Hazard [Health Devices Nov 1982;12(1):22-5]

Problem

ECRI received from member hospitals two reports of fatal cerebral air embolism immediately following routine recalibration of an invasive arterial blood pressure monitoring system. In both cases, the equipment was calibrated by pressurizing the system with air using a sphygmomanometer cuff inflation bulb and comparing the pressure monitor display with the pressure indicated by a mercury manometer. Apparently, a bolus of air was infused into the patient, either because the stopcock between the catheter and pressure tubing had not been closed or because air had been inadvertently introduced into the transducer dome or continuous flushing device, then fast-flushed into the patient's artery after calibration.

Blood pressure transducers and electronic amplifiers have had a history of calibration problems; therefore, many users have instituted procedures for checking system calibration to ensure that blood pressures are measured accurately. The monitoring system is usually calibrated during periodic inspections and immediately before connection to the patient. This well-known technique compares the electronic pressure display to the pressure indicated on a mercury manometer (after zeroing the transducer) and is relatively simple to perform with inexpensive equipment. However, in some hospitals, this same method is also used to recalibrate the system while it is connected to the patient at eight-hour intervals (work-shift changes) and after suspicious or clinically significant readings. Using this technique, the clinician must turn off the stopcock between the transducer and patient to pressurize the system with the cuff inflation bulb.

Most clinicians recognize that air embolism is possible if this step is omitted. However, it can be overlooked. When it is, air pressure greater than the blood pressure will force air into the transducer dome and tubing system and, possibly, into the patient. Under these conditions, the calibration pressure (typically 100 or 200 mm Hg) will not be reached, and the clinician may try additional squeezes of the inflation bulb, further increasing the likelihood of air embolism. Air can also be introduced into the system or patient if the patient stopcock is opened before the air pressure is released.

Another potential mechanism for air embolus formation is fast-flushing of the system when air is present, using continuous flush valves. Depending on its duration, flushing can cause significant retrograde flow in the arterial vessels because the flush pressures are typically greater than the arterial pressures. Even small amounts of air, inadvertently introduced into the system, can be flushed from the radial artery, up the brachial artery and aorta to the cerebral and coronary blood supply, where they can cause an air embolus. (For example, a small amount of air can be introduced into the system, even with the patient stopcock closed during calibration, if the calibration pressure exceeds that of the pressure infuser flushing system. Air might also be inadvertently introduced after blood sampling.)

In general, the benefits of continuous flush valves to maintain an open catheter lumen outweigh the risks of retrograde flow during fast-flushing. The likelihood of significant retrograde flow can be reduced by minimizing the time that the fast-flush valve is activated. (This is especially relevant when the system's fidelity is checked by observing the dynamic response to a step function generated by quick-releasing the fast-flush mechanism.) In any event, it is imperative to keep the system free of air at all times.

To more safely insure accurate readings while monitoring, most hospitals rely on periodic rezeroing by opening the dome stopcock to atmosphere and resetting the zero baseline of the monitor when the transducer is positioned at the level of the heart. The electronic balance of the monitor, sometimes referred to as electronic "cal," is also checked. Unfortunately, this function only simulates a transducer imbalance corresponding to a specified pressure and may not always detect a faulty transducer. Most monitor manufacturers and some transducer manufacturers recommend rezeroing and electronic calibration (although one manufacturer's instruction manual still describes an air-pressurization technique); these procedures have proved to be adequate for most purposes and present little risk of air embolism. In fact, results of our evaluation of pressure transducers (Health Devices, Vol. 8, p. 199) support the adequacy of these procedures since we did not find significant drift in transducer sensitivities. Transducer failure is most likely to occur during cleaning or other handling. Sudden transducer failure or significant sensitivity changes are unlikely while the transducer is undisturbed during monitoring.

We believe that many clinicians have unrealistic expectations for the accuracy of the monitoring system during arterial blood pressure monitoring. System accuracy is more limited by the dynamic response of the hydraulic system (tubing, stopcocks, and catheter) than by the electronic equipment. Resonance in the hydraulic system can cause overshoot of the pressure pulse, resulting in errors of up to 20 mm Hg in the systolic pressure value (see Health Devices, Vol. 8, p. 201). Our accuracy criterion for the monitoring system allows errors of 2 mm Hg or 10%, whichever is less, when static pressures are applied.

We believe that improvements in pressure transducers and monitors and the standardization of transducer sensitivities and monitor gain have significantly reduced the need for routine recalibration of the system during use. Initial zeroing and calibration and periodic rezeroing and electronic calibration checks are still required. However, we believe that the risk of air emboli caused by forgetting to close the patient stopcock during air-pressurized calibration procedures outweighs the risk of inappropriate diagnosis or therapy from slight system inaccuracies.

1. Review hospital procedures for invasive blood pressure monitoring and criteria for recalibration intervals.
2. Eliminate routine recalibration procedures using air-pressurizing equipment (i.e., cuff inflation bulbs) while the system is in use.
3. If rezeroing and electronic calibration cannot resolve a clinical discrepancy, consider an alternative recalibration technique (see discussion below) after discussing system accuracy requirements and infection control policies with hospital staff.

Alternative In-Use Calibration Techniques

There may be rare instances where a blood pressure monitoring system in use needs recalibration with a known pressure reference.  For example, the transducer may be jarred or the monitor's gain adjustment (if so equipped) may be inadvertently changed. A physician may want the system recalibrated before taking action to confirm that a significant blood pressure change has occurred.

There are methods of applying reference pressures for those special circumstances that minimize the risk of air embolism. One technique is to use a vertically oriented length of tubing connected to the transducer dome and flushed with solution from the pressure infuser to produce a water manometer. For example, a saline column within a raised one-meter length of extension tubing will generate about 73 mm Hg. (Three- and four-foot tubing lengths will generate pressure of 67 mm Hg and 90 mm Hg, respectively.) This technique can be performed rapidly and will not introduce air into the system. However, this calibration pressure may not be adequate for confirmation of high systolic pressures, and the technique may be awkward in some settings. Slight errors may result if the tubing is not held vertically.

If disposable diaphragm domes are used, the user can disconnect the transducer from the sealed hydraulic system and reconnect the transducer to another dome, which is then used for calibration.  After calibration, the transducer can be reconnected to the hydraulic system, rezeroed, and quickly put back into operation. However, some manufacturers believe that inconsistent dome couplings caused by variations in dome application techniques or coupling differences between dome lots, may result in transducer sensitivity changes of up to 3%.

Another rarely used technique is to adapt a liquid isolator device for recalibrating the system. We have had limited success in tests of one such device.  The isolator, a balloon membrane within a syringe barrel, is connected between the transducer dome and manometer guage system, and the system is pressurized using the pressure infuser by activating the "fast-flush" of the continuous flush valve. The manometer and monitor readings can then be compared at various pressures. This system also minimizes the risk of air embolism. However, pressure will not be accurately transmitted to the manometer if the balloon collapses. Also, failure to close the stopcock to the patient before actiivating the fast-flush mechanism can cause retrograde flow into the blood vessel or overinfusion of the flush solution. 

Other manufacturers have developed devices for calibration of pressure monitoring systems. One such device automatically generates air pressures for calibrating the monitoring system and displays those pressures on a digital display. this device is appropriate for calibration before use or during inspection or maintenance, but may cause air embolism if used while the system is in use and the patient stopcock is not closed.

Another calibration device may be safely used under certain circumstances. This device uses a piston/cylinder arrangement that generates pressure when the piston is moved within the cylinder. This mechanism limits the total volume of air that can be introduced. However, the risk of air embolism does exist if this volume is greater than the volume within certain monitoring kits (i.e., when short tubing lengths are used with miniature transducers mounted on the patient's limb). The risk is much less for IV pole-mounted systems with longer tubing lengths, which have greater volumes. Users of this device should also consider that it generate expected pressures based on compression of a "known" volume of air and cannot be used as a second pressure standard (i.e., like a mercury manometer) to compare with the monitor reading. Unexpected compliances (e.g., caused by excessive microbubbles or air-filled tubing lengths used to connect the device and system) may cause calibration errors with this device.

The recalibration technique must also minimze the risk of contaminating the systems's sterile fluid pathway. Stopcock and dome ports can become contaminated using any recalibration or rezeroing technique. It is usually desirable to interface calibration equipment and the monitoring system with a low-cost sterile component that can be discarded later, such as a stopcock, short length of tubing, or filter designed for this purpose. However, the use of a filter will not necessarily reduce the risk of air embolism when air pressurization calibration is performed. The flush solution manometer (e.g., PVC tubing) and the fluid isolator device use prepackaged sterilized disposables, and, therefore, aid in reducing the risk of contamination during recalibration.   

Edmonds JF, Barker GA, Conn AW. Current concepts in cardiovascular monitoring in children. Crit Care Med 1980;8(Oct):548-553.

Lantiegne KC, Civetta JM. A system for maintaining invasive pressure monitoring. Heart Lung 1978;7(Jul-Aug):610-21.

Lowenstein E, Little JW, Lo HH. Prevention of cerebral embolization from flushing radial-arterial cannulas. N Engl J Med 1971;285(Sep):1414.

Smith RN. Invasive pressure monitoring. Am J Nurs 1978;78(Sep):1514-21.

UMDNS Term

Pressure Monitors, Blood, General/Invasive [16-764]

Device factor: Device interaction

User errors: Inappropriate reliance on an automated feature; Incorrect clinical use

Support system failure: Lack or failure of incoming and pre-use inspections

Embolism (gaseous)