Book Text

CAPACITIVE COUPLING

The direct connection of two circuit elements by a wire or resistor is known as resistive coupling. It is the easiest kind of coupling for an anesthesiologist to recognize

Figure 87-4 Schematic diagram of a parallel-plate capacitor. C, Capacitance in farads; f, frequency of alternating current.

and understand. The connection is visible and the effects of the connection are apparent for circuits carrying DC or AC at all frequencies. More subtle and abstract is the concept of capacitive coupling, which applies only to AC current. This concept is relevant to the anesthesiologist because it can account for electrical connections being present at high electrical frequencies. If the alarm of an LIM goes off only when the electrosurgical electrode delivers high-frequency current to the patient, the dangerous pathway to the ground may be caused by capacitive coupling, and there may be some power outlets that are safe for low-frequency currents but not for high-frequency currents. Grounding pads, connections to electrical plugs, and equipment may need to be separated even farther from each other before the problem is remedied. The rationale follows directly from basic principles governing capacitive coupling.

The parallel-plate capacitor ( Fig. 87-4 ) is a circuit element that permits the temporary storage of electrical charge and the passage of AC. Parallel-plate capacitors, however, do not permit the passage of DC. The electrical impedance of an object is quantified by the total number of ohms it has for obstructing AC or DC. This concept, which is a generalization of electrical resistance, permits frequency-dependent assignments of ohms to all circuit elements: capacitors, inductors, and resistors. For capacitors, the ohmic calculation is done with a simple formula. The impedance (Z, in ohms) of a pure parallel-plate capacitor is a function of the AC frequency (f): In

this instance, capacitance (C, in farads) is directly proportional to the area of the capacitor plates and is inversely proportional to the space between the plates. As frequency (f) increases, impedance to the flow of current decreases. For DC, f equals 0, and the impedance becomes infinitely large.

At very high frequencies, capacitive coupling to the ground occurs through fewer ohms of impedance than at low frequencies. Because any two conducting objects in a room have surface area and an average distance of separation, they form a kind of capacitor. Any two objects have a capacitive coupling. This means that each of the two output lines of an isolation transformer is always coupled to the ground at AC frequencies.

Although such capacitive coupling is usually insignificant, it can sometimes be significant at frequencies of

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60 cycles/sec (i.e., 60 Hz). More commonly, capacitive coupling becomes a problem during electrosurgery, which occurs at higher frequencies, typically hundreds to thousands of kilocycles per second. Capacitive coupling, which lets high-frequency currents pass easily, is commonly used by manufacturers in two connections: between the electrosurgical unit and the electrosurgical pencil (i.e., cutting or coagulation tip) and between the electrosurgical unit and the large-area "grounding pad" that attaches to the patient. The formal term for this large-area conducting pad is dispersive electrode. Capacitive coupling gives high-frequency electrosurgery currents a low-impedance path to and from the patient. At the same time, by presenting high-impedance paths at low frequencies, capacitive coupling prevents the electrosurgical pencil and the large-area conducting pad from participating in dangerous low-frequency grounding of the patient.

Capacitive coupling in an electrosurgical unit might have prevented a serious burn caused by the DC of a 9.5-volt battery within an anesthesiologist's damaged nerve stimulator.^[9] An older electrosurgical unit was used in that case, and it was not capacitively coupled to the large-area grounding pad on the patient. The damaged nerve stimulator had a connection to the ground contact, and this permitted completion of a DC circuit (i.e., patient through faulty connection to ground and then ground through large area pad back to patient). The DC persisted throughout the case, even when the stimulator was not pulsing. In laparoscopic and endoscopic surgery, it is important for surgeons to avoid unwanted capacitive coupling between the unipolar cautery tool and neighboring metal conductors, such as trocar cannulas. Stray currents from unwanted capacitive coupling can involve and injure organs such as the bile duct and bowel.^[22] ^[23] Anesthesiologists need to be aware of such potential surgical complications, particularly if they may be erroneously attributed to anesthesia.

Capacitive coupling can be responsible for severe patient burns from conventional pulse oximetry (with nonfiberoptic cable) in patients anesthetized for magnetic resonance imaging (MRI).^[24] ^[25] ^[26] However, the possibility of being burned by a faulty pulse oximeter probe exists independently of capacitive coupling or radiofrequency currents induced by the MRI environment.^[27] ^[28] In such a burn, the patient is external to a malfunctioning electrical circuit that causes the probe (i.e., the unit that contains the photodiode and attaches to the patient) to overheat and burn tissue. Such a catastrophe can be caused by connecting the wrong kind of probe (e.g., one from another type of unit) to an oximeter console.

It has long been known a perfectly safe conventional pulse oximeter (with nonfiberoptic cable) can be transformed into a dangerous, burn-causing device if it is attached to a patient being scanned in an MRI magnet. The problem arises from changing magnetic and electric fields that occur at megahertz frequencies (in radiofrequency coils) and at kilohertz frequencies (in gradient coils) during MRI data acquisitions. Capacitive coupling can connect a patient to wires in a pulse oximeter cable and to metal in a pulse oximeter probe. In such a situation, the patient becomes part of a high-frequency loop circuit, with currents entering and exiting at the points of capacitive coupling, particularly at the site of probe attachment. ^[29] ^[30] ^[31] ^[32] Fortunately, anesthesiologists now know to use only pulse oximeter cables having long, nonconducting fiberoptic connections between the photogenerating and photodetecting parts. Connections of the console to the cable are outside the magnet, distanced from pulsing MR coils, and only nonconducting components are connected to the patient in the magnet. The absence of metal or conducting components to the cable or probe attachment eliminates the possibility of patient burns from induced currents during MRI.

Anesthesiologists monitoring patients with pulse oximetry in MRI magnets should use only units that are MRI compatible—those with a long fiberoptic cable as described previously. Severe burns can occur if for some reason the anesthesiologist decided to take a conventional pulse oximeter from an operating room to the MRI suite and attach it to a patient in an MRI magnet.