What is the purpose of a surgical tourniquet instrument?
The purpose of a surgical tourniquet instrument is to safely and accurately supply and regulate the pressure in a tourniquet cuff. The tourniquet instrument provides the compressed air to inflate the cuff. A cuff is typically pneumatically connected to the instrument via a length of tubing and specialized connectors to ensure a stable pneumatic connection. The tourniquet instrument can also monitor and regulate the pressure in a tourniquet cuff, and will provide alarms and alerts related to pressure and time control.
Description of personalized surgical tourniquet instruments
Personalized tourniquet instruments are state-of-the-art, modern pneumatic tourniquet instruments. Click here to learn about the history of tourniquets. They include automatic means of estimating the Limb Occlusion Pressure (LOP) of each patient, permitting individualized setting of safer and lower tourniquet pressures [1]. Click here to find out how Limb Occlusion Pressure (LOP) minimizes cuff pressure and increases patient safety. Personalized tourniquet instruments have an intuitive user interface; accurate and automatic pressure control for one or more pneumatic channels; audiovisual alarms; and numerous features to improve usability, reduce errors and ultimately increase patient safety. To facilitate LOP measurement and adaptation of tourniquet operation during surgery, some personalized tourniquet instruments include provision for connection of the tourniquet instrument to physiologic monitors [2, 3]. Personalized tourniquet instruments are also becoming more integrated with a wide range of pneumatic cuffs that are connectable to them, to optimize the performance of the overall system for greatest safety, accuracy and reliability [4-6]. Figure 1 shows elements of a personalized tourniquet system.
The user interface, pressure control, and safety features of a typical personalized tourniquet instrument are outlined below.
Personalized pressures – limb occlusion pressure
What is Tourniquet “Limb Occlusion Pressure” (LOP)?
Limb Occlusion Pressure (LOP) is the minimum pressure required, at a specific time in a specific tourniquet cuff applied to a specific patient’s limb at a specific location, to stop the flow of arterial blood into the limb distal to the cuff. Many studies and reviews published in the clinical literature have shown that higher tourniquet pressures and higher pressure gradients are associated with higher risks of tourniquet-related injuries, e.g. [7-13]. Studies have also shown that lower tourniquet pressures are associated with lower complications and pain, e.g. [14-15]. Therefore, when a tourniquet is used in surgery, the best practice is to use the lowest tourniquet pressure that is safely possible and to use a tourniquet cuff that is designed to produce the lowest pressure gradients.
LOP is personalized to each individual patient and each individual surgical procedure. Setting the tourniquet pressure on the basis of LOP minimizes the pressure and related pressure gradients applied by a cuff to an underlying limb, which helps to minimize the risk of tourniquet-related injuries.
Pressure gradient beneath a tourniquet cuff
Pressure gradient may be defined as the variation in pressure applied by a pressurized tourniquet cuff to the underlying limb across the width of the cuff. It is affected by the tourniquet pressure level and by the design and width of the pressurized cuff.
Figure 2 presents the pressure profiles across the widths of 3 tourniquet cuffs at tourniquet pressure levels required to achieve arterial occlusion in the limb. As shown in Figure 2, the pressure gradient is the slope of the curve on each graph. Pressure gradient (slope) increases as the applied pressure increases (y-axis) or as the cuff width decreases (x-axis). For example, a wide pneumatic tourniquet cuff (Cuff A) requires a lower tourniquet pressure level to achieve arterial occlusion in the limb (250 mmHg), and distributes the applied pressure over a wider cuff width (10 cm), resulting in lower pressure gradients being applied to the underlying soft tissues (up to 50 mmHg/cm). In comparison, narrow, non-pneumatic tourniquet cuffs (Cuffs B and C) require substantially higher tourniquet pressure levels to achieve arterial occlusion in the limb (up to 450 mmHg), and the applied pressure is distributed over a shorter cuff width (2 cm), resulting in much higher pressure gradients being applied to the underlying soft tissues (450 mmHg/cm). As noted above, higher tourniquet pressures and higher pressure gradients are associated with higher risks of tourniquet-related injuries [7-13].
The primary mechanism of tourniquet-related nerve injury is the application of a high pressure gradient along the length of the nerve underlying a tourniquet cuff [7, 11, 12, 16, 17]. A high pressure gradient results in an axial force being applied along the nerve, effectively stretching, invaginating, and buckling the myelin sheath which surrounds the nerves and displacing the nodes of Ranvier, thus injuring the nerve cells underlying the cuff [11, 12]. The location of such tourniquet-related nerve injuries is typically beneath the cuff location, and often near the distal edge of the cuff location, where the applied pressure gradient was greatest [12].
The hazard of high pressure gradients is mitigated in some modern tourniquet systems by (1) use of personalized tourniquet pressures, optimized to automatically identify the lowest tourniquet pressure level needed for individual patients, and (2) use of personalized tourniquet cuffs, designed to match the limb shape and optimally distribute cuff pressure from one cuff edge to the other, and circumferentially, thereby producing the lowest pressure gradients beneath the cuff when used in conjunction with personalized tourniquet pressures [8, 13].
Setting tourniquet pressure based on LOP
The best practice for setting tourniquet pressure is to set a personalized pressure that is based on LOP. The currently established guideline for setting tourniquet pressure based on LOP is that an additional safety margin of pressure is added to a measured LOP, to account for physiologic variations and other changes that may be anticipated to occur normally over the duration of a surgical procedure [18]. A safety margin of 40-100 mmHg above the LOP has been suggested in the literature [18, 19, 20]. Typically the safety margin is lower for lower LOP levels and higher for higher LOP levels. A higher margin may be selected if a considerable amount of manipulation of the limb and/or large blood pressure rises are expected to occur during the procedure. One cuff pressure setting method that has been used successfully in clinical studies is LOP + 40 mmHg for LOP levels less than 130 mmHg, LOP + 60 mmHg for LOP levels between 131 – 190 mmHg, and LOP + 80 mmHg for LOP levels greater than 190 mmHg [19, 20]. This method of setting personalized pressures is commonly used in commercial systems that can measure LOP. In future, as tourniquet-related research continues, it may be possible to safety establish a personalized safety margin that is substantially lower than currently accepted levels.
If possible, LOP measurement should be made while the blood pressure is stabilized at a level expected during surgery. Depending on the anesthetic technique, the LOP measurement may therefore occur before or after induction of anesthesia. Blood pressure at the time of LOP measurement should be noted.
Surgical staff can measure LOP manually by palpation or Doppler ultrasound, or automatically using a distal photoplethysmographic sensor or a dual-purpose tourniquet cuff.
Manual measurement of LOP using Doppler ultrasound
One technique for manual measurement of LOP based on monitoring arterial pulsations as an indicator of arterial blood flow is as follows: tourniquet cuff pressure is increased by an operator slowly from zero while monitoring arterial pulsations in the limb distal to the cuff until the pulsations can no longer be detected; the lowest tourniquet cuff pressure at which the pulsations can no longer be detected can be defined as the ascending LOP. A second manual technique is that an operator can slowly decrease tourniquet cuff pressure while monitoring to detect the appearance of arterial pulsations distal to the cuff; the highest pressure at which arterial pulsations are detected can be defined as the descending LOP. The accuracy of such manual measurements of LOP is highly dependent on the sensitivity, precision and noise immunity of the technique for detecting and monitoring arterial pulsations, on the time taken to slowly increase and decrease cuff pressure, and on operator skill, technique and consistency. Under the best circumstances considerable elapsed time is required on the part of a skilled, experienced and consistent operator, using a sensitive and precise technique for detecting and monitoring pulsations as an indicator of distal blood flow, to accurately measure LOP by manual means.
Automatic measurement of LOP using distal photoplethysmography probe
An automated plethysmographic system built into the tourniquet instrument that measures LOP in about thirty seconds at the beginning of an operative procedure has been developed and is widely used in surgical settings [21, 22]. The automated system incorporates a plethysmographic probe placed on a digit of the patient’s operative limb (See Figure 3). The probe detects the presence of arterial pulsations in the limb distal to the tourniquet cuff as an indicator of arterial blood flow past the cuff and into the distal limb. By automatically inflating the tourniquet cuff in predetermined steps having consistently defined durations, the minimum cuff pressure at which arterial blood flow past the cuff is stopped can be accurately determined. This cuff pressure is the patient’s personalized limb occlusion pressure (LOP). Upon the determination of LOP, the plethysmographic probe is removed.
A study by Younger et al. [20] demonstrated that use of the automated plethysmographic system and the wide contoured cuff reduced average pressure by 33% to 42% from typical pressures. The study also found that systolic blood pressure was not correlated well to LOP, and should not be used to set the cuff pressure.
A randomized controlled study of 164 patients by Olivecrona et al. [19] demonstrated that the measurement of LOP with the automated plethysmographic system provides a significant reduction in tourniquet pressure and more individual cuff pressures when used with wide contour cuffs. Although the study method demonstrated no difference in postoperative pain between the LOP and control groups, the authors noted that patients with cuff pressures below 225 mmHg had fewer postoperative complications.
In another study, McEwen et al. [22] used the automated plethysmographic system to measure LOP on the lower legs of adult volunteers. The study showed that, based on the volunteer results, LOP plus a safety margin of 40, 60, 80 mmHg (for LOP < 130, 131-190, or 190+ respectively) with a standard width cylindrical cuff had an average cuff pressure of 223 mmHg (range 170-299 mmHg, SD 36), which is 11% lower than typical practice and up to 80 mmHg (32%) lower on some patients. Using a wide, contoured cuff reduced the average cuff pressure further to 195 mmHg (range 160-280 mmHg, SD 33), which is 22% lower than typical practice and a reduction of up to 90 mmHg (36%).
Reily et. al. [23] conducted a study on pediatric patients aged 10 to 17 comparing the quality of surgical field using standard cuffs with a standard pressure of 300 mmHg to using wide contour cuffs with a pressure of LOP plus a safety margin of 50 or 100 mm Hg (depending on the level of the measured LOP). They found that the use of an automatic LOP measurement with the use of wide contour cuffs significantly reduced the mean tourniquet cuff pressure in pediatric patients from typical practice of 300 mmHg to 151 mmHg [23].
Automatic measurement of LOP using a dual-purpose tourniquet cuff
The best practice for setting tourniquet pressure is to set a personalized pressure that is based on LOP. However the routine use of personalized tourniquet settings based on LOP has been somewhat limited by practical difficulties of manual LOP determination using Doppler ultrasound, and because of limitations inherent in the current technique of automatic LOP measurement [24]. These limitations include: a distal blood flow sensor is required; the sterile field may be affected; perioperative workflow and time may be impacted affected, and the success rate of LOP measurement is dependent on affecting measurement of low blood flow distal to the cuff such as cold digits or poor peripheral circulation.
A new technique for measuring LOP has been developed in an effort to facilitate routine use of personalized tourniquet pressures, using a tourniquet cuff with continuous pneumatic passageway surrounding the limb as a dual-purpose patient sensor and pneumatic effector, and no distal sensor. A study on 143 pre-surgical and post-surgical patients found that the new technique of LOP measurement has surgically acceptable accuracy that is comparable to LOP measurement by a Doppler ultrasound, and that the new technique is feasible for incorporation into improved personalized tourniquet systems [24]. Further, many limitations of present techniques of LOP measurement are overcome with the new technique, for example: no distal blood flow sensor is required; the sterile field is unaffected; perioperative workflow and time are less affected as this technique allows measurement of the LOP while the limb is elevated and being prepared for surgery; and the success rate of LOP measurement should be substantially greater because the new technique is not dependent on variables affecting measurement of low blood flow distal to the cuff such as cold digits or poor peripheral circulation.
Currently, there are no surgical instruments that measure LOP using dual-purpose cuffs. These instruments are being developed and should be introduced in the surgical setting in the near future. Once introduced, they will improve patient safety, just as some existing types of surgical tourniquet instruments using distal sensors do very well, by allowing accurate and reliable LOP measurements and pressure regulation at lower, safer, personalized tourniquet pressures. In addition, they will have less impact on perioperative workflow and time, while increasing the success rate of LOP measurements.
Beyond the surgical setting, this technology has been used with great success for Personalized Blood Flow Restriction Rehabilitation (PBFRR) in the perioperative setting. Typically, a person is required to lift loads at or above 65% of their one repetition maximum to have noticeable increase in muscle size and strength [25]. However, many postoperative patients are either limited by their inability to attain these requisite loads or are limited by their need to protect their postoperative extremity [26]. PBFRR is being increasingly used to substantially accelerate the rehabilitation of orthopaedic patients, injured professional athletes and wounded soldiers by allowing the patients to increase their muscle strength and mass while exercising at low-intensity with only 20%-30% of their one repetition maximum load [27]. Click here to learn more about PBFRR.
User interface
Personalized tourniquet instruments have digital displays and easy-to-use user interfaces. Typically, the tourniquet instrument allows the perioperative staff to set the tourniquet pressure, inflate or deflate the tourniquet cuff, and adjust safety settings such as the maximum allowed pressure and time limits. The digital display and control buttons give the surgical staff a straightforward method of monitoring the tourniquet pressure and time, and for changing the various tourniquet settings. Personalized tourniquet instruments have non-volatile memory to enable surgical staff to store specialized settings most appropriate for certain surgeries (e.g. paediatric and hand surgery), relevant data about significant surgical events related to tourniquet usage, and to enable a data printout or transfer to an operating room information network. In addition, personalized tourniquet instruments use intuitive audio-visual alarms to alert the surgical staff of events related to patient safety.
Autoregulation
Compressed gas source
The tourniquet cuff bladder requires a source of compressed gas to supply a carefully controlled amount of tourniquet pressure. The gas used may be ambient air, nitrogen, or some other gas. Most modern tourniquet systems utilize low-pressure gas provided by pumps, while a few systems use high-pressure gas sources. Note that nitrous oxide or oxygen should never be used to inflate the tourniquet cuff, because of the increased risk of fire.
Personalized tourniquet systems utilize an internal electrical pump to compress ambient air to a low pressure; these systems do not require external high-pressure sources, such as portable canisters, portable tanks, or built-in hospital systems.
Pressure regulator
The pressure regulator adjusts and controls the gas pressure in the cuff bladder. Older, non-computerized tourniquet systems utilize valves that attempt to respond mechanically to changes in pressure. For example, if pressure in the cuff bladder falls, a valve may open to allow more gas to enter the regulator from the gas source; if pressure exceeds a certain level, the pressure may force a release valve to open and expel gas into the environment. Sometimes, the pressure levels at which these two valves turn on and off are quite different and cuff pressure may fluctuate within a certain range above and below the selected pressure. Due to the sensitive mechanical components of these systems, it is very important to follow the manufacturer’s instructions regarding frequent testing, and calibration and to perform these checks before each surgical procedure as recommended. In general, tourniquet systems with mechanical regulators are now considered to be inaccurate, unreliable, and are not suitable for incorporation with modern tourniquet safety features.
In personalized tourniquet instruments, the internal electrical pump, pressure display, and pressure regulator are combined in a single instrument in which a microprocessor continuously monitors and compensates for changing levels of pressure in the cuff bladder. Regulation does not rely on mechanical (pressure) forces to turn valves off and on. Instead, the microprocessor can detect extremely small changes in the cuff pressure and automatically regulate the flow of gas to control the pressure. Typical surgical-grade pressure regulators can accurately autoregulates cuff pressure to be less than 15 mmHg different than the target pressure during a 1 second period [7, 8].
Single and dual port instruments
Most surgical tourniquet instruments available on the market are comprised of single port technology. This means that there is one connection between the tourniquet instrument and the tourniquet cuff. All pressure functions are controlled through this connection, including cuff inflation, deflation, regulation around the set pressure, and the sensing/monitoring for alarm conditions (e.g., leaks, line occlusion, over pressurization, etc.)
Some advanced, personalized tourniquet systems use a sophisticated dual port system for improved performance and added safety. The dual port system provides the most accurate control of cuff pressure and the fastest response to pressure changes. The advantages of a dual port tourniquet system were first described by McEwen in 1981 [7]. A dual port tourniquet instrument comprises of the same main port as in the single port instruments; however, it also has an additional second port solely dedicated to pressure sensing. The tourniquet cuff is connected to the instrument via two independent pneumatic hose connections.
Dual port functionality
A dual port tourniquet instrument contains an additional pressure transducer, which is a device that measures the pressure of a fluid/gas, that is dedicated solely for pressure sensing. This additional sense port allows for a more accurate and precise system; control of the cuff pressure is facilitated through the main port (inflating, deflating, regulating, sensing), while the second ‘sense’ port is continuously sensing the actual cuff pressure.
Single port functionality
In a single port instrument, there is only one pressure transducer to control and monitor cuff pressure. In order to actively control cuff pressure through the one port, ‘sensing’ is briefly paused whenever the pump valve is open, which occurs during cuff inflating, deflation, or active pressure regulation. The delay incurred by pausing the pressure sensing can lead to poorer pressure regulation and reduced accuracy in the single port system compared to a dual port system.
Dual port advantages – safety and efficacy
Separating out two ports on a cuff connected to two ports on an instrument means there is complete independence between the monitoring of the cuff pressure through one port and the control of cuff pressure through the other port which improves accuracy, regulation, faster responses, and better detection of alarm conditions.
Another core advantage of the dual port technology is its advanced line occlusion detection capability. Short bursts of air are sent through the control port, and the sense port looks for a response via a pressure increase. If the instrument does not detect the expected pressure via the sense port, there is likely an occlusion within the system, and an alarm will alert the user.
Safety features
Below is a list of safety features found in the most advanced tourniquet instruments [2, 28-32]:
- Automatic measurement of the Limb Occlusion Pressure (LOP) to enable individualized setting of safer and lower tourniquet pressures
- Self-calibration to establish correct pressure readings
- Self-check to make sure the system is operational prior to use
- Ability to set the maximum tourniquet pressure and inflation time limits
- Automatic timer to provide an accurate record of tourniquet inflation time
- Backup battery to allow the instrument to continue to operate during an unanticipated power interruption or during patient transport
- A cuff hazard interlock to avoid inadvertent power off of the instrument while a cuff is still inflated
- Interlocks to help prevent unintended cuff deflation during IVRA procedures and bilateral limb procedures. Click here to find out more about Intravenous Regional Anesthesia
- Positive-locking ports to ensure complete pneumatic link with the pneumatic tubing and to prevent accidental disconnections
- Interfaces to information systems in the operation room to remotely capture cuff pressures, inflation times and potentially hazardous events, and
- Audio-visual alarms for:
- Reaching the maximum pressure,
- Reaching the maximum inflation time,
- Low cuff pressure,
- High cuff pressure,
- Occlusion in the pneumatic system,
- Leakage in the pneumatic system,
- Low battery, and
- Failure of the cuff to depressurize when deflation is intended.
Deflation protocols
Rapid deflation of a tourniquet cuff is the typical deflation method for most surgical cases. Rapid deflation of the tourniquet allows for immediate venous return, prevents possible engorgement of the limb, and limits the amount of time the limb is compressed by the cuff.
However, there are some cases for which alternative deflation protocols may be appropriate to achieve other clinical benefits. These deflation methods are based upon clinical feedback relayed from surgeons and perioperative nurses.
Cyclic deflation can allow for the rapid detection and ability to cauterize bleeding vessels prior to wound closure. Cyclic deflation can be achieved by rapidly deflating, pausing, and reinflating the tourniquet cuff in several cycles over a short period of time. This enables a controlled, brief reperfusion period that allows the surgeon to achieve improved hemostasis.
Step deflation can allow for a safe, gradual cuff deflation near the end of a surgical procedure to allow for a gradual return of arterial flow 1) without significantly increasing tourniquet time and 2) without causing significant venous congestion. Step deflation can be achieved by gradually decreasing the set pressure in defined steps, until the set pressure ultimately reaches a fully deflated state (0 mmHg). Step deflation may simplify and standardize the detection and management of bleeding vessels and allow for a gradual release of built-up metabolites.
Deflation protocol techniques can be achieved by manually adjusting the tourniquet instrument’s pressure set-point. However, for more precise control and to reduce the impact on clinical workflow, automatic deflation protocols programmed directly into the tourniquet instrument are preferred to achieve more precise control of the tourniquet time and pressure. Further, automatic deflation protocols substantially simplify the clinical workflow, freeing up the tourniquet operator’s time to focus on other tasks in the OR.
Through anecdotal evidence, we know that deflation protocols are being used in clinical procedures today. However, there is limited research regarding optimal protocol parameters and their specific clinical benefits, and further studies are necessary to better understand their potential uses and outcomes.