The amplitude of pulse-synchronous oscillations varies with the level of intramuscular pressure in simulated compartment syndrome

Subjects

Seven healthy subjects (four females and three males) with a mean age of 28 (range
23–38) years and a mean body mass index (BMI) of 23 (range 20–26) kg/m² volunteered to participate in the study. The study was performed on 12 legs of these
subjects. All the subjects gave their informed written consent prior to participation.
The study protocol was approved by the regional Research Ethics Committee.

IMP recordings

IMP was recorded for 5 minutes for baseline data, 10 minutes during a model of abnormally
elevated IMP (simulated compartment syndrome), followed by 5 minutes of recovery after
the model was removed. The subject lay supine with his/her heel placed in a padded
footrest. The function of the footrest was to keep the leg at heart level and to ensure
that no external compression from the bed was applied to the calf. The heart level
was defined as 5 cm below the manubrium sterni. The foot was kept in a relaxed neutral
position to avoid effects on the IMP (Weiner et al. 1994]). Measurements were made in both legs in five subjects but on different occasions.
In two subjects, measurements were only made in one leg.

IMP measurement systems

A fiber-optic pressure transducer with a diameter of 0.42 mm and an estimated volume
of 0.072 mm³ (Samba 420 LP, Samba Sensors AB, Göteborg, Sweden) was used for the IMP measurement.
The sensing area is placed at the distal end of the fiber and is forward sensing.
The transducer was connected to a Samba 3200 control unit (Samba Sensors AB, Göteborg,
Sweden) set to measure relative pressure in mmHg. The transducer was calibrated at
room temperature before insertion. The Samba control unit was connected to a computer
(PC) equipped with data acquisition hardware from National Instruments (National Instruments,
Austin, TX, USA) and custom-made LabView based software. All units were turned on
at least 30 minutes before the start of IMP measurements to minimize internal temperature
drift. Data were collected at 20 Hz. In addition, a Stryker Intra-Compartmental Pressure
Monitor was used with a 1.3?×?60 mm disposable needle with a sideport and interconnected
with tubing. The 20 cm long tubing and a 3 ml syringe were prefilled with room-tempered
saline. During measurements, the transducer of the monitor was kept at the same level
as the tip of the needle to avoid hydrostatic offset. The IMP values from the Stryker
system were used as a reference and were not analyzed further.

Model of abnormally elevated IMP (simulated compartment syndrome)

The IMP was abnormally elevated by venous obstruction induced by a thigh tourniquet
on a casted leg. The plaster cast was applied over two layers of cotton padding extending
from slightly below the knee joint to the distal part of the leg. The proximal part
of the plaster was modified to make sufficient room for two catheters used for IMP
measurements. A 145 mm wide pneumatic tourniquet was placed around the thigh and inflated
to 65 mmHg to obstruct venous return from the leg.

This method of elevating IMP has previously been evaluated in healthy subjects (Styf
and Wiger 1998]) and used to simulate compartment syndromes in the human leg (Zhang et al. 2001]; Wiger and Styf 1998]; Wiger et al. 2000]; Styf and Wiger 1998]). The model elevated IMP to levels that are seen in patients with compartment syndromes
and elicited sensory dysfunction, muscle weakness, a mild throbbing pain and oscillations
of the IMP (Styf and Wiger 1998]). In this study, the model was applied for only 10 minutes to elevate the IMP without
provoking any additional symptoms associated with compartment syndromes.

Measurement catheter insertion

A local anesthetic (1–2 ml of xylocain 10 mg/ml with epinephrine, 5 ?g/ml) was injected
subcutaneously approximately 7 cm below the knee joint and 2 cm lateral to the tibial
tuberosity of the test leg. Under sterile conditions, a Venflon introducer (1.3?×?45
mm) was inserted through the skin into the anterior tibial muscle fascia in a distal
direction at an angle of 30 degrees from the plane of the skin, while the subject
kept his/her ankle joint dorsiflexed. The tip of the needle was then retracted into
the sheath of the introducer and the set was bluntly advanced parallel to the muscle
fibers in the relaxed muscle with the foot in a neutral position. The angle of insertion
was kept as parallel to the muscle fibers as possible to reduce pain, trauma to the
muscle and local edema that may affect pressure measurements (Styf 1989]). The needle was then removed and the optic fiber was inserted 45 mm into the Venflon
tubing. The sensor element of the optic fiber was thereby positioned at the end of
the tubing. A Luer lock Tuohy Borst Adapter with a gasket ensured sealing and fixation
of the optic fiber. The Stryker needle was inserted using the same procedure but 10 mm
lateral or medial (every other time) and parallel to the optic fiber. Since IMP varies
with depth (Nakhostine, 1993), both transducers were introduced to the same depth.
The depth of the catheter tip and the angle between the catheter and the overlying
fascia were measured by sonography (Acuson CV-70, Siemens Medical Solutions USA, Inc.,
CA, USA).

Blood pressure and pulse rate

Blood pressure and pulse rate were measured repeatedly with a non-invasive blood pressure
manometer (NAIS, Matsushita, Electronic Works, Japan) in the left arm. Mean arterial
pressure (MAP) was calculated by adding one third of the pulse pressure to the diastolic
blood pressure. Local perfusion pressure was calculated as MAP minus IMP.

Statistical method

Wilcoxon signed rank test was used for comparisons and significance was set at p??0.05.
Correlations are given with Pearson’s r. Unless otherwise stated, all results are
given as the mean and standard deviation (SD).