Myoglobinuria

Two features characterize the lithotomy positions: the legs are elevated above the level of the aorta and a capillary bed is compressed between the supported member (ie, heel or calf) and the supporting device (ie, sling, stirrup). Compression of capillary beds compromises tissue perfusion and invites the development of pressure injury if the support persists for a long time (eg, several hours). (1) This article describes a three-part analysis undertaken to increase understanding of the occurrence of pressure ulcers in lithotomy positions. In two parts of the analysis, an innovative device placed under participants' heels and calves was used to measure capillary perfusion in the standard and high lithotomy positions.

THE LITHOTOMY POSITIONS AND PRESSURE INJURIES

There are four lithotomy positions: low, standard, high, and exaggerated (Figure 1), and two support sites: the heel and the calf. In the high and exaggerated positions, the calf is in a more vertical position, and the support site is the heel. Typically, a sling is placed around the heel or a stirrup supports the heel. In the low and standard lithotomy positions, the calf is parallel to the surface on which the patient lies so the leg also can be supported at the calf by a sling, rather than using the heel as the sole weight-bearing support.

[FIGURE 1 OMITTED]

Pressure sores can occur at heel-support sites, especially when the legs are supported by the heel in the standard, high, or exaggerated lithotomy position for many hours. The high and exaggerated positions are likely to produce the most serious lesions because the mean arterial pressure at the support site is much less than aortic pressure, thereby reducing capillary perfusion. Typical capillary perfusion pressure is approximately 30 mmHg. (2) Capillary-support pressure above this level arrests capillary perfusion; however, producing tissue injury requires the cessation of blood flow for a prolonged period.

LITERATURE REVIEW

There appears to be a relationship between the amount and duration of pressure necessary to produce tissue injury, but the details of this relationship have not been established. (3) One researcher found that a pressure of 70 mmHg applied to the skin for more than two hours produced irreversible tissue injury. (4) Added to the difficulty of specifying the pressure-time relationship to produce an injury are patient-related factors, such as body weight, presence of vascular disease, environmental temperature, and situations in which patients are anesthetized or paralyzed and, therefore, unable to move or complain.

In one study, researchers measured the difference between brachial artery and dorsalis pedis artery systolic pressure for various lithotomy positions on eight adult participants. (5) In the standard lithotomy position, mean systolic brachial artery pressure was 109 mmHg [+ or -] 3 mmHg and mean dorsalis pedis systolic pressure was 83 mmHg [ or -] 3 mmHg, a difference of 26 mmHg. The difference was much higher for the exaggerated lithotomy position (ie, 51 mmHg).

Bilateral heel injuries were reported by one researcher after a radical vulvectomy and bilateral inguinal lymphadenectomy to remove cancer from the vulvae of a 93-lb female patient." The OR bed was cushioned with silicone-filled pads. The patient was supine for 90 minutes and in the lithotomy position for 90 minutes. General and epidural anesthesia were used and the patient's systolic blood pressure remained between 85 mmHg and 90 mmHg during the procedure.

The patient was unable to move her legs until the second postoperative day. On the third postoperative day, severe pressure sores were found on both of her heels. In commenting on the procedure, the researcher said,

A serious complication of the exaggerated lithotomy position was reported by researchers in 1997 (7) The patient was a 50-year-old man diagnosed with prostatic carcinoma. The procedure performed lasted seven hours. Twelve hours postoperatively, the patient's urine output decreased, with an increase in blood urea nitrogen to 40 mg/dL and creatinine to 2.7 mg/dL. The patient complained of pain in his thighs. Bilateral swelling with tenderness at the same site was noted, and peripheral pulses were intact.

The patient was transferred to the surgical intensive care unit. A diagnosis of rhabdomyolysis (ie, destruction of muscle fibers) with acute renal failure was made because of the patient's tense, swollen thighs; high creatine kinase; and worsening renal function. Compartment syndrome (ie, increased pressure in the fascia surrounding muscles) was suspected.

Compartment pressures in both thighs were measured, and values between 35 mmHg and 40 mmHg were obtained fie, pressures above capillary perfusion pressure of 30 mmHg). The calf compartmental pressures were normal;. The patient continued to complain of increasing pain in his thighs and was returned to the OR for bilateral thigh fasciotomies. The fasciae were under tension, and the muscles, although viable, were bulging before the incisions were made. In commenting on the case, the authors said,

In another study, heel and calf capillary-support pressures were measured on participants in the supine position on a hard surface and a regular bed.(8) The researcher found that the average calf capillary-support pressure for the hard surface and regular bed were 23 mmHg and 14 mmHg respectively. The average heel capillary-support pressure for the hard surface and regular bed were 125 mmHg and 115 mmHg. It is noteworthy that the mean heel and calf capillary-support pressures were well above the normal value of 30 mmHg with no heel elevation.

THEORY

It is necessary to establish the relationship between 1 ft of elevation and the mean decrease in perfusion pressure. One inch equals 25.4 mm; therefore, 1 ft equals 304.8 mm (ie, 12 inches multiplied bv 25.4 mm). The density of blood is approximately 1 g/mL and the density of mercury is 13.6 g/mL; therefore, 1 ft of blood is equal to 22.4 mmHg (ie, 304.8 mm divided by 13.6 g/mL).

METHODS AND MATERIALS

Three analyses were performed. These included

* ankle blood pressure with heel elevation,

* calf and heel capillary-support pressures in standard lithotomy position, and

* heel capillary-support pressures as the foot was elevated incrementally to the high lithotomy position.

ANKLE BLOOD PRESSURE WITH HEEL ELEVATION, This protocol was designed to show the reduction in ankle blood pressure with the heel elevated in 6-inch increments up to 2 ft above a mat on which participants were lying. The study involved 11 adult participants (ie, male and female) ranging in age from 20 to 82 years. Ankle pressure was measured with a commercially available oscillometric instrument connected to a cuff with a width of 40% of the ankle circumference. (9) To ensure the cuff was settled, the first measurement for each participant was discarded.

Figure 2 illustrates ankle systolic and diastolic pressure versus heel height in inches for a typical participant. For all 11 participants, both systolic and diastolic pressure decreased as the height of the heel increased. The mean decrease for systolic pressure was 20.6 mmHg per foot of elevation. The mean decrease in diastolic pressure was 18.8 mmHg per foot of elevation. These values are only slightly lower than the theoretical value of 22.4 mmHg per foot.

[FIGURE 2 OMITTED]

HEEL AND CALF CAPILLARY-SUPPORT PRESSURE IN THE STANDARD LITHOTOMY POSITION. Ten adult men and five adult women, ranging in weight from 125 lbs to 200 lbs, participated in this analysis. In all cases, participants lay on a 1.5-inch foam mat with their femurs vertical and lower legs parallel to the mat on the floor. Calf and heel slings were used to keep the lower leg in position. To measure calf and heel capillary-support, an innovative capillary pressure-measuring device was placed between the heel and the sling and the calf and the sling.

The capillary-support pressure-measuring device was modified from a design described in 1971. (8) The device consists of a rectangular, rubber bladder removed from a blood-pressure cuff with electrode arrays mounted on opposite walls inside the bladder (Figure 3). When the bladder is deflated, the electrodes in the arrays come into contact with each other, and the electrical resistance measured between them essentially is zero. When the bladder under the limb is slowly inflated, the electrodes break contact with each other, which is indicated by an abrupt increase in electrical resistance measured with an ohmmeter, and the pressure is noted. Then the bladder is slowly deflated until the electrode arrays make contact, and the pressure is noted. The capillary-support pressure is the average of the pressure measurement when contact is broken and the measurement when contact is re-established.

[FIGURE 3 OMITTED]

Two bladders were used, a large one for the calf (ie, 22 cm by 12 cm) and a smaller one for the heel (ie 6.5 cm by 7 cm). These sizes were chosen because they completely covered the supported area.

The capillary-support pressures for calf and heel support in the standard lithotomy position are presented as a function of body weight (Figure 4). In all cases, capillary-support pressure was higher for the heel-support than for the calf-support site because the calf provides a larger support area. For both support sites, the support pressure increased with increasing body weight.

Even for the calf-support site, only one of the 21 measurements was as low as the typical capillary-perfusion pressure (ie, 30 mmHg).

From the data in this analysis, it is possible to calculate the mean capillary-support pressure for the heel and calf in the standard lithotomy position for the range of body weights. For the 125-lb participant, heel capillary-support pressure was 88.8 mmHg, and calf capillary-support pressure was 37.4 mmHg. For the 200-lb participant, heel capillary-support pressure was 124 mmHg, and calf capillary-support pressure was 52.4 mmHg. Lower capillary-support pressure at the calf indicates that the calf is the most desirable support site for the standard lithotomy position.

HEEL CAPILLARY-SUPPORT PRESSURE IN THE HIGH LITHOTOMY POSITION. In this part of the analysis, heel capillary-support pressure was measured as the heel was elevated 6 inches, 12 inches, 18 inches, and 24 inches above horizontal (ie, 0 inches). Fourteen adult men and two adult women participated. The capillary-support pressure measuring device was placed between the heel and a sling that was connected by a rope to a ceiling-mounted pulley to raise the heel. In all cases, participants were positioned so that the rope connected to the heel sling was at a right angle to the lower leg. Measurements were taken for each participant as the heel was raised, and repeat measurements were taken at each height as the heel was lowered. The pressure readings at each heel height were averaged.

The results are presented in Figures 5 and 6. As the heel-support height increased, capillary-support pressure increased. Even with the heel at zero (ie, no elevation), the mean capillary-support pressure was 115 mmHg, well above the typical capillary perfusion pressure of 30 mmHg. This indicates that patients are susceptible to pressure ulcer development even when they are in the supine position, if they are in that position for a prolonged period of time.

[FIGURES 5-6 OMITTED]

SUMMARY OF RESULTS

The following results were found in this three-part analysis.

* For every 1 ft of ankle elevation above the aorta, ankle systolic and diastolic pressure decreased by approximately 20 mmHg.

* Heel and calf capillary-support pressure increased with body weight.

* For a given body weight, calf capillary-support pressure was less than heel capillary-support pressure.

* When a leg is elevated by a heel sling, the heel capillary-support pressure increases with the height of the heel above the surface on which the patient lies. [t is important to note that the mean heel capillary-support pressure without elevation was 115 mmHg, well above capillary perfusion pressure (ie, 30 mmHg).

* For a given height, the larger the supporting area, the lower the capillary-support pressure.

CONCLUSION

The risk of developing pressure sores is present when support pressure is greater than capillary perfusion pressure. These analyses found that support pressure for the heel is greater than capillary perfusion pressure when patients are lying flat, and the support pressure increases as the elevation of the lithotomy position increases. Five implications for perioperative personnel can be drawn from this information:

* reposition or turn patients frequently,

* use the lithotomy positions only when necessary,

* use the lowest heel elevation allowed by the procedure,

* apply support over the largest surface area possible, and

* maintain the patient in the lithotomy position for the shortest possible duration.

Positioning

Injury risks and safety considerations for patients in the lithotomy position

AORN's "Recommended practices for positioning the patient in the perioperative practice setting" offers some safety considerations to be followed when placing patients in the lithotomy position.

Risks

Hip and knee joint injury Lumbar and sacral pressure Vascular congestion

Neuropathy of obturator nerves, saphenous nerves, femoral nerves, common peroneal nerves, and ulnar nerves

Restricted diaphragmatic movement

Pulmonary region

Safety considerations

* Place stirrups at even height.

* Elevate and lower legs slowly and simultaneously from stirrups.

* Maintain minimal external rotation of hips.

* Pad lateral or posterior knees and ankles to prevent pressure and contact with metal surface.

* Keep arms away from chest to facilitate respiration.

* Arms on arm boards at less than 90-degree angle or over abdomen.

Reprinted with permission from Standards, Recommended Practices, a Guidelines, AORN, Inc, Denver, 2005.

RESOURCES

Lord, E V. Patient Pasitioning: Competency Assessment Module (Denver: Certification Board Perioperative Nursing, 2001).

Standards, Recommended Practices, and Guidelines (Denver: AORN, Inc, 2005).

NOTES

(1.) L A Geddes, Medical Device Accidents and Illustrative Cases, second ed (Tucson: Lawyers and Judges Publishing Co, 2002).

(2.) C J Wiggers, Physiology in Health and Disease, fifth ed (Philadelphia: Lea & Febiger, 1949).

(3.) B Brooks, G W Duncan, "Effects of pressure on tissues," Archives of Surgery 40 (April 1940) 680-709.

(4.) M Kosiak, "Etiology of decubitus ulcers," Archives of Physical Medicine and Rehabilitation 42 (January 1961) 19-29.

(5.) J R Halliwill et al, "Effect of various lithotomy positions on lower-extremity blood pressure," Anesthesiology 89 (December 1998) 1373-1376.

(6.) J L Shah, "Lesson of the week: Postoperative pressure sores after epidural anesthesia," British Medical Journal 321 (October 14, 2000) 941-942.

(7.) S Biswas et al, "Exaggerated lithotomy position-related rhabdomyolysis," American Surgeon 63 (April 1997) 361-364.

(8.) J A Pearce, "Skin-pressure distributions on three methods of patient support," in Air Fluidized Bed, Clinical and Research Symposium, ed C P Artz, T S Hargest (Charleston, SC: Medical University of South Carolina, 1971) 85-89.

(9.) W M Kirkendall et al, "Recommendations for human blood pressure determination by sphygmomanometers. Subcommittee of the AHA Postgraduate Education Committee," Circulation 62 (November 1980) 1146A-1155A.

Rebecca A. Roeder, PhD, is a postdoctoral research associate, Weldon School of Biomedical Engineering, Purdue University, West Lafayette, Ind.

Leslie A. Geddes, PhD, is the Showalter Distinguished Professor Emeritus of Bioengineering, Weldon School of Biomedical Engineering, Purdue University, West Lafayette, Ind.

Neal Corson, BS, is an MD/PhD student at the University of Chicago, Chicago.

Carrie Pell, BS, is a veterinary technician at Indiana University-Purdue University, Indianapolis.

Michael Otlewski, BSE, is a research assistant, Weldon School of Biomedical Engineering, Purdue University, West Lafayette, Ind.

Andre Kemeny, BSEE, is a research assistant, Weldon School of Biomedical Engineering, Purdue University, West Lafayette, Ind.

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