Water flows downhill. Notice how the streams on the map cut perpendicularly through the lines of constant height. It is possible to map voltages in the same way, as shown in figure b. The electric field is strongest where the constant-voltage curves are closest together, and the electric field vectors always point perpendicular to the constant-voltage curves. Near the charge, the curves are so closely spaced that they blend together on this drawing due to the finite width with which they were drawn.
Some electric fields are shown as arrows. Imagine that figure a represents voltage rather than height. Figure c shows some examples of ways to visualize field and voltage patterns. East Timor Timor-Leste. El Salvador. Equatorial Guinea. Faeroe Islands.
Falkland Islands. French Guiana. Gabon Gabonese Republic. Articles and Info Why Use Diesel? New vs. Recently, left atrial bipolar endocardial voltage mapping Fig. Contemporary electro-anatomic mapping platforms used during AF ablation allow hundreds to thousands of voltage points to be mapped onto a geometric model of the atrial endocardium. It is proposed that low atrial bipolar voltage amplitude is a surrogate marker for the presence of native atrial fibrosis and that atrial fibrosis plays a key role in maintaining AF.
As such, several clinical studies have described approaches for isolating areas of left atrial low voltage, showing promise as methods to reduce arrhythmia recurrence after AF ablation. However, the methodology for defining low voltage areas has not been standardised and a clear voltage threshold for abnormality has never been histologically validated.
These observations, and the potential for widespread clinical use of these new treatment strategies, necessitate a thorough understanding of the techniques, benefits and challenges of using voltage mapping to define and target the AF substrate.
Left atrial voltage mapping. Panel a includes points. Panel b includes points. In this review article, we first summarise the contemporary studies of voltage-guided ablation. Recognising the variable outcomes from these clinical studies, we then examine the technical aspects of recording the extracellular electrical field potential voltage during clinical procedures, and the relationship between these signals and recognised components of the AF substrate.
Finally, we discuss recent technical developments that may overcome some of the limitations of bipolar voltage mapping for defining the AF substrate. Rolf et al. In their non-randomised study, patients underwent pulmonary vein isolation plus additional isolation of low voltage areas.
Compared to an historical control group of 26 patients with low voltage areas undergoing pulmonary vein isolation alone, patients receiving voltage-guided ablation had significantly greater arrhythmia-free survival. Subsequently, seven further observational studies have compared outcomes with low voltage area-based ablation with standard ablation [ 2 , 3 , 4 , 5 , 6 , 7 ].
Broadly, these studies fall into two categories: those comparing standard ablation to low voltage-guided ablation in the subset of patients with atrial low voltage, and those comparing ablation strategies standard ablation vs.
Although these studies demonstrate a variable impact of voltage-guided ablation on arrhythmia recurrence, a recent meta-analysis has shown an overall improvement in outcomes with low voltage area-guided ablation [ 8 ].
However, there was significant heterogeneity in mapping strategies, patient selection and low voltage area prevalence between these studies and therefore direct comparisons are difficult to make. In particular, rhythm during mapping, electrode size and mapping resolution all varied between studies.
For example, Yagishita et al. Similarly, Jadidi et al. Schreiber et al. More recently, two randomised controlled clinical trials of voltage-informed intervention have been performed, with contrasting outcomes [ 10 , 11 ]. Kircher et al. However, a low success rate for persistent AF patients in the control group may have contributed in part to the difference between these groups. In contrast, Yang et al. However, procedure times were significantly shorter with significantly less ablation and shorter fluoroscopy times using the low voltage area-based approach.
Approaches to low voltage area ablation. Strategies described by Kircher et al. Clinical outcomes of randomised studies comparing voltage-guided ablation to standard ablation. The figures were adapted with permission.
Given the conflicting results of the clinical trials, two important questions arise. To answer these questions, it is necessary to first examine the technical aspects of bipolar voltage mapping. The local extracellular potential near the surface is changed in response to an electrical activation wave propagating through the cardiac tissue beneath it. Such a wave is commonly thought of as a pair of propagating dipoles, one representing the depolarising activation wave-front and the other representing the repolarising wave-back Fig.
Extracellular field created by an activation wave-front. At the exact region of the wave-front, axial current flows from the activated tissue to the resting tissue, creating a system representative of a propagating dipole. Transmembrane current then flows out of the membrane ahead of the wave, to return at the wave-back via newly activated sodium channels, thus forming a current loop.
Consequently, the extracellular field is positive ahead of the wave-front and negative behind it. This potential gradient drives a flow of intracellular axial current through the gap junctions activating the neighbouring downstream cell by an outward positive capacitive current which begins to increase V m i.
The intracellular space remains positive with respect to the extracellular spacing during the action potential plateau with a V m during the plateau of approximately 0—20 mV. Thus, this spatial region of transition from negative to positive at the wave-front can be represented by a positive dipole, pointing in the same direction as wave propagation.
It can be shown mathematically that the extracellular potential recorded at a particular location due to a propagating dipole depends on. A positive dipole, propagating in the direction of the wave, will generate a positive signal recorded by an electrode at a location as the dipole approaches the recording site, corresponding to the positive potential ahead of the wave-front.
The recorded signal will then switch to a negative signal as the dipole propagates away from the recording site, corresponding to the negative potential behind the wave-front. When the dipole passes exactly in line with the recording site, the signal is zero.
These basic biophysical phenomena, as recorded at the cellular level, describe the shape of unipolar electrograms recorded from the surface of the atria by contemporary mapping catheters. Bipolar signals are taken as the difference between two neighbouring unipolar signals, either with the use of a differential amplifier or via post-processing of unipolar signals.
Extra-cardiac signals, i. However, as the wave-front will be at a different distance from the two unipolar recording sites, there will be a temporal offset between the two unipolar signals, depending on exactly when the wave-front passes beneath the recording site.
This difference is reflected in the bipolar electrogram morphology. Voltage signals recorded from individual electrodes are converted by contemporary electro-anatomic systems into colour-coded voltage maps, providing a static representation of time-dependent electrical activation of the atrium. As such, a recording window must be specified for analysis.
In the case of regular atrial rhythms, this window of interest is set around a fixed temporal reference, usually set to include a single cycle of activation. In the case of irregular rhythms i.
AF , the window of interest is set as a duration e. Voltage amplitude is then defined as the maximal peak-to-peak voltage within the window of interest. It is important to recognise the raw data constituting a voltage map consists of three-dimensional Cartesian co-ordinates of the electrode positions at the time of recording together with a single number for each position representing the voltage.
These data are transformed, typically in real time, into a voltage map by two processes: projecting the recording co-ordinates onto the atrial shell and interpolating the voltage data across the surface of the shell. To the best of our knowledge, the proprietary algorithms used for projection and interpolation are not publicly available, though the operator is generally able to define the degree of interpolation between voltage points.
To define low voltage areas, the final step requires the user to set a threshold defining low voltage, typically less than 0. Based on the above principles, several non-substrate factors Table 2 can theoretically influence electrogram voltage, some of which have been demonstrated during in vivo mapping. The relationship between the orientation of the recording bipole and the wave-front dipole will influence the arrival time of the activating wave-front at each electrode.
Therefore, hypothetically, orienting the recording bipole exactly parallel to the wave-front creates identical electrograms on each unipole electrode and the resulting bipolar voltage is zero. In contrast to predictions from simulated data, these effects are highly variable indicating that voltage change could not be used, for example, to infer activation direction [ 16 ]. For a fixed conduction velocity, the distance separating the bipole electrodes determines the temporal offset between the wave-front arrival time at each electrode.
The exact temporal offset thus determines the amplitude and morphology of the bipolar signal. In computer simulation studies, increasing interelectrode distance leads to increasing voltage which plateaus in healthy, but not diseased, tissue with spacing greater than 4 mm [ 17 ].
Further, the contribution of farfield signal may be greater when electrodes are more widely spaced [ 18 ]. Electrodes covering a larger surface area have been shown to record larger amplitude signals [ 19 ], while in others smaller electrodes on mapping catheters generated statistically significant increases in mean amplitude [ 20 ]. The characteristics of the underlying atrial tissue are likely to modulate these effects.
For example, in one study, the presence of fibrosis, larger electrodes may summate voltages from both healthy and fibrotic areas and therefore record lower, not higher, amplitude signals [ 21 ]. Considering the above effects, the specifications of individual catheters are clearly important for interpreting bipolar voltage signals Table 3 especially given that it appears size and spacing of electrodes have different contributions to the overall voltage [ 22 ].
The wide array of electrode sizes and spacings used in clinical studies and the lack of direct comparisons between mapping catheters prevents a sound appraisal of the effect electrode size and spacing has on the identification of low voltage areas in clinical studies at present.
However, there appear to be different effects depending on whether it is healthy or fibrotic tissue being measured. Although adequate and consistent contact with myocardium is required to record reliable voltage signals, the effect of ever-increasing contact force is less certain. For example, one study assessing voltages recorded with a contact force sensing catheter found a weak correlation between force and voltage but only at very low contact forces [ 23 ]. Once contact force was increased above 5 g 0.
Bandpass filtering may change the amplitude of any given electrogram, and thus the specific filter settings used may consequently alter the peak-to-peak amplitude of the recorded bipolar signal. During clinical mapping, bipolar electrograms are typically bandpass filtered with a high pass of 1—30 Hz and a low pass of — Hz with a notch filter at 50—60 Hz.
To our knowledge, the direct relationship between filter settings and voltage amplitude for contact mapping has not been quantified. However, using non-contact mapping, Lin et al. We suggest that a consensus on the ideal voltage mapping strategy electrode size, spacing, point density and filter settings could contribute significantly to minimising variation between studies. Given these limitations, the next section will discuss the available evidence demonstrating the extent to which voltage mapping, in its present form, identifies a substrate for AF.
A major challenge in evaluating the diagnostic performance of voltage mapping for identifying the AF substrate is the lack of a clear consensus on the form of that substrate. Atrial fibrosis has been identified histologically in patients with AF [ 29 ] and patients with risk factors for AF [ 30 , 31 ].
However, histological validation between low voltage and native atrial fibrosis is currently lacking. Cardiac magnetic resonance remains the only available technique for non-invasive assessment of atrial fibrosis. Late gadolinium enhancement has been correlated with atrial fibrosis by histological assessment in a small number of patients [ 32 ] and several studies have compared bipolar voltage with late gadolinium enhancement.
Spragg et al. Another study of 21 patients with paroxysmal AF correlated increased signal intensity with progressively lower bipolar voltage [ 34 ]. Not all studies demonstrated such a clear relationship. For example, in a study of 18 patients with persistent AF, Jadidi et al.
However, more recently, Khurram et al. Increasing atrial size is associated with lower mean atrial voltages, suggesting a link between atrial wall stress and morphological changes [ 37 ]. There is evidence from invasive and echocardiographic measurements that atrial pressure inversely correlates with mean atrial voltage [ 38 ] and in a study of 20 patients, Hunter et al.
Additionally, acute atrial dilatation has also been shown to increase the prevalence of low voltage zones, especially in the posterior wall [ 40 ], suggesting an alternative mechanism than the presence of fibrosis.
Therefore, low conduction velocity is likely to coexist with low voltage areas. Indeed, total activation time is greater in patients with persistent AF compared to those with paroxysmal AF and correlates inversely with mean left atrial voltage [ 46 ]. However, in one study of AF patients with a minimum voltage of greater than 0. A number of studies have hypothesised that low voltage areas may identify trigger regions for AF by comparing voltage amplitude to high dominant frequency, complex fractionated electrograms and rotors.
Patients with AF triggers from the pulmonary veins have lower voltage in the pulmonary vein antra and similarly patients with AF triggers in the superior vena cava show reduced voltage in the right atrium [ 49 ]. However, voltage is not significantly lower at specific areas with increased dominant frequency [ 50 ], and it has been demonstrated that the majority of high dominant frequency sites in AF do not correlate with low voltage areas recorded in sinus rhythm [ 51 ].
Conversely, a study using basket catheters found that voltages were significantly lower at high dominant frequency sites compared to non-high dominant frequency sites, but this only held for voltages measured during AF and not during sinus rhythm [ 52 ].
Complex fractionated electrogram sites in AF have previously been reported to represent critical sites for AF initiation and maintenance, but their relationship to low voltage areas in sinus rhythm is complex.
A small study of persistent AF patients showed only a small overlap between complex fractionated electrogram sites and low voltage areas [ 14 ].
Further, when recording in AF, sites of complex fractionated electrogram sites are associated with normal bipolar voltage in sinus rhythm [ 51 ]. Indeed, voltages may be higher at complex fractionated electrogram sites than non-fractionated when recorded in sinus rhythm [ 53 ]. Narayan et al. Given these conflicting observations and varied possible mechanisms for the genesis of complex fractionated electrograms, it is challenging to identify a direct relationship between low voltage areas and electrogram fractionation.
Rotors, representing stable but meandering spiral waves, can anchor to areas of anatomic discontinuity such as fibrosis [ 55 ] and therefore voltage mapping may reveal sites important to maintaining atrial rotational activity. This indicates the MAP sensor is responding to changes in vacuum.
Digital MAP sensors are best tested with an oscilloscope. However, they can also be tested with a tachometer, which is a type of frequency counter. The MAP voltage readings should range between 0. This vehicle is high at times, indicating pressure pulses in the intake. Erratic RPM and MAP sensor readings normally indicate an internal misfire condition due to improper sealing of the cylinder. Vacuum at idle is always high and typically ranges from 16 to 20 inches Hg in most vehicles.
If the ratio is incorrect, ignition inside the engine will occur at an improper time in the combustion cycle. This leads to excessive fuel consumption, poor fuel economy, and possibly detonation.
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