Compartmental Syndromes.

Tissue pressure and its measurement

Pressure measurement techiques

Several methods have been described for the measurement of tissue pressure, only a few of which are clinically useful.

The infusion technique is a reliable method for continuously monitoring tissue pressure in the clinical situation.

The continuous infusion and wick techniques give similar pressure readings for intramuscular tissue pressures in animal and human model systems.

Tissue pressure within a limb may significantly exceed the pressure applied externally to the limb.

To be reliable and thus clinically useful, any tissue-pressure-measurement technique should be practiced in normal subjects before it is used for evaluation of a patient with a possible acute compartmental syndrome.

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• Infusion technique (1.63 MB)

Definition of tissue pressure

By definition, increased tissue pressure is the primary pathophysiological factor in compartmental syndromes. We must therefore define tissue pressure and attempt to resolve some of the confusion that has resulted from previous usage of this term.

A nonhomogeneous and anisotropic material such as tissue cannot be thought of as having a pressure in the same sense as a liquid or gas. This ambiguity is resolved somewhat by considering the two contexts in which the term "tissue pressure" might be invoked. The first concerns the exchange of fluid across a capillary wall. 1 2 This fluid movement is related to:

K (PC - PT ( H) + R OT - R OC) [ 1 ]

where K is a constant, PC is the capillary blood pressure, PT ( H) is the hydrostatic pressure of tissue fluid, R is the capillary membrane reflection coefficient, OT is the oncotic pressure of tissue fluid, and OC is the oncotic pressure of blood plasma.

The second situation in which the concept of tissue pressure might be invoked is in the consideration of forces operating on a vessel wall. The law of Laplace has been applied to this situation:

PI - PO = T/R [ 2 ]

where PI is the pressure exerted on the inside of the vessel wall, PO is the net force per unit area exerted on the outside of the vessel wall, T is the tension in the vessel wall, and R is the vascular radius.

Neither PT (H) nor PO is simple. Because extracellular fluid may exist in a free form, in a gel, and perhaps in other forms, PT (H) should actually refer to the physical chemical activity of extracellular fluid. 4 By contrast, PO is the resultant of several different elements. A positive contribution to PO may result from interstitial fluids, gels, and matrices, as well as from fibers and cells under compression. A negative contribution to PO may arise from cells and fibers under tension. Thus, in the general case it cannot be assumed that the two "tissue pressures" [PT (H) from equation 1 and PO from equation 2] are equal. 5- 6 To appreciate how they may differ, we have only to consider the analogy of a beaker that contains water and ball bearings. The hydrostatic pressure of water at the bottom of the beaker [analogous to PT(H)] is equal to the height of water in the beaker (H). The net force per unit area on the beaker bottom (analogous to PO) is equal to the total weight in water of the ball bearings (W) divided by the surface area of the beaker bottom (A) plus the hydrostatic pressure: W/A + H.

In discussing the effects of increased tissue pressure on local blood flow, the "tissue pressure" of primary interest is PO, the net force per unit area exerted on the outside of a vessel wall. This is the force that affects the pressure in and the flow through collapsible vessels. The mechanisms by which increased tissue pressure compromises local blood flow will be discussed in greater detail in Chapter 3.

Tissue pressure measurement

Because tissue pressure plays a central role in compartmental syndromes, it is appropriate to review some of the described techniques for tissue pressure measurement.

The capsule method employs a porous capsule surgically implanted in the tissue to be studied. After several weeks the fluid in the capsule reaches equilibrium with the surrounding interstitial fluid. The pressure of the fluid within the capsule is then measured with a pressure transducer. 5- 7- 8 This method has the clinical disadvantages of requiring surgical implantation and a prolonged period for equilibration. Stromberg and Wiederhielm 8 have criticized the capsule method on the basis that the observed pressure is influenced by the osmotic gradient between the fluid inside and the fluid outside the capsule.

Collapsible segment methods measure the pressure inside a flaccid-walled structure located within the tissue. 9 10 These methods are based on equation 2: when the walls of a fluid-filled structure are flaccid (the tension of the walls [ T ] is zero), the pressure of the fluid inside (PI) is equal to the pressure outside (PO). ; Thus, the measuring of the fluid pressure inside this structure ; yields the tissue pressure. Although some of these methods have l the disadvantage of requiring surgical implantation, Ryder et al. 11 and Kjellmer l2 have described variations using an in situ vein as the collapsible segment. Although the method of Ryder et al may be clinically useful for measuring subcutaneous tissue pressure, it is impractical for the measurement of intramuscular pressure in a X traumatized limb because it would require the cannulation of a deep intramuscular vein and repeated raising and lowering of the limb relative to the heart.

A servonull technique with micropipettes has been described by Wiederhielm. 4 In this method no net fluid is injected into the tissue, yet a continuous fluid column between the transducer and the tissue is maintained by a servosystem. Although this method is highly accurate and responsive, it appears to be too delicate and complicated for routine clinical use.

The injection technique measures the pressure necessary to inject a small quantity of fluid into the tissue through a f needle. l3-l7 Although this method has the advantage of using inexpensive equipment, it has a disadvantage in that a steady-state reading is not attained. Thus, it may be somewhat awkward in practice because a fluid manometer and an air-water meniscus must be observed simultaneously to detect the pressure at which fluid first begins to flow into the tissue. In an animal model where tissue pressure was elevated by fluid infusion at a known pressure, Hargens et al found that the injection technique overestimated low tissue pressures and underestimated high tissue pressures. Clayton et al 9 evaluated the injection technique by applying known pressures to the extremities of six rabbits with a pneumatic cuff. A good linear correlation was obtained with a slope of 1.03 (r = 0.99).

The wick technique employs strands of wettable material extending into the tissue from a fluid-filled catheter connected to a pressure transducer. 6,7, 20-22 The wick increases the surface area in contact with the tissue. To protect its fibers, the wick catheter is inserted through a larger cannula, which is then withdrawn. Clotting around the fibers is minimized by heparinization of the fluid within the catheter. Various materials have been used to make wick catheters; these include cotton and polyglycolic acid suture. The latter is most commonly used in the clinical situation. Zeluff 23 pointed out, however, that polyglycolic acid suture has a short shelf life after sterilization and suggested that Dacron (DuPont) may be a more suitable material.

Continuity of the fluid column between the tissue and the transducer is necessary for accurate pressure measurement. This continuity may be verified by observing a sharp increment in the observed pressure when the tissue overlying the catheter is pressed manually. If catheter patency cannot be assured, the catheter may be flushed with a small volume of heparinized saline. Mubarak et al 22 found that the wick catheter accurately reflected pressures applied by fluid infusion in dog limbs. We obtained reproducible results with the wick catheter when known increments of pressure were applied externally to rabbit and human limbs as long as the wick catheter remained patent. 24

In the continuous infusion technique, the patency of a hypodermic needle or intravenous catheter inserted into the tissue is maintained by the slow but continuous infusion of nonheparinized saline solution. The pressure of the fluid within the needle or catheter is continuously monitored with a standard blood pressure transducer. Since its original descriptions this technique has been improved through the use of noncompliant tubing, a simplified fluid path, and an ordinary needle or catheters

For continuous pressure monitoring, an infusion rate of 0.7 cc per day is used. Laboratory studies have demonstrated that the pressure measured is relatively independent of the rate of infusion: an acute 40-fold increase in the infusion rate from 0.7 to 29 cc per day produced only a 4-mm Hg increase in measured pressure. 25 It could be argued theoretically that even a rate of infusion as low as 0.7 cc per day could be hazardous to the patient. For example, Hargens et al 2 found that the acute infusion of 2 cc of plasma into a canine anterolateral compartment (volume of 40 cc) raised the intracompartmental pressure from 30 to 45 mm Hg. The pressure increment from saline infusion is unlikely to be a problem clinically, however, for two reasons: saline is absorbed three times more rapidly than plasma, 2 and three days of pressure monitoring would be necessary to infuse the volume of 2 cc. Furthermore, most human compartments are well over 10 times as large as the canine anterolateral compartment. The data obtained by Whitesides et al 7 from a limb amputated for sarcoma of the femur indicated that over the range of intracompartmental pressures from 10 to 50 mm Hg, the infusion of 1 cc of saline into the anterior compartment of the leg produced a 1-mm Hg increment in intracompartmental pressure. Thus, even assuming the worst possible case in which saline absorption is zero (i.e., a totally ischemic compartment), three days of continuous pressure monitoring with an infusion rate of 0.7 cc per day would give rise to an increment in tissue pressure of only 2 mm Hg.

We have demonstrated the accuracy and dependability of the continuous infusion technique in rabbit and human model systems where known increments of pressure were applied to living limbs.

Results of different tissue pressure measurement t

Earlier in this chapter we discussed the fact that there are at least two different "tissue pressures": the PO in the law of Laplace and the PT(H) in the capillary filtration equation. Because these two quantities cannot in the general case be expected to be identical, it would not be surprising if different tissue-pressure-measuring techniques yielded somewhat different values.

To determine whether or not the wick and continuous infusion techniques yielded significantly different results, we conducted side-by-side studies in rabbit and human model systems in which increments of external pressures were applied. We found that as long as wick catheter patency was closely monitored and any obstruction was cleared by flushing with a minimal volume of fluid, the two methods yielded virtually identical results in compressed rabbit and human muscle.

Even when no pressure was applied, our side-by-side comparison yielded similar pressure measurement values in human tibialis anterior muscle with the wick and the infusion techniques: 7+3 mm Hg and 9+3 mm Hg, respectively. 24 These values do not appear to be significantly different from those obtained with the wick technique by Mubarak et al 22 in normal forearm and leg muscle: 4 +4 mm Hg. Thus, in our hands, there is no practical difference between the results of the wick and the infusion techniques for intramuscular pressure measurement.

I prefer the continuous infusion technique for clinical tissue pressure measurement for the following reasons:

1. No specially prepared catheter is required; any needle or small intravenous catheter will serve. In compartments of the forearm and leg we routinely use a standard 22-gauge intravenous catheter with a 19-gauge inserting needle. This is considerably smaller than the 14- and 16-gauge placement units recommended for use with the wick catheters 22 We have used 25- and 27-gauge needles to measure pressure within the interosseous compartments of the hand.
2. Heparinization of the fluid within the catheter is not required; thus, the possibility of enhanced local bleeding is eliminated.
3. Catheter patency is continuously maintained by a volume-controlled infusion. As a result, catheter obstruction has not occurred in our clinical use of this monitoring method. Continuous pressure monitoring may be carried out for periods of at least 72 hours without the need for adjusting or manually flushing the system.
4. The pressure can be read at any time from the meter of the transducer monitor.
5. The equipment for the technique is available in most hospitals. Anesthesiologists are well acquainted with the calibration, zeroing, and operation of pressure transducers and can be of great assistance in setting up the system.
6. The results are accurate and reproducible.
7. On removal of the catheter or needle, there need be no concern about retained wick elements. 18

Like all other techniques for tissue pressure measurement, the continuous infusion method requires attention to detail and practice before proficiency and the necessary bedside efficiency are attained. Thus, I believe it is unwise to try to learn this or any other tissue pressure-measuring technique when confronted with a possible acute compartmental syndrome. The experience is likely to be frustrating, and the pressure reading obtained is unlikely to be reliable. An incorrect pressure reading is worse than none at all because it may distract from important clinical findings.

For those interested in being prepared to perform reliable pressure measurements, I would suggest practicing the technique on normal subjects until consistent readings in the normal range are obtained. If desired, one can apply a known pressure to the limb with an air splint to see if the measured pressure increases by the expected increment.

Relationship of applied and measured pressure

It has been generally assumed that the pressure applied to the outside of a limb is distributed essentially unaltered throughout the tissue. However, in our initial studies of the infusion technique, 25 we identified a phenomenon of "summation": the intramuscular pressure measured when an external pressure is applied to a limb is equal to the measured intramuscular pressure before pressure application plus the externally applied). More recent data 24 indicated an additional feature in the variance of applied and measured pressure: the increment in measured pressure within muscle exceeds the increment in applied pressure by a factor ranging from 1.02 to 1.3, depending upon the model system, as indicated by slopes of the plots of measured and applied pressure. This geometric augmentation of applied pressure is referred to as "amplification." 24 This phenomenon was found to be most striking in the anterior compartment of the rabbit leg, where the amplification factor obtained with both the wick and infusion techniques was 1.3 (i.e., the increment in measured pressure was 30% higher than the increment in externally applied pressure). At present we have not identified the mechanism of amplification. The fact ; that in the rabbit anterior compartment the amplification factor was dramatically diminished by fasciotomy suggests that the nonyielding fascia may have a significant effect on the pressure field induced by externally applied pressure.

Both summation and amplification can result in a significant difference between the pressure applied to an extremity and the pressure measured within it. This difference may be important in e interpreting experiments on the amount of pressure required to arrest blood flow, 33 in deciding the safe limits for air splints and pressure dressings, 34 and in interpreting the results of sphygmomanometry. 35 The magnitude of the apparent discrepancy is significant. The observed relationship of externally applied pressure.

10 surgery questions for your surgeon before having surgery

Surgery for Compartmental Syndromes at the University of Washington, Department of Orthopaedics and Sports Medicine, Seattle, Washington

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