Intraarterial pressure monitoring is most often performed with miniature disposable pressure transducers connected to monitors that display time tracings. This practice is common, safe, and so reliable that clinicians may question if it really is required to periodically check intraarterial pressure measurements against those obtained from a standard blood pressure cuff (1,2). This case report describes a situation where the patient benefited from simultaneous comparisons of the two measurements.
A 45-yr-old woman arrived in our Interventional Radiology Department one day after having a subarachnoid hemorrhage from a ruptured cerebral aneurysm. She was scheduled for endovascular embolization under general anesthesia, for which intraarterial monitoring was indicated to avoid excessive blood pressure (BP) fluctuations. Routine noninvasive monitors were also used, and these included a BP cuff on the upper right arm. Although intraarterial catheters in such cases are usually placed before inducing anesthesia, we delayed placement until afterwards because this patient was very anxious, and her initial BP was stable. Standard induction and maintenance infusions of propofol and remifentanil were titrated to produce moderate reductions in cuff pressures, these being obtained every 2.5 min during controlled face mask ventilation.
The arterial catheter transducer (Transpac IV Monitoring Kit, Abbott Critical Care Systems, North Chicago, IL) was zeroed to atmospheric pressure and the tubing cleared of bubbles. After observing that movement of the tubing produced appropriate changes in the tracing on the monitor (AS/3 Anesthesia Monitoring System, Datex-Engstrom-Ohmeda, Andover, MA), the radial arterial catheter was placed on the left side, which is the side away from the radiologists. The intraarterial pressure tracing seemed normal and had a dicrotic notch, but the pressure readings were approximately half of those obtained from the BP cuff. BP cuff pressures on the left arm were then checked, and these were identical to the BP cuff reading on the right arm. Laryngoscopy was performed twice so as to first spray the trachea with 4% lidocaine (4 mL) approximately 3 min before inserting the endotracheal tube. There was no substantial change in arterial catheter or cuff pressure response to tracheal intubation.
We returned to the matter of reconciling the two pressure readings. Our differential diagnosis had two pathways: one related to dysfunction of electronic systems, and the other related to unequal distal perfusion of the right and left arms. A second arterial catheter was inserted in the right radial artery, and the cable from the monitor to the first intraarterial pressure transducer was switched to display pressures from the new arterial catheter. Intraarterial pressure catheters were equal in both arms, just as the cuff pressures had been. Next, we changed the Datex circuit boxes, but all readings were unchanged. Finally, we changed the electrical cable for the arterial catheter, which resulted in tracings that brought the arterial catheter values upwards into perfect agreement with simultaneous measurements from the cuff. Inspection of the cable revealed corrosive deposits on the socket wires that connect to the telephone wire cable from the transducer (Fig. 1A). The remainder of the case proceeded uneventfully, and the aneurysm was successfully treated.
We took the corroded cable to another operating room (OR) where a recorder module could be used to print the arterial catheter pressure on a strip chart, and we documented that the use of the corroded cable again produced decreased pressure readings. The cable was then referred for testing to the Clinical Bioengineering Group for our OR suites. They cleaned the metal pins with cotton applicators containing rubbing alcohol. After complete drying, the cable, together with a monitor, performed perfectly for a standard two-point calibration—zeroing and appropriate tracings from a calibration box that simulates the transducer output by generating a square 100-mm Hg wave form from the transducer. We then returned to the OR with the recorder module and documented the repair. The formerly corroded cable read 134/80 mm Hg after cleaning compared to 38/23 mm Hg before cleaning (Fig. 2).
Continuous monitoring of transduced pressures, arterial blood pressures, in particular, is part of the standard of anesthesiology care for many procedures (3). Associated equipment problems, such as total failure of a cable or transducer, are usually easily recognizable. However, this case illustrates a subtle, treacherous type of failure, which is artifactual attenuation of the transducer output voltage without modulation of the shape of its wave form. In this case, the intraarterial catheter pressure tracing indicated pressures ≈50% of the cuff value, heightening suspicion of artifact. Had the error been smaller and a cuff pressure not measured, we might have thought the BP was low and increased it pharmacologically, risking an aneurysmal re-bleed and a devastating stroke. Several possible artifacts associated with intraarterial pressure monitoring are described in the literature (4). However, we did not find a report of false pressures from cable failure such as the one we describe. Intracranial pressure (ICP) is also often transduced in our neuroangiography suites, and the faulty cable in this case could very likely have been used for that purpose. Although ICP is often managed via an intraventricular catheter that has an associated manometer, in general, there is no quickly available second system for checking ICP as there is for checking arm and leg arterial pressures. Falsely low ICP readings in neurosurgical cases could lead to disastrous outcomes.
Basic details about miniature disposable pressure transducers, sketched in Figure 3, explain how false pressure readings can arise from corrosion in the connecting cable. Corrosion occurs immediately if the sensitive gold-plated copper contacts are exposed to blood or saline. Miniature disposable pressure transducers are produced using microelectromechanical system technology, the same process used for making integrated circuits and tiny integrated devices that combine mechanical and electrical components, all ranging in size from a few microns to a few millimeters. The first miniature disposable pressure transducers were made in 1982, approximately a year after the appearance of techniques for micromachining silicon. Before then, pressure transducers were large, costing more than $600 each, requiring sterilization before reuse. They were also potentially dangerous because it was possible for certain types of failure to result in microshock currents that could travel back to the heart through the arterial tree causing ventricular fibrillation (5). Today it is not uncommon to encounter research work using catheter-based micropressure transducers that have the same Wheatstone bridge design but are smaller than 0.1 in. in diameter (6). Figure 3A shows the schematic diagram of a miniature piezoresistive pressure transducer, which starts out as a silicon wafer in which a thin diaphragm is created by micromachining. Afterwards, piezoelectric material attached at deflection points is incorporated into a Wheatstone-bridge circuit, as shown schematically in Figure 3B. Applied pressure causes a bulge in the diaphragm, which in turn produces an ohmic change in one resistor (designated as X) in the Wheatstone bridge. This in turn brings about a change in output voltage, which is typically a few microvolts when the input voltage is a volt or two. If corrosion increases ohmic resistance in the input or output lines, the Datex unit will be presented with a lower output voltage from the Wheatstone-bridge, and therefore, it will assign lower pressures to the detected signal. However, this is but one scenario for generating artifacts. There are many ways to disturb a Wheatstone bridge. For example, saline or other substances in the connection can couple one corner of the Wheatstone bridge circuit to another, even shorting it out. Because the formula in Figure 3B has one term being subtracted from another, it is evident that it is even possible for the electronics to artifactually conclude that the applied pressure is negative when in fact it is very positive.
Our report shows that perfectly shaped pressure tracings on the monitor can provide a false sense of security. The standard check of a pressure transducer setup involves confirmation of the zero level and the measurement of calibrated sources. At our hospital, the cables for pressure monitoring are periodically checked by the Clinical Bioengineering Group when it checks the amplifiers in the Datex units. This is performed with a small, battery-powered calibration box from Abbott Critical Care Systems (Fig. 4). Anesthesiologists and workroom personnel do not normally carry or use such calibration boxes, which provide the ultimate test of the whole pressure monitoring setup. It might be prudent to have calibration boxes in several locations. However, that would be helpful only if someone took responsibility for assuring that they all functioned similarly and correctly. In any case, an important lesson for us was that, regardless of the number of such calibration boxes in a department, there should be one that is always immediately available for bedside use.
There are some obvious, time-honored remedies to the circumstances that led to use of a damaged cable. One involves the training of technicians who clean and set up equipment for anesthesiologists. Pressure cables can easily suffer mechanical abuse as a result of being vigorously pulled or stretched inadvertently or being crushed by heavy rolling equipment. However, it is often possible at the end of wet cases for disconnected pressure cables to have blood or saline in a socket. At the end of a case, cables that are disconnected from pressure transducers should be properly stored. Before use, anesthesiologists should routinely perform visual and tactile cable inspections, searching for mechanical damage and corroded contacts. Had all this been done in our case, the incident we described would most likely not have occurred. During a case, the anesthesiologist should make sure that there is a good seal and proper orientation to the protective housing that keeps blood away from the connection between the transducer cable and the monitor cable, i.e., between the two ends shown in Figure 1. Proper checkout of equipment should also include the testing of standard BP cuffs.
Two important points summarize our experience in which technologically advanced monitoring was accompanied by a technologically advanced artifact. The first is to have independent, redundant indicators and thus avoid reliance on a single physiological indicator. Whenever monitoring BP with an arterial catheter, a second independent monitor must be used. Usually this is a standard noninvasive cuff. In general, any type of pressure tracing should be verified by an independent method. Second, electrical cables need to be inspected before their use, especially in remote anesthetizing locations where mechanical damage or chemical modulation is likely to occur between uses.
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2. Pace NL, East TD. Simultaneous comparison of intraarterial, oscillometric, and finapres monitoring during anesthesia. Anesth Analg 1991; 73: 213–20.
3. Keegan MT, Atkinson JL, Kasperbaver JL, Lanier WL. Exaggerated hemodynamic responses to nasal injection and awakening from anesthesia in a Cushingoid patient having transphenoidal hypophysectomy. J Neurosurg Anesthesiol 2000; 12: 225–9.
4. Bur A, Hirschl MM, Herkner H, et al. Accuracy of oscillometric blood pressure measurement according to the relation between cuff size and upper-arm circumference in critically ill patients. Crit Care Med 2000; 28: 371–6.
5. Hull CJ. Electrocution hazards in the operating theatre. Br J Anaesth 1978; 50: 647–57.
6. Walter B, Bauer R, Fritz H, et al. Evaluation of micro tip pressure transducers for the measurement of intracerebral pressure transients induced by fluid percussion. Exp Toxicol Pathol 1999; 51: 124–9.
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