Anesth Analg 2006;102:1752-1757
© 2006 International Anesthesia Research Society
doi: 10.1213/01.ane.0000209018.00998.24
TECHNOLOGY, COMPUTING, AND SIMULATION
A New Method for Estimating Arterial Occlusion Pressure in Optimizing Pneumatic Tourniquet Inflation Pressure
Bahattin Tuncali, MD,
Ayse Karci, MD,
Binnur Erdalkiran Tuncali, MD,
Omur Mavioglu, MD,
Mustafa Ozkan,
Abdul Kadir Bacakoglu,
Hakan Baydur, MD,
Ahmet Ekin, and
Zahide Elar
Department of Anesthesiology and Reanimation, Department of Orthopedics and Traumatology-Division of Hand Surgery, Department of Public Health, Dokuz Eylul University, Izmir, Turkey
Address correspondence and reprint requests to Bahattin Tuncali, Inonu cad. No: 582/5 Poligon, 
zmir-Turkey. Address e-mail to bahattin.tuncali{at}deu.net.tr.
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Abstract
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To reduce pressure-related injuries resulting from pneumatic tourniquet use, the lowest possible inflation pressure is recommended. Arterial occlusion pressure (AOP) is a measure of the cuff pressure required to maintain a bloodless surgical field. However, its determination method is time consuming, requires operator skill, and is therefore seldom used in current practice. An AOP estimation can be made by knowing the pressure transmitted to the underlying soft tissues. We measured upper and lower extremity tissue pressures under the tourniquet cuff at 100, 200, and 300 mm Hg of tourniquet inflation pressures in 30 anesthetized living adult patients. All patients received general anesthesia with neuromuscular relaxation. A Stryker intra-compartmental pressure monitor was used to measure tissue pressures under the tourniquet cuff. In all patients, the soft tissue pressures were consistently lower than the applied tourniquet inflation pressures. Our results revealed tissue padding coefficients for extremities 20 to 75 cm in circumferences. An estimation method of AOP was developed [AOP = (systolic blood pressure + 10)/Tissue padding coefficient]. The new AOP estimation method may be a simple, rapid, and clinically practical alternative to the AOP determination method.
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Introduction
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The use of a pneumatic tourniquet to produce a bloodless surgical field is a well-established practice in orthopedic surgery, but it is not without its complications (1). Because pressure-related injuries to the underlying skin, nerve, muscle, and blood vessels are dependent on both the duration and pressure of tourniquet inflation, use of the lowest possible tourniquet inflation pressure (TIP) is recommended (1–5). Arterial occlusion pressure (AOP, the minimum cuff pressure that stops arterial blood flow distal to the cuff) is a measure of the cuff pressure required to maintain a bloodless surgical field and has been shown to be useful in optimizing cuff pressures (6–9). However, its determination method is time consuming and requires considerable operator skill to be accurate and precise, and it is therefore seldom used in current practice (10). Direct measurement of AOP at the time of cuff application considers factors such as the type of cuff and the properties of the patient’s soft tissues and vessels under the tourniquet cuff (11). An estimation of AOP can be derived with knowing the pressure transmitted to the underlying soft tissues.
Investigations with animals and human cadavers showed that the pressure in the soft tissues under a tourniquet cuff may vary widely from the inflation pressure of a pneumatic tourniquet (12–14). We have not found any study in which the tissue pressure under the tourniquet (TPUT) was measured in anesthetized living adult patients. Therefore, the actual pressure that the arteries, nerves, and muscles under the tourniquet are exposed to in anesthetized living patients is unknown. It is possible that living patients with complete muscle relaxation and a lower tissue turgor may require lower TIP than those suggested by the animal and cadaver experiments.
The aim of this study was to determine the relationship between the TIP and the pressure in the underlying soft tissues in anesthetized adult patients undergoing upper and lower limb surgery.
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Methods
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After Hospitals Ethics Committee approval, informed consent was obtained from 30 adult, ASA physical status I–II patients scheduled for upper (Group 1, n = 15) or lower (Group 2, n = 15) extremity surgery. The gender and age of the patients, body mass index values and circumference of the extremity were recorded.
On arrival at the operating suite, an 18-gauge IV catheter was inserted in the arm that was not operated. All subjects were positioned supine. Peroperative monitoring included noninvasive arterial blood pressure, electrocardiogram (ECG), Spo2, and end-tidal carbon dioxide (ETco2). Anesthesia was induced with propofol (1.5–2.5 mg/kg IV bolus) and remifentanil (0.5 µg · kg–1 · min–1 continuous IV infusion), and endotracheal intubation was facilitated with vecuronium (0.1 mg/kg). Anesthesia was maintained with propofol (4–5 mg · kg–1 · h–1) and remifentanil (0.3–1 µg · kg–1 · min–1) continuous infusion. The lungs were ventilated with 50% O2 + 50% N2O to maintain an ETco2 at approximately 30 mm Hg. After the endotracheal intubation, limb circumferences were measured at the longitudinal midposition of the tourniquet cuff.
In the upper extremity group, the arm circumference was measured 10 cm proximal to the olecranon with the elbow joint in extension. Then the tourniquet cuff was placed around the arm with the distal edge 5 cm proximal to the olecranon. In the lower extremity group, the thigh circumference was measured 20 cm proximal to the superior pole of the patella with the knee in extension. Then the tourniquet cuff was placed around the thigh with the distal edge 15 cm proximal to the proximal pole of the patella. An 11 cm straight operating room tourniquet was applied to all patients.
A Stryker intra-compartmental pressure monitor system (Stryker 295–1 Pressure Monitor, Kalamazoo, MI) was used to measure the TPUT cuff. The monitoring kit with a disposable syringe filled with 3.0 mL of 0.9% saline and a sterile needle was connected to the diaphragm transducer, and the whole assembly clipped into the monitor specifically designed for measuring tissue pressure. The syringe was pressed until a drop of saline exuded from the needle tip to prime the diaphragm and ensure a solid column of fluid from the needle tip to the transducer. The monitor was zeroed at a 45° angle. After the skin preparation with povidone iodine, the pressure probe (18-gauge side ported needle 6.4 cm long) was inserted 45° at the distal edge of the pneumatic tourniquet cuff and positioned directly to the center of the limb. The tip of the pressure probe was placed adjacent to humerus or femur in upper and lower extremity subjects, respectively. A small amount of saline was injected. After allowing a few seconds for the reading to equilibrate, basal tissue pressure was recorded. The tourniquet was then pressurized. Pressures were increased in increments of 100 mm Hg, starting at 100 mm Hg and ending at 300 mm Hg. The soft tissue pressures were measured with a precision of 1 mm Hg and recorded at, 100, 200, and 300 mm Hg of tourniquet pressures.
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Results
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Thirty patients were included in the study. All patients received general anesthesia for the operation. The demographic characteristics (age, BMI, gender, circumference of the extremity) are given in Table 1.
The results of this study are presented in Table 2. In all patients, the TPUT ranged between 9 and 16 mm Hg when TIP were 0 mm Hg (basal TPUT). However, in all patients the soft tissue pressures were consistently less than the applied TIP at 100, 200, and 300 mm Hg. This difference in pressure was small in limbs of lesser diameter but increased as the size of the extremity increased.
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Table 2. Tissue Pressures Under Tourniquet (TPUT) at 0 (Basal), 100, 200, and 300 mm Hg of Tourniquet Inflation Pressures of the Patients.
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The plot of the data demonstrates that the relationship between TPUT values and extremity circumferences is nonlinear in both the arm and the leg. Therefore, this relationship can be revealed by adding a power curve between the TPUT values. The entire sample was pooled to develop a regression model of TPUT expressed as a percentage of TIP (KTP) versus limb circumference (Fig. 1). This yields the following equation:
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Figure 1. Relationship between tissue pressure under tourniquet (TPUT) values expressed as a percentage of tourniquet inflation pressure (TIP) (KTP) and extremity circumferences. The range of pressure varies with limb circumferences.
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where Y = TPUT, ß0 = constant for TIP, t = extremity circumference (cm), and ß1 = constant for extremity circumference.
According to the power law, we estimated TPUT values for extremities of 20 to 75 cm in circumference using the formulas given below.
Figure 2 shows the actual TPUT versus the estimated TPUT values in our patients.
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Figure 2. Actual tissue pressure under tourniquet (TPUT) versus estimated TPUT values based on our equation at 100, 200, and 300 mm Hg of tourniquet inflation pressures.
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TPUT is consistently lower than TIP as a result of tissue padding beneath the tourniquet cuff. Thus, the term "Tissue padding coefficient (KTP)."
If we formulate this:
The major determinants of KTP are shape and circumference of the extremity. In our study, as we measured TPUT at 100, 200, and 300 mm Hg of TIP using the same tourniquet cuff, we can estimate KTP for extremities based on the limb circumferences. Our results revealed KTP for extremities 20 to 75 cm in circumference (Table 3).
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Table 3. Estimated Tissue Pressure Under Tourniquet (TPUT) Values Expressed as a Percentage of Tourniquet Inflation Pressure (KTP) According to Power Curve in Limbs of 20 to 75 cm in Circumference.
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Because the systolic blood pressure (SBP) of the patient is a major factor to determine AOP, a soft tissue pressure must be selected and maintained slightly higher than the arterial blood pressure (safety margin) in order to maintain a bloodless field. In other words, if we really want to reach AOP, a safety margin should be added to SBP to obtain minimal TPUT not the TIP. For example, minimal TPUT to occlude blood flow can be achieved by adding 10 mm Hg to SBP. Thus, TPUT cuff will be 10 mm Hg higher than the patient’s SBP at any applied TIP. If we formulate this, TIP will be AOP if TPUT = SBP + 10 mm Hg.
To determine AOP, we have 2 formulas:
Then,
For example, in a patient whose extremity circumference and SBP is 37 cm and 100 mm Hg, respectively [AOP = (100 + 10)/0.78 = 141.0 mm Hg], an AOP of 141 mm Hg should be enough to occlude blood flow.
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Discussion
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Pneumatic tourniquets are routinely used to establish a bloodless surgical field in extremity surgery. Maintaining a bloodless surgical field makes dissection easier, renders surgical procedures less traumatic, and generally shortens the time required to complete the surgical procedure (1). However, there is extensive literature on nerve injuries associated with the use of tourniquets, ranging from paraesthesia to complete paralysis. Many reported cases of preventable nerve damage, limb paralysis, and other injuries are believed to have resulted from over-pressurization of the cuff (1–5).
Approximately 100 years have elapsed since Cushing (1904) introduced the pneumatic tourniquet, but the safe limits for duration of tourniquet ischemia and TIP are still being discussed (1). Evidence indicates that excessive pressures have been used for many years. Recommendations have evolved regarding the optimal TIP (15): some authors advise pressures of 250–300 mm Hg and 350–500 mm Hg for upper and lower extremity cases, respectively. Others recommend twice the SBP or SBP plus 30–100 mm Hg (7).
Ideally, a pneumatic tourniquet should be inflated to the minimum supra-systolic pressure required to maintain a bloodless surgical field distal to the cuff. Thus, one practical approach would be to pressurize the cuff to a supra-systolic pressure with a reasonable safety margin. This is called minimum TIP (SBP plus a safety margin). Using this technique, Newman and Muirhead (15) used "SBP plus 35 mm Hg" and obtained satisfactory results with "166 ± 19.6 mm Hg" inflation pressures for the arm tourniquet. The Association of Operating Room Nurses recommends "SBP plus 50–75 mm Hg" and "SBP plus 100–150 mm Hg" for the arm and leg tourniquet, respectively (16).
Some investigators recommend setting the TIP according to a direct determination of the minimum cuff pressure needed to occlude blood flow. In this setting, it is the clinical technique described for determination of AOP. AOP can be determined by slow cuff inflation to pulse cessation with diagnostic equipment such as a Doppler flowmeter or a pulse oximeter (6–9). With this technique, minimal TIP can be achieved by adding a safety margin to AOP. Using this technique Van Roakel and Thurston (6) suggested that in a normotensive, average-sized patient, a TIP of 26.7 kPa (200 mm Hg) should be adequate for the upper limb and 33.3 kPa (250 mm Hg) should be adequate for the lower limb. Levy et al. (8) and Reid et al. (9) recommended 202.3 ± 34.2 and 190 mm Hg, respectively.
However, the current "gold standard" AOP determination method is time consuming and requires considerable operator skill to be accurate and precise and is therefore seldom used in current practice (10). Moreover, studies showed that AOP varies widely relative to SBP. This variability suggests that cuff pressure based on SBP alone will not be optimal for many patients. SBP is only one variable affecting AOP, and correlation between SBP and AOP is not always strong. Another variable affecting AOP is the width of the tourniquet cuff. Therefore, we used the same tourniquet cuff in all patients. Our study is limited in that only one size tourniquet was used (11 cm, single-bladder); results may be different with wider or narrower tourniquets or with dual-bladder tourniquets. The other variables affecting AOP include limb circumference and limb shape, which produce a tissue padding effect and create a difference between applied TIP and TPUT cuff (11). Therefore, this model of limb circumference only accounted for approximately 50% of the variability in TPUT. As a result, pneumatic TIPs do not accurately reflect the TPUT.
Previous studies which have described soft tissue pressure distribution under a pneumatic tourniquet cuff have used a variety of experimental models and measurement techniques. Shaw and Murray (12) examined tissue pressures beneath a thigh tourniquet in hip disarticulation specimens and found that the soft tissue pressure tended to decrease as the probe was placed in the limb from a subcutaneous position to a location nearer the bone. They concluded that the central pressure in the limb is consistently lower than the TIP by an amount proportional to the circumference of the limb. Similar findings have been noted in studies using animal models (13). Graham et al. (14) evaluated the pressure transmitted to the major peripheral nerves of the arm by an externally applied pneumatic tourniquet using a biomedical pressure transducer placed adjacent to radial, median, and ulnar nerves in six cadaver upper extremities of average dimensions. They found a pressure gradient between perineural areas under the edges and midpoint of the pneumatic cuff, especially at high inflation pressures. The results of our study confirm previous studies that the inflation pressure of a pneumatic cuff may not represent the actual pressure in the soft tissues under the cuff.
In our study, we developed an AOP equation [AOP = (SBP + 10)/KTP] based on SBP and extremity circumferences. The new AOP estimation method may be a clinically practical alternative to the AOP determination method. Moreover, using constant TIP during surgery, this formula clearly shows that if we want to reach minimal inflation pressures, the patient’s SBP should be kept as low as possible and maintained stable during operation. In a previous study, using a controlled hypotension and minimal TIP technique, we reached TIP significantly smaller than those recommended in the literature. With this technique minimal TIP ranged between 110–140 mm Hg (mean, 118.2 mm Hg) and we obtained a bloodless surgical field in almost all cases in upper limb surgery (17). However, the patient’s arterial blood pressure may fluctuate during surgery. Ideally, an arterial blood pressure-adjusted pneumatic tourniquet that beat-to-beat responds to the patient’s SBP based on our formula should allow not using a safety margin added to AOP. For this purpose, we are developing a pneumatic tourniquet that beat-to-beat responds to the patient’s SBP based on our formula. Until then we can not call these pressures the real minimal TIP.
In conclusion, our study showed that the soft TPUT cuff are lower than the applied TIP as a result of the tissue padding effect in anesthetized living patients. Our results revealed KTP for extremities 20 to 75 cm in circumference. The new simple and rapid AOP estimation method based on the equation [AOP = (SBP + 10)/KTP] may be a practical alternative to AOP determination.
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Footnotes
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Accepted for publication January 19, 2006.
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References
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Abstract
Introduction
