OVERVIEW
 
A previously healthy 33-year-old woman underwent left upper lobe segmentectomy for resection of a malignant mediastinal mesenchymal tumor. The surgery was complicated by an injury to the subclavian artery, hemostasis being achieved by opening the pericardium. The postoperative period was uneventful, and the patient was discharged from the ICU. However, she was readmitted to the ICU on postoperative day 5 because of respiratory failure, which was managed with continuous noninvasive ventilation (NIV). Further evaluation ruled out pulmonary congestion, infection, and thromboembolism. A chest X-ray showed an elevated left diaphragm, raising the suspicion of diaphragm dysfunction (DD). This suspicion was corroborated by extensive complementary evaluations,(1,2) including the following:
 
1.            Pulmonary function testing (PFT) disclosed a restrictive pattern (a substantial drop in FVC and FEV1). Unfortunately, PFT was not performed in the supine position.
2.            Reduced inspiratory muscle strength on volitional tests (reduced MIP and sniff nasal inspiratory pressure) and nonvolitional tests (significantly reduced left twitch transdiaphragmatic pressure [TwPdi] but only slightly reduced right TwPdi), together with a paradoxical drop in gastric pressure during inspiration.
3.            Increased recruitment of extradiaphragmatic inspiratory muscles (the scalene and sternocleidomastoid muscles), as assessed by surface electromyography.
4.            Thoracoabdominal asynchrony (a phase angle of 180° indicating a paradoxical pattern), as assessed by respiratory inductance plethysmography.
 
Diaphragm ultrasound (DUS) confirmed the suspicion of DD. DUS showed markedly reduced left diaphragm mobility (during quiet and deep breathing), including paradoxical motion during sniffing. Yet, the left diaphragm was thin (reduced thickness), with reduced inspiratory thickening. The right diaphragm showed slightly reduced deep breathing motion, although thickness and thickening remained unaltered.(3)
 
CASE SUMMARY
 
Our patient had DD caused by bilateral traumatic injury to the phrenic nerve during open-heart surgery, DD being more severe on the left side. Dyspnea was relieved by NIV and can be explained by bilateral DD, given that unilateral DD can be asymptomatic. The fact that the patient was progressively weaned off of NIV suggested recovery of diaphragm function.
 
Phrenic nerve dysfunction has been described in open-heart surgery, being caused by hypothermia (topical cardiac cooling), mechanical stretching of the phrenic nerve by the sternal retractor, or a combination of the two. Phrenic nerve palsy is an uncommon complication after cardiac surgery, usually affecting only the left phrenic nerve and resolving completely in almost all cases.(4)
 
CLINICAL MESSAGES
 
DD remains underdiagnosed because of its nonspecific presentation and the difficulty in diagnosing it. Once DD is suspected, ancillary tests can be ordered to confirm it or rule it out.(2)
 
Unexplained dyspnea (particularly orthopnea), an elevated diaphragm on imaging, a restrictive pattern on PFT, and reduced MIP may raise the suspicion of DD. Diagnostic tests for DD include surface electromyography, respiratory inductance plethysmography, and measurement of TwPdi; however, these are largely unavailable, with measurement of TwPdi having the additional disadvantage of being an invasive test.(2) DUS, on the other hand, has many advantages, including its availability, its repeatability, and its being a noninvasive test.(5)
 
The following DUS findings can help confirm a diagnosis of DD, suggesting diaphragmatic paralysis(5):
 
•              absent mobility during quiet and deep breathing, as well as absent mobility or paradoxical motion during sniffing
 
•              reduced diaphragm thickness (a thin, atrophic diaphragm), as well as absent diaphragm inspiratory thickening
 
 
•              Normal diaphragm thickness in the presence of reduced diaphragm thickening suggests acute or subacute diaphragmatic paralysis.
 
The following DUS findings are diagnostic of diaphragm weakness:
 
•              reduced diaphragm mobility and thickness, as well as reduced diaphragm inspiratory thickening (lower than the lower limit of normal in healthy individuals, sex and body position being taken into account)

 
AUTHOR CONTRIBUTIONS
 
The authors contributed equally to this work.
 
CONFLICTS OF INTEREST
 
None declared.
 
REFERENCES
 
1.            Caruso P, Albuquerque AL, Santana PV, Cardenas LZ, Ferreira JG, Prina E, et al. Diagnostic methods to assess inspiratory and expiratory muscle strength. J Bras Pneumol. 2015;41(2):110-123. https://doi.org/10.1590/S1806-37132015000004474
2.            Laveneziana P, Albuquerque A, Aliverti A, Babb T, Barreiro E, Dres M, et al. ERS statement on respiratory muscle testing at rest and during exercise. Eur Resp J. 2019;53(6):1801214. https://doi.org/10.1183/13993003.01214-2018
3.            Caleffi-Pereira M, Pletsch-Assunção R, Cardenas LZ, Santana PV, Ferreira JG, Iamonti VC, et al. Unilateral diaphragm paralysis: a dysfunction restricted not just to one hemidiaphragm. BMC Pulm Med. 2018;18(1):126. https://doi.org/10.1186/s12890-018-0698-1
4.            Aguirre VJ, Sinha P, Zimmet A, Lee GA, Kwa L, Rosenfeldt F. Phrenic nerve injury during cardiac surgery: mechanisms, management and prevention. Heart Lung Circ. 2013;22(11):895-902. https://doi.org/10.1016/j.hlc.2013.06.010
5.            Santana PV, Cardenas LZ, Albuquerque ALP, Carvalho CRR, Caruso P. Diaphragmatic ultrasound: a review of its methodological aspects and clinical uses. J Bras Pneumol. 2020;46(6):e20200064. https://doi.org/10.36416/1806-3756/e20200064