Diagnostic value of visual stethoscopes for detecting bronchospasm in a patient under general anesthesia - A case report -

Abstract

Background

Bronchospasm is a rare but potentially life-threatening complication during anesthesia that requires prompt recognition and management. Traditional auscultation plays a key role but is limited in objective interpretation and continuous monitoring.

Case

We report a case of intraoperative bronchospasm during laparoscopic surgery, detected early through real-time acoustic visualization using a digitalized esophageal stethoscope. The visualization of lung sounds facilitated the rapid identification of expiratory wheezing and abnormal spectrogram patterns characteristic of bronchospasm. Immediate intervention with a bronchodilator resolved the condition without further complications.

Conclusions

This case demonstrates the utility of visual stethoscopes in enhancing perioperative respiratory management. Real-time visualization and quantification of lung sounds offer anesthesiologists a valuable tool for early diagnosis and collaborative decision-making during critical respiratory events.

Bronchospasm is a rare but potentially life-threatening complication that can occur during any stage of general anesthesia. Various causes include allergic or anaphylactic reactions, drugs such as antibiotics and neuromuscular blockers, and bronchial hyperreactivity [1]. Prompt recognition and immediate management are crucial, with auscultation playing a fundamental role in early detection, which can be further enhanced through visualization techniques [2]. A spectrogram is a visual representation of lung sounds that illustrates changes in power across different frequencies over time, capturing dynamic variations during various phases of respiration. In this case report, the patient experienced bronchospasm during laparoscopic liver segmentectomy, with lung sounds monitored using a digitalized esophageal stethoscope. The recorded acoustic signal and spectrogram provided an immediate and distinct representation of wheezing, highlighting the potential diagnostic values of visual stethoscopes [3]. This method can identify specific frequency characteristics associated with different respiratory conditions, such as asthma and chronic obstructive pulmonary disease (COPD), by detecting sound generation or transmission changes resulting from structural changes in the bronchi and surrounding lung tissue [4,5].

CASE REPORT

A 45-year-old male patient (89.3 kg, 171.3 cm) underwent a liver lateral segmentectomy for hepatocellular carcinoma. He had a history of allergic rhinitis and paranasal sinusitis with postnasal drip. He had undergone surgery for chronic rhinosinusitis with bilateral nasal polyps 18 months earlier. He had a 20-pack-year history of smoking. Preoperatively, the patient was advised to cease smoking; however, neither steroid pretreatment nor bronchodilator administration was performed. Preoperative laboratory findings and chest X-ray findings were within normal limits. Standard preoperative preparation was provided, including nil per os after midnight and respiratory exercises.

Upon arrival in the operating room, his peripheral oxygen saturation (SpO₂) was 99%, systolic/diastolic blood pressure was 135/80 mmHg, and heart rate was 65 bpm. Physical examination revealed clear lung sounds and no other abnormality. Anesthesia was induced with 180 mg of propofol and 100 mg of rocuronium and maintained with desflurane (4−5 vol%) and remifentanil via target-controlled infusion. Continuous arterial blood pressure was monitored via a 20G radial artery catheter. The ventilator was set to pressure support ventilation (PSV) mode with an inspiratory pressure of 20 cmH₂O, positive end-expiratory pressure of 3 cmH₂O, and a respiratory rate of 12 breaths per min. Arterial blood gas analysis demonstrated no abnormalities, with a fraction of inspired oxygen (FiO₂) of 50%, end-tidal carbon dioxide (EtCO₂) of 34 mmHg, and SpO₂ of 100%. Following induction, the patient was positioned in the classical lithotomy position with hip extension for laparoscopic surgery. Carbon dioxide (CO₂) insufflation at 15 mmHg was utilized for pneumoperitoneum, resulting in an approximately 2 mmHg increase in peak inspiratory pressure without other hemodynamic or respiratory abnormalities. As part of a previous research protocol (IRB No. 2022-0987), a high-fidelity microphone sensor system was connected to the tip of the esophageal stethoscope [6]. Digitalized phonocardiogram signals were analyzed in real time using the in-house developed software SignalTAB (Signal House Co., Ltd.) [7]. This Android-based application collects and records multiple biometric signals and waveforms simultaneously.

Bronchospasm occurred 60 min into the surgery and 30 min after initiating CO₂ insufflation. While maintaining PSV mode, peak inspiratory pressure (PIP) transiently increased from 15 cmH₂O to 20 cmH₂O. EtCO₂ slightly decreased from 32 mmHg to 31 mmHg, and the capnography waveform changed from a rectangular shape to a ‘shark-fin’ appearance, suggesting obstruction. The patient's blood pressure dropped sharply from 135/80 mmHg to 96/54 mmHg, and SpO₂ significantly reduced from 99% to 96%. Analysis of the recorded visual and acoustic signal and its spectrogram revealed erratic waveforms and increased intensities in higher frequencies of the heart-lung sounds during expiration, as shown in both sets of images in Fig. 1. Transthoracic auscultation revealed expiratory wheezing, which correlated with the visual images generated by the digitalized esophageal stethoscope, characteristic of bronchospasm.

Fig. 1.

Visual stethoscope signal, end-tidal CO₂, airway pressure, and spectrogram. The left panel illustrates a sudden episode of bronchospasm, showing a high-frequency band in the visual stethoscope signal and spectrogram. The right panel shows recovery following bronchodilator treatment, with the disappearance of the high-frequency band in the visual stethoscope signal and spectrogram. Notably, end-tidal CO₂ and airway pressure returned to normal ranges within 30 min.

Based on these findings, a bronchodilator was prioritized, and metered-dose salbutamol (5 puffs) was administered through the endotracheal tube with an adaptor. Subsequently, the PIP and the airway pressure decreased to pre-complication levels. EtCO₂ temporarily increased to 34 mmHg. Blood pressure and SpO₂ rapidly normalized within 30 min and were maintained until the end of the surgery. The visual and acoustic signal of the lung sounds and its spectrogram demonstrated smoother and more regular waveforms without the characteristic wheezing signals of bronchospasm after treatment, as shown in Fig. 1B.

The total surgery duration was 5 h and 45 min without further incidents. After surgery, 0.5 mg ramosetron hydrochloride and 200 mg sugammadex were administered. The patient was transferred from the recovery room to a general ward and discharged on postoperative day 8. As shown in Fig. 2, the postoperative chest X-ray revealed left pleural effusion, subsegmental atelectasis, and bilateral lower lung changes, common findings following liver lateral segmentectomy. The patient recovered without significant complications.

Fig. 2.

Serial chest radiographs of the patient. (A) The perioperative radiograph shows normal findings. (B) On postoperative day 2, a slight left pleural effusion and subsegmental atelectasis are observed. (C) At the 2-month outpatient follow-up, the pleural effusion and atelectasis have resolved.

DISCUSSION

Despite its low incidence, bronchospasm remains a significant concern, particularly during general anesthesia's induction and maintenance phases. In a computer-aided incidence study of 136,929 patients, Olsson et al. [8] reported a bronchospasm rate of 1.7 per 1,000 anesthesia cases. The overall incidence among the general surgical population is approximately 0.2%, but it can rise as high as 20% in high-risk patients [8]. Risk factors include obesity, smoking, COPD, asthma, allergies, and upper respiratory tract infections [1]. However, bronchospasm during general anesthesia often occurs in patients without a history of airway-related conditions. Potential causes include pharmacological agents, anaphylaxis, or inadequate depth of anesthesia. Bronchospasm can be triggered by various procedures, including laryngoscopy, tracheal intubation, exposure to cold-inspired gasses, and interventions that cause visceral stretching, leading to vagal stimulation and increased airway tone [1]. While multiple factors may have contributed to intraoperative bronchospasm in this patient, desflurane and pneumoperitoneum with CO₂ insufflation are suspected to be the likely triggers. Despite implementing pharmacologic and mechanical preventive measures, unexpected events can still occur during anesthetic management. Among 103 bronchospasm incidents reported in an ongoing national study of voluntary, anonymous reporting of unintended incidents across 90 hospitals, 78% were unrelated to allergy or anaphylaxis, underscoring that bronchospasm under general anesthesia mainly occurs as an independent phenomenon [9].

Clinical signs of bronchospasm include a prolonged expiratory phase, elevated peak inspiratory pressure, and reduced chest excursion. Hypoxia, hypercapnia, and hypotension may manifest, but wheezing or increased inflation pressure are primary indicators that initiate assessment and management. Chest auscultation should be performed to confirm the characteristic breath sounds of bronchospasm, such as wheezing and prolonged expiration; however, lung sounds may be absent in severe cases. Previous literature [3] indicates that airway obstruction during anesthesia produces distinct acoustic changes that can be visualized through spectrogram analysis. Wheezing is characterized by continuous, musical sounds with dominant frequencies above 100 Hz, often associated with airway narrowing. In this case, spectrogram analysis (Fig. 1) demonstrated similar high-frequency variations and irregular patterns, supporting the role of real-time acoustic monitoring in detecting intraoperative airway events.

While most cases of bronchospasm do not result in severe complications, untreated bronchospasm can lead to hypoxia, hypotension, and, in severe cases, brain injury, with mortality rates as high as 70% [10]. Therefore, early detection and prompt management are critical. For suspected perioperative bronchospasm, inhaled selective beta-2 agonists, such as salbutamol, are the first drug of choice. Immediate measures include administering 100% oxygen, discontinuing the surgical procedure or other precipitating factors, and deepening anesthesia. A chest radiograph should be ordered during postoperative care and chest evaluations should be performed to rule out pulmonary edema and pneumothorax [1]. Vigilant monitoring is crucial for early detection and management. Evidence suggests that continuous auscultation could have prevented up to 54% of reported incidents in one study [9]. The quantification of acoustic signals enables rapid and cost-effective sound data processing, facilitating further research in intraoperative auscultation [11]. Furthermore, by implementing noise cancellation techniques, extraneous noise in the operation room that interferes with main heart and lung sounds can be effectively filtered, providing a more informative tool for assessing breath sounds in real clinical settings [6].

Although auscultation has been the standard method for identifying bronchospasms during general anesthesia, conventional auscultation has limitations. It is subjective, requires clinician expertise, and is limited to a single user without recording or sharing capabilities. Moreover, due to rapid changes in cardiopulmonary parameters and the need to manage multiple critical tasks, anesthesiologists may prioritize immediate interventions, such as salbutamol administration, over auscultation. In contrast, visual stethoscopes address many of the limitations of traditional auscultations. They allow clinicians to observe rapid changes in the shape of the visualized breath sounds in real time. Studies have shown that spectral analysis of breath sounds using an esophageal stethoscope can effectively detect airway obstructions by highlighting characteristic frequency shifts in a spectrogram [12]. This finding supports the utility of real-time spectrogram visualization in identifying airway obstructions, such as wheezing patterns in bronchospasm. Additionally, multiple clinicians can simultaneously view the shared computer screen, facilitating collaborative discussion and improved patient care plans, leading to earlier treatment and improved outcomes. Visual changes in lung sounds can be detected before they are audible, enabling a proactive approach [13]. Furthermore, spectrograms provide a cost-effective method for quantitative acoustic analysis, allowing for retrospective evaluation. While arterial blood pressure, pulse oximetry, capnography, and ECG are more commonly used for cardiopulmonary monitoring in surgical settings, these devices lack sensitivity, provide delayed indications of complications during general anesthesia, and often rely on invasive methods, making them suboptimal for detecting bronchospasm and other pulmonary complications during general anesthesia [14].

Although esophageal stethoscopes have been used during anesthesia for over 45 years, they are primarily employed for core temperature monitoring during surgeries, while their use for auscultating heart and lung sounds has been less common [11]. The less common use of esophageal stethoscopes for auscultating heart and lung sounds is partly due to the limited availability of data for interpretation and challenges in achieving optimal placement for accurate acoustic signals [15]. However, recent studies have highlighted the usefulness of acoustic monitoring with visual stethoscopes to detect the position of the tracheal tube by comparing amplified bilateral and unilateral lung sounds displayed on a computer screen [13]. Additionally, spectral analysis of breath sounds has shown promise as a more sensitive tool for detecting intratracheal secretions compared to other monitoring indices, such as peak airway pressure [14].

In conclusion, this case highlights the importance of vigilant monitoring and prompt detection of bronchospasms during general anesthesia. Advanced devices, such as visual stethoscopes and spectrogram analysis, enable real-time, quantitative lung sound monitoring, facilitating faster diagnosis and improved treatment. Novel cardiopulmonary monitoring technologies will enhance patient safety and improve anesthesiologists’ management of perioperative complications.