results obtained essentially by gross observation. The basic equipment for the mass of work he had previously turned out consisted of a mercury manometer with a float and a cross arm to record blood pressure on a smoked drum which was turned by a spring-driven motor, a Van Slyke-Neill manometric blood gas apparatus for the determination of carbon dioxide and oxygen content of the arterial and venous blood, and a Benedict-Roth spirometer to measure oxygen consumption. By figuring the difference in the oxygen content of the arterial and venous blood and knowing the oxygen uptake of the animal, the cardiac output was calculated using the Fick formula. There was also a Sahli hemoglobinometer and a metric scale to weigh the animals. In the past months, I had become fairly adept in the use of the Van Slyke apparatus, which was the most difficult to master. I was using it on an almost daily basis.
The mercury manometer was a U-shaped glass tube filled a little less than halfway with mercury. The blood pressure was obtained by connecting a piece of saline-filled rubber tubing to one side of the U-tube. The other end of the rubber tubing was then connected to a glass cannula previously inserted in the femoral or carotid artery. The pressure in the artery would be exerted upon the column of mercury and the difference in the levels of the mercury in the two columns represented the blood pressure. To record the blood pressure, a fine rod of aluminum with a hard rubber float on one end was inserted into the opposite side of the U-tube, float down. An aluminum cross arm with one end arrow-shaped was affixed to the upright rod by the use of a small piece of cork through which the two rods were passed at right angles to each other. The record proper was made on smoked, glazed paper. This paper was fastened around an open-ended drum or cylinder about six and a half inches in diameter. Inside this drum was fastened a hollow shaft which extended six to eight inches from one end. The paper was smoked by holding this shaft and rotating the drum over a kerosene lamp with a four-inch wick which gave off a cloud of dense black smoke. The hollow shaft portion of the drum with the smoked paper was then put onto the upright rod of a kymograph power unit. The motor in this unit was spring wound and could be adjusted to turn the drum at various speeds of from one inch per hour to a full turn in one minute. By placing the mercury manometer close to the