M.D. Wongworawat and A.R. Boody reply:
The arterial line manometer is an electronic pressure transducer used in
our operating suites and telemetry-monitored units. The tubing and manometer
are single-use items packaged and manufactured by Maxxim Medical (Athens,
Texas). The electronic portion of the device and the monitor screen are made
by Hewlett-Packard (Houston, Texas). The mechanical pressure is transformed to
a visual waveform and a number (mm Hg), which is read from the monitor. There
is no diameter listed for the tubing itself, but we "zeroed"
(tared) the device at the level of measurement prior to data collection, so
the differential diameter of the needle and tubing should not be an issue. We
did not change the tubing in the arterial line manometer system for any of the
measurements.
The slit catheter is 18 gauge. It is inserted into tissue with use of a
16-gauge needle, which is removed prior to use.
Regarding the question pertaining to temperature, it is true that
temperature affects the pressure of any system. However, the effect of varying
temperatures on fluid density is nearly negligible from a clinical standpoint
(0.998 g/cm3 at room temperature and 0.993 g/cm3 at body
temperature, which represents a 0.5% change); see reference 20 in our
manuscript. The primary reason for the use of warmed saline solution was to
keep the muscle tissue in a physiologic environment so as to avoid the
development of muscle stiffness associated with excessive cooling. While the
temperature in the column was not measured with each trial, warm saline
solution was added in an incremental fashion throughout the experiment; we
believe that this sufficed in keeping the system at near physiologic
conditions.
Dr. Barnes is absolutely correct regarding the effect (or lack of effect)
of atmospheric pressure on our measurements.