We thank Dr. Barnes for his letter and respond to his comments below.

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, TX). The electronic portion of the device and monitor screen are made by HP (Houston, TX). The mechanical pressure is transformed to a visual waveform and a number (mmHg) 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 differential diameter of needle and tubing should not be an issue. We did not change tubing in the arterial line manometer system for any of the measurements.

The slit catheter is 18-gauge. It is inserted into tissue using 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 using warmed saline 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 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 thereof) of atmospheric pressure on our measurements.

To The Editor:

This very worthwhile, comprehensive and long overdue paper addresses important aspects about the accuracy of compartment pressure measurements. While there are some interesting explanations about the importance of a good tissue model and the effect of different needles and catheters, the authors leave unanswered a number of important methodologic questions.

Comprehensive details and references are given about where to find further information about the Stryker system and the Whitesides method, but only the phrase “arterial line manometer” is used to describe the third device. What does this term mean? Is it a pressure transducer and, if so, we need more details about its manufacturer and its characteristics. How is it connected to the needles?

This is important information to know because all the associated external plumbing with these systems can have a significant effect on the pressures recorded (possibly greater than the differences between the three methods that were compared).

It is clearly stated that the needles used were 18g. but the diameter of the slit catheter is not stated. The diameter may have a significant effect on the pressure measured.

The authors are obviously aware of the effect that temperature changes can have on the pressure recorded, but it is not clear how these were taken into account. They state that normal saline was heated to 37°C to mimic physiological conditions. Was the whole system then maintained at 37°C throughout the measurements, or was the temperature of the saline measured each time a pressure measurement was recorded? Did the entire experimental set up gradually cool to room temperature as the pressure was increased? The authors were measuring small differences between the different devices, but the effects of changes in temperature could far outweigh these measured differences.

Immediate atmospheric pressure changes could not have affected the results. Since all the pressures were measured with respect to atmospheric pressure, they are not absolute values. If there was a change in atmospheric pressure then it would affect both the measurement device and the source in exactly the same way. I look forward to a clarification of these issues from the authors.