Patient Grounding - Then and Now
Andrew Goldstein, BS, CNIM & Brett Netherton, MS, CNIM, FASNM
Current patient monitoring equipment standards require the use of isolated patient inputs including ground to minimize risk of harm due to unintended electrical currents. In the 1970s, increased awareness of electrocution risks led regulatory bodies to create standards phasing out the use of earth ground at the point of patient connection, as it was for previous generations of equipment (UL 544 1972, IEC 601-1 1977, ANSI/AAMI SCL 1978). However, many of the methodological conventions associated with the use of earth ground are still embedded in the performance of neurodiagnostics. As a field, we need to remove these outdated conventions from our practice and understand ground for true benefits and limitations.
Many of the problems encountered in discussing ground originate from the generic use of the term to represent several related but different concepts. We have earth ground, chassis ground, signal ground, isolated ground and technical ground. A discussion of all of the intricacies of the various types of ground is beyond the scope of this note so we will focus on earth ground and signal ground. These two concepts are the most relevant when discussing the issues of electrical safety and signal noise which are generally our main concerns regarding ground.
Earth ground refers to an electrical reference connected to the surface of the earth (see figure 1 below). In modern commercial and residential wiring, the ground pin of an electrical outlet is connected through wiring and/or the structure of the building to a conductor sunk physically into the ground. This is often water supply pipes, although there is some variation as the use of plastic plumbing elements becomes more common. At one time all grounds in electrical instrumentation were tied to an earth ground. The intent was to place various pieces of equipment at the same voltage potential avoiding the dangerous currents that could flow between equipment (through the patient) when differing voltages are encountered. The earth ground also had the capacity to shunt away unwanted electrical signals and reduce noise. In practice however, the earth ground introduced problems. Having everything referenced to the same earth ground, meant that if a break developed in a ground conductor, electrical current would find another path back to ground. This was especially a concern in wet environments as often encountered in operating rooms where there was a high probability of the current finding an easier path to ground through the patient. The noise reduction capacity of the earth ground was also compromised as more devices were attached to the ground conductor. The multiple resulting currents flowing through the ground introduced rather than reduced noise.
To counter these issues, isolation was introduced (see figure 2 below). Isolation is the breaking of the electrical pathways between two parts of a circuit. Through isolation the physical and electrical connection to earth ground is eliminated removing the path for currents to flow to a point of lower potential (the earth) through the patient. Multiple
levels of isolation exist in modern medical equipment resulting in there being no electrical pathway between any patient connection and earth ground. It is beneficial to understand that when the patient is no longer referenced (electrically linked) to earth, any voltages present on the patient no longer seek to drive currents to the lowest impedance pathway back to earth. The opportunity for dangerous currents and ground loops related to earth ground no longer exist with modern equipment. The patient connection labeled as ground on modern neuromonitoring equipment, sometimes referred to as isolated ground is more appropriately referred to as signal ground.
The signal ground does not have the high shunting capability that an earth ground had. Placing it on the patient in a region of high electrical noise will not cause the noise to be shunted or “grounded.” The main purpose of the signal ground to provide a common mode reference for the so-called active and indifferent electrodes that constitute the inputs to an amplifier channel. For this reason, the signal ground should be placed so that it sees the same noise signals as the active and indifferent electrodes to ensure that noise is optimally rejected.
What does this mean in practical terms?
The signal ground has no bearing on electrical safety. Furthermore, connecting any patient lead (including the one labelled ground) to an earth ground will actually create a safety hazard since it will defeat the isolation and reintroduce earth ground as a reference point.
Ground loops must also be thought of differently than in the past. The ground lead of each isolated circuit is a separate entity and having multiple grounds from separate circuits will not cause ground loops. Having multiple grounds from a single circuit however, can cause the noise problems associated with ground loops. Since it is possible to have multiple isolated circuits from the same device it is important to know the circuit configuration in order to place appropriate grounds. For example, some common 32 channel IONM systems consist of two separately isolated 16 channel amplifiers each of which have multiple places to connect the ground. It is important that a ground electrode be placed for each amplifier, and also that multiple grounds not be connected to the same amplifier.
Note: This article was originally published in The ASNM Monitor Newsletter (June, 2014). We are reposting it in our blog to give ASNM members convenient access to this important educational material. Please feel free to leave questions and comments.