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Table of Contents    
Year : 2022  |  Volume : 70  |  Issue : 8  |  Page : 117-122

A New Surface Technique for Phrenic Nerve Conduction Study

Department of Neurology, Sanjay Gandhi Post Graduate Institute of Medical Sciences, Lucknow, Uttar Pradesh, India

Date of Web Publication11-Nov-2022

Correspondence Address:
Sunil Pradhan
Prof. and Head, Department of Neurology, Sanjay Gandhi Post Graduate Institute of Medical Sciences, Lucknow, Uttar Pradesh
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/0028-3886.360904

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 » Abstract 

Objective: To report a new patient friendly and convenient technique for phrenic nerve conduction with alternative sites of stimulation and recording.
Methods: Phrenic nerve conduction was performed in forty volunteers and ten patients of peripheral neuropathy. Active recording electrode was placed in tenth intercostal space 2.5 cm away from para-spinal muscles (mid-scapular line), reference electrode in eighth intercostal space just medial to subcostal margin with ground between stimulating and recording electrode. Stimulation was done at the level of crico-thyroid space near or under the posterior margin of sternocleidomastoid muscle. This new method was compared with existing ones.
Analysis: Data was analysed using SPSS 23 version. Correlation between height, weight, body mass index, age, and chest expansion was done using bi-variate correlation. Mean latency and amplitude of the study method were compared with other methods using MANNOVA test.
Results: Total of forty subjects were studied. Thirty-seven were male subjects. Mean age was 28.03 ± 9.63 years, height 168.0 ± 9.60 cm and chest expansion 3.53 ± 0.64 cm. Right sided phrenic nerve mean latency was 5.99 ± 0.629 ms and amplitude 1.088 ± 0.178 mV. Left sided phrenic nerve conductions showed mean latency of 6.02 ± 1.82 ms, amplitude of 1.092 ± 0.2912 mV. These standard deviations were smaller than what were observed with other methods suggesting increased consistency of our results. There was no correlation between phrenic nerve conduction with age, height, gender or chest expansion.
Conclusion: This study method gave a better as well as consistent morphology, higher amplitude and required lower amount of current strength. It was superior to previously reported methods in consistency of normative data.

Keywords: India, new technique, normative data for phrenic nerve conduction, phrenic nerve, phrenic nerve conduction
Key Message: This new technique is patient friendly and easy to perform. Due to low maximal current requirement, consistently high amplitude and sharp onset point of the CMAP, it is likely to become a part of routine electrical assessment of phrenic nerve in neurogenic respiratory dysfunction.

How to cite this article:
Pradhan S, Anand S. A New Surface Technique for Phrenic Nerve Conduction Study. Neurol India 2022;70, Suppl S2:117-22

How to cite this URL:
Pradhan S, Anand S. A New Surface Technique for Phrenic Nerve Conduction Study. Neurol India [serial online] 2022 [cited 2022 Dec 3];70, Suppl S2:117-22. Available from: https://www.neurologyindia.com/text.asp?2022/70/8/117/360904

The study of the truncal nerves involved in respiration are not as frequently performed as that of peripheral nerves despite their possible utility in the assessment of respiratory dysfunction in any peripheral nerve disorder. The reasons could be many and may include more painful stimulation of deeply situated nerves, inconvenience of marking the recording sites, poor take-off point to mark the latency, positive-negative rather than sharp negative take-off, and inconsistencies in the normative data with which the abnormalities are needed to be compared.[1],[2],[3],[4],[5] To overcome these difficulties, after placing recording electrodes at various locations over the surface anatomy of diaphragm and stimulating the phrenic nerve at various possible sites in the neck, we discovered some better stimulation and recording sites, which addressed several of the adversities mentioned earlier. Accordingly, the technique is standardized in normal individuals and applied to some patients with peripheral neuropathy to observe whether it helps in detecting abnormalities or not.

 » Methods Top

To get the best stimulation point for the phrenic nerve, we stimulated the nerve from the level of angle of mandible on lateral border of sternocleidomastoid muscle down to the level just above the clavicle along the medial border of the muscle. To get the best recording site, we first did ultrasonography of the chest above the subcostal margin over the anterior intercostals spaces. The point where good diaphragm movements were visible [Figure 1] and the point where diaphragm was closely approximated to the chest-wall during expiration with maximum during phrenic nerve stimulation [Figure 2], were selected for placement of recording electrode and then the point for the placement of indifferent electrode was selected by observing the best and consistent compound motor action potential (CMAP) on repeated stimulation.[15],[16]
Figure 1: Ultrasound of the chest-wall showing the position of diaphragm (arrows) with respect to chest-wall during end of expiration (a), middle of inspiration (b), and at the end of inspiration (c)

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Figure 2: Ultrasound of the chest-wall during phrenic nerve stimulation showing the position of diaphragm (arrows) with respect to chest-wall during end of expiration (a) and immediately after the stimulation (b)

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Healthy volunteers, without any evidence of peripheral neuropathy, pulmonary disease, chest deformity, abdominal surgery, or injury were the subjects for this study. Written and informed consent was taken in every subject. Their height, weight, and chest expansion were noted. Phrenic nerve action potential recording was done in supine position. As the recording electrode was needed to be applied on the back, first a lateral position was adopted to place the active recording electrode. Active recording electrode was placed over the tenth intercostal space, 2.5 cm lateral to lateral border of para spinal muscles (that corresponded nearly to mid-scapular line). In subjects with difficulty in ascertaining intercostal space, arms were rested over the head to make intercostal spaces prominent. Then, the patient was made to lie in a comfortable supine position for the rest of the procedure. Reference recording electrode was placed just medial to the subcostal margin at a point close to the eighth intercostal space. The ground electrode was placed in between stimulating and recording sites [Figure 3]a. Phrenic nerve conductions were also performed using two other standardized techniques, i.e., Newsom-Davis's[1] and Markand's.[3] For Markand's technique, active electrode was placed at xiphoid and reference electrode at a point 16 cm away along the subcostal margin. In Newsom-Davis's method, electrodes were placed in eighth intercostal space 3.5–5 cm away or in eighth and ninth intercostal spaces with anterior recording electrode in anterior axillary line.
Figure 3: (a) Active electrode (yellow) placed over tenth intercostal space, 2.5 cm lateral to lateral border of para spinal muscles (mid scapular line), reference electrode (red) placed just medial to the subcostal margin at a point close to the eighth intercostal space and ground electrode (green) placed in between stimulating and recording site. (b) Stimulation is done near or just under the lateral border of sternocleidomastoid muscle at the level of cricothyroid space

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The method of study was explained in detail before performing the conduction. This ensured proper relaxation and quite breathing throughout the conduction study. Subjects were instructed to lie calm and avoid change of position during the study.

Stimulation was performed near or just under the posterior margin of sternocleidomastoid muscle at the level of crico-thyroid space [Figure 3]b. Stimulation just above the clavicle (lateral to or between the two heads of sternocleidomastoid muscle) was easier but avoided as it resulted in co-stimulation of brachial plexus. To ensure that the diaphragm is fully relaxed, phrenic nerve was stimulated at the end of the expiration. Phrenic nerve stimulation was confirmed by hiccup sensation felt by the subjects. As this hiccup denotes sudden jerky movement of the diaphragm, it confirmed proper stimulation of the phrenic nerve and it did not interfere in recording as the overlying intercostal muscles were relaxed during the phase of end-expiration. Near diaphragm surface recording was made from both right and left sides at the stipulated points.

Entire experiment was performed on Synergy electromyography machine product version copyright 2013 Natus. Supramaximal stimulation was done using square wave pulse of 0.2 ms duration. Gain was kept at 500 μV/div and sweep time at 10 ms/div. The latency of the compound muscle action potential (CMAP) was marked at the point of sharp take-off from the baseline and amplitude was measured from base to peak.

The study was approved by the ethical committee of the Institute (IEC code: 2017-227-IP-EXP).


Data analysis was done by SPSS 23 version software. Mean, median, and standard deviation were calculated. Correlation between height, weight, body mass index, age, and chest expansion were done using bivariate correlation. P value <0.05 was taken as significant. Mean latency and amplitude of the study method were compared with other methods using MANNOVA test.

 » Results Top

In total, 40 healthy subjects were studied. Out of these, 37 were male subjects. Mean age was 28.03 ± 9.63 years, height was 168 ± 9.60 cm, and chest expansion was 3.53 ± 0.64 cm.

The best site of phrenic nerve stimulation was just lateral to the cricothyroid space, where the stimulus was at enough distance from brachial plexus not to cause any stimulation of the latter. The best recording site discovered with the method described here was over the tenth intercostal space, 2.5 cm lateral to lateral border of para spinal muscles.

CMAP on phrenic nerve stimulation was recorded in all the healthy subjects without any practical difficulty. The action potentials had a negative-positive wave form in all cases with a sharp take-off point that was helpful in accurately measuring the latency of the potential. None of the subjects had bifid peak or multiphasic waveform. The CMAP morphology recorded after stimulating right-sided phrenic nerve from five healthy subjects have been shown in [Figure 4]. Right-sided phrenic nerve conduction showed mean latency of 5.99 ± 0.629 ms and amplitude of 1088 ± 178.1 μV. Left-sided phrenic nerve conduction showed mean latency of 6.02 ± 1.82 ms and amplitude of 1092 ± 291.2 μV. Mean duration (measured from baseline to return of negative peak to the baseline) was 20 ± 1.8 ms.
Figure 4: Compound muscle action potentials (CMAPs) recorded from three methods. (a) Study method; (b) Newsom-Davis', and (c) Markand's method, in five representative healthy subjects as recorded after right phrenic nerve stimulation. Gain is 500 μ/div in (a) because of higher amplitude and 200 μ/div in (b) and (c)

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There was no significant difference between right and left sided conductions in term of amplitude and latency. There was no correlation between weight, height, age, sex, and chest expansion with either amplitude or latency [Table 1] and [Table 2].
Table 1: Correlation of latency with various parameters

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Table 2: Correlation of amplitude with various parameters

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The mean values of latency and amplitude of phrenic CMAP by the three methods were compared with each other using MANNOVA [Table 3].
Table 3: Mean latency and amplitude obtained in three methods of phrenic nerve conduction

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These standard deviations were smaller than what were observed with other methods suggesting increased consistency of our results. There was a significant difference in term of amplitude and latency (P value < 0.001).

Phrenic nerve conduction was also performed in ten patients of peripheral polyneuropathy like Guillain-Barre syndrome (GBS) and chronic inflammatory demyelinating polyneuropathy (CIDP). Out of nine patients of GBS, four were acute immune mediated demyelinating polyneuropathy (AIDP) and five were acute motor axonal neuropathy (AMAN). The clinical profile and pattern of nerve involvement is described in detail [Table 4]. These patients had characteristic changes in phrenic nerve conduction correlating with the pattern of peripheral nerves.
Table 4: Clinical characteristics, nerve conduction study (NCV) of the peripheral nerves and phrenic conduction in peripheral neuropathy patients

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 » Discussion Top

The biggest concern in acute neuromuscular disorder is the development of respiratory distress due to disease severity related involvement of nerves, neuromuscular junction, or the muscles related to breathing.[17],[18] It makes the concerned treating physician much more comfortable if he/she gets early information about impending respiratory dysfunction. Though the spirometry can tell about severity of respiratory dysfunction at that moment, it fails to reveal information about the nature, site, and severity of lesion, which are important in predicting impending respiratory dysfunction.[19] Thus, the study of the nerves supplying respiratory muscles shall have an important role in the management of these patients. Intercostal and phrenic nerves have been studied in detail due to their utility in the electrical assessment of the respiratory dysfunction. However, the study of both these nerves faces the basic problem of access to stimulation and recording sites. On the stimulating site, there is problem of deeper location of the nerve for surface stimulation requiring higher than usual current with a technical possibilities of wider spread. On the recording site, there is a problem of deeper location of the muscle under evaluation such as diaphragm with no clarity about the end plate zone, which is the preferred site for recording.[20],[21],[22]

In this study, we tried to overcome these difficulties by using inching technique to find out the best stimulation and recording sites for the study of phrenic nerve at the initial planning stage. Upon realizing the advantage of our finally proposed sites of stimulation and recording over the existing techniques, we planned this systematic study to compare our technique with others in terms of sharp onset-point to mark the CMAP minimal latency and the amplitude. As is shown in [Table 3], minimal latency of M-wave with our method was shorter by >1 ms compared with other methods described by Newsom Davis and Markand. This is expected because we stimulated phrenic nerve distal to the site of stimulation chosen by previous authors and placed our recording electrodes much posteriorly compared to the other methods and it was much nearer to the diaphragm supplied by shorter posterolateral branch rather than anterolateral branch. The relative sharp take-off point may also have contributed to reliable measurement of the latency. We observed following advantages of this study method for phrenic nerve conduction:

  1. Good CMAP morphology

  2. The morphology of the CMAP was predominantly negative with a sharp take-off as is true for classical peripheral nerve M wave rather than positive–negative or with poorly defined take-off point as is sometimes seen with other known methods of phrenic nerve conduction. In general, our findings suggest that this new technique could be superior to others in several ways. The lesser current is required to achieve the maximum amplitude of the CMAP. It is noteworthy that in this technique we did not fix the distance between active and the reference electrode as is customary in standard nerve conduction techniques. This is because of our observation that the body morphology-related surface points were giving more consistent results as far as sharp negative take-off and the magnitude of the amplitude of the CMAP was concerned. However, this deviation from the standard fixed interelectrode distance recording is not likely to produce any significant adverse effect because using the surface anatomical points of recording yielded only in mild interindividual difference. Moreover, using a fixed interelectrode distance creates a bigger problem in using this technique in individuals with very small (including children) or very large thoracic girth. We stimulated phrenic nerve at the level of cricothyroid cartilage, which is much above the clavicle and therefore far away from brachial plexus. That is why we did not encounter any visible movement of the arm or shoulder during stimulation suggesting no significant stimulation of brachial plexus.

  3. Reliability of the study method

  4. For any test to be reliable, one needs the CMAP morphology obtained to be reproducible when observed at several occasions in the same subject. Using nearly the same sites of stimulation and recording, this method gave a consistent result in term of amplitude and latency.

  5. Lower amount of current needed to get required CMAP

  6. The stimulation of phrenic nerve involves neck, which is a sensitive area and contraction of diaphragm is confirmed by hiccup; therefore, the amount of current requirement becomes important in phrenic nerve conduction. In our study, the current strength required for getting maximum CMAP was lower compared to the earlier studies. CMAP was obtainable even at 15 mA and maximum current required was less than 40 mA. Previous study done by Resman et al. also showed lower amount of current requirement however the site of stimulation was different.[5] The method of stimulating phrenic nerve in between two heads of sternocleidomastoid was avoided as the nerve may be deeper resulting in a painful technique. The lower requirement of current in our study can be explained by understanding the course of phrenic nerve in neck and stimulation site used. Phrenic nerve is a mixed nerve which arises from anterior cervical rami of C2 to C4 segments (sometimes C3 to C5 segments). It then passes over anterior border of anterior scalene muscle, which is deep to sternocleidomastoid muscle. Earlier studies have been done with stimulation in the upper part of the neck where the phrenic nerve is deep to sternocleidomastoid muscle and therefore it required a little insinuation of stimulating electrode under the muscle. However, we observed that in the middle of the neck at the level of cricothyroid space, it was possible to stimulate phrenic nerve just at the posterior margin of sternocleidomastoid muscle without much effort to find the nerve. This was probably more specific site because of direct stimulation and thus lower amount of current gave the required CMAP.

  7. Higher amplitude of CMAP

  8. Diaphragm consists of two parts which are central tendon and peripheral muscles. Peripheral muscles originate from sternum, ribs and spine and converge over central tendon part. The sternal part attaches to posterior aspect of xiphoid process. Costal part attaches to inferior six ribs and their costal cartilages, and the spinal part attaches to lumbar vertebra.[6] Phrenic nerve divides into anterolateral and posterolateral trunks; anterolateral trunk lies in eighth intercostal space whereas posterolateral trunk supplying lumbar part of diaphragm lies over the lumber vertebrae.[7],[8],[9] In our study, we used tenth intercostal space for placing active electrode, and on the vertical plane, it was 2.5 cm away from paraspinal muscles, which corresponded to mid scapular line. Hemmerling et al. studied anterior and posterior parts of diaphragm separately. For anterior part, they placed electrodes in seventh or eighth intercostal space at anterior or mid-axillary line. The posterior part was studied using electrode placement lateral to T12 or L1–L2 vertebral level.[10] In a previous study using ultrasound, the best visualisation of diaphragm was in the same region as was our recording site.[11] Active recording site being more superficial, lying directly over posterolateral trunk of phrenic nerve as well as absence of overlying para-spinal muscle bulk gave a better amplitude. Other possible explanation is that this may also be a site for end plate zone of diaphragm muscle, thus giving the better result. The latency recorded in our study was shorter in comparison to previous studies because the recording electrodes in our study were placed more posteriorly and these may be at a shorter distance with respect to electrical pathway of the phrenic nerve. It is worth noting that we recorded the CMAP at the end of expiration and diaphragm is closer to the surface at the end of expiration and this may have resulted in a better amplitude.

  9. Abnormalities in peripheral neuropathy

Phrenic conductions were also recorded in patients of Guillain-Barre syndrome and a patient of CIDP. They showed prolonged latency and decreased amplitude. This pattern was consistent with peripheral pattern of involvement as shown in previous studies.[12] As normal respiration involves both phrenic and intercostal nerves, at times there may be discrepancy in the breathing pattern and severity of the involvement of the phrenic nerve. This is because the intercostal nerves responsible for respiration may have been unaffected or recovered before phrenic nerve.[13],[14] Detailed assessment of these nerves is thus important for the prognosis.

 » Conclusion Top

Phrenic nerve stimulation is best recorded with active electrode at tenth intercostal space 2.5 cm away from para-spinal muscles (mid-scapular line) and reference electrode just medial to eighth subcostal space. This site gives consistent and better CMAP morphology in phrenic nerve conduction study.

Declaration of patient consent

The authors certify that they have obtained all appropriate patient consent forms. In the form the patient(s) has/have given his/her/their consent for his/her/their images and other clinical information to be reported in the journal. The patients understand that their names and initials will not be published and due efforts will be made to conceal their identity, but anonymity cannot be guaranteed.


We would like to thank Mr. Anil Kumar for the illustrative diagrams and Ms. Pranjul Asthana for the technical help.

Financial support and sponsorship


Conflicts of interest

There are no conflicts of interest.

 » References Top

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Markand ON, Kincaid JC, Pourmand RA, Moorthy SS, King RD, Mahomed Y, et al. Electrophysiologic evaluation of diaphragm by transcutaneous phrenic nerve stimulation. Neurology 1984;34:604-14.  Back to cited text no. 3
Chen R, Collins S, Remtulla H, Parkes A, Bolton CF. Phrenic nerve conduction study in normal subjects. Muscle Nerve 1995;18:330-5.  Back to cited text no. 4
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Dunst CM, DeMeester SR. Surgical anatomy of esophageal hiatus. In: Swanstrom LL and Dunst CM (eds.) Antireflux Surgery Springer: New York; 2015. p. 3-8.  Back to cited text no. 9
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Sarwal A, Walker FO, Cartwright MS. Neuromuscular ultrasound for evaluation of the diaphragm. Muscle Nerve 2013;47:319-29.  Back to cited text no. 11
Gourie-Devi M, Ganapathy GR. Phrenic nerve conduction time in Guillain-Barré syndrome. J Neurol Neurosurg Psychiatry 1985;48:245-9.  Back to cited text no. 12
Taylor A. The contribution of the intercostal muscles to the effort of respiration in man. J Physiol 1960;151:390-402.  Back to cited text no. 13
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Zibly Z, Averbuch S, Deogaonker M. Emerging Technologies and Indications of Neuromodulation and Increasing Role of Non Invasive Neuromodulation. Neurol India 2020;68(Supplement):S316-S321.  Back to cited text no. 19
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  [Figure 1], [Figure 2], [Figure 3], [Figure 4]

  [Table 1], [Table 2], [Table 3], [Table 4]


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