In current practice, oxygen saturation has been used as the standard for assessing adequate ventilation in postoperative patients [1]. However, in the setting of decreased ventilation, oxygen diffusion still takes place within the functional residual capacity of the lung, thereby consuming the oxygen present [2]. As there is no replacement of the oxygen due to the decreased minute ventilation, a patient’s oxygen saturation will decrease rapidly and with little warning. There is latency in pulse oximetry that coincides with the plateau in the sigmoid oxygen dissociation curve [3]. During this time frame, diminished minute ventilation can be detected before the pulse oximetry begins to plummet. In patients with opioid or propofol induced respiratory depression, inadequate reversal of muscle relaxation, or emergence from anesthesia, monitoring of minute ventilation in real time, can be accomplished with the ExSpiron 1Xi. This monitoring device may prove to be a critical patient safety device, by allowing more timely notification of the underlying physiology, thereby allowing for earlier intervention than current pulse oximetry [4]. In postoperative patients, deciding when to extubate can be greatly facilitated by going beyond the information provided by oxygen saturation. While patients are often times extubated in the operating room, others receive a trial extubation or require extended ventilation in the post-anesthesia care unit (PACU). It has been demonstrated that in patients with traumatic brain injury, outcomes are negatively impacted by extubation failures, highlighting the importance of having a means of monitoring ventilation parameters during a trial extubation [5]. Weissman, et al. found that “hypoxemic episodes are common in the PACU…(and) timely resolution of desaturation events outside of the operating room is challenging and protracted, potentially placing patients at increased risk” [6] (Figure 1).

Figure 1: Monitoring tidal volume.

We present a morbidly obese 49 year old female patient with obstructive sleep apnea who was postoperatively reaccumulating insufflated CO₂ and had a low tidal volume, detected by real-time noninvasive monitoring. This occurred despite the use of an opioid sparing multimodal pain regimen, appropriate reversal of muscle relaxation, and early reduction and shut off of volatile anesthetic gases. The multimodal pain regimen involved the intravenous administration of 1 g acetaminophen at the start of the case and 30 mg ketorolac once the surgeon began closing. Minimal opioids were used during the case, with a total of 100 mcg fentanyl upon induction and bolus of 50 mcg during the two and half hour procedure. The last dose of intermediate acting muscle relaxant, rocuronium, was administered one hour prior to close. A 4/4 TOF with sustained tetanus was observed prior to a dose of 70 mcg/kg neostigmine administration. Despite these measures, signs of respiratory depression were still present as measured by a low tidal volume. The borderline low minute ventilation and tidal volume detected by the ExSpiron device at the end of the case, correlated within 6-10 cc of the anesthesia machine, thus confirming the accuracy of the device. This allowed the anesthesia care team to make the decision to transport the patient to the PACU intubated, where she was later extubated having met all criteria, but now with the benefit of real time objective evidence of the adequacy of her post-extubation minute ventilation and tidal volumes. No naloxone was administered to the patient in the OR nor PACU as the total fentanyl administered was less than 1.1 mcg/kg. This highly accurate device enabled the anesthesiologists and nurses to use the objective criteria of minute ventilation throughout the post-extubation period, irrespective of the presence of anesthetics, sedatives, or secured airway, reducing the likelihood of an adverse event.

The patient was safely extubated in the PACU with evidence of her ability to maintain an adequate minute ventilation and tidal volume. She was evaluated postoperatively after being extubated and found to be breathing comfortably with a respiratory rate of 18, tidal volume between 350-400, and an oxygen saturation of 100. She was alert and her minute ventilation had returned to normal, allowing her to be safely discharged from the PACU without an increased length of stay.

Measuring a decrease in tidal volume and minute ventilation were factors crucial to delaying the extubation of this patient, which prevented her from becoming hypoxemic in the PACU. Additionally, had she received a trial of extubation, detection of this hypoxemia would likely have taken longer to have been detected by pulse oximetry than by the ExSpiron device as the patient had been extubated on 100% FiO₂ [7]. When the functional residual capacity (FRC) has been filled with 100% FiO₂, this masks the inadequacy of ventilation until the patient is on the steep portion of the oxygen dissociation curve. This latency of detection in pulse oximetry would likely have led to an emergent reintubation, which is an undesirable adverse event and outcome. Compounding the problem, a patient on supplemental oxygen will have a higher oxygen saturation that will not indicate the problem of low minute ventilation [8]. As put forward by Williams et al. [9] real time ventilation monitoring has previously proved useful for determining the necessity of respiratory interventions surrounding the extubation of a patient with a mass in the upper airway. Supporting this view, the Joint Commission has suggested that the monitoring of ventilation is a key safety measure in patients receiving opioid analgesia [10]. It is important to note the combination of obesity and obstructive sleep apnea in this patient are both risk factors for postoperative complications. In addition to reaching a decision regarding an appropriate time to safely extubate a patient, monitoring minute ventilation also assists in evaluating patients with sleep apnea for apnea/hypopnea frequency postoperatively [11]. Minute ventilation as measured by non-invasive real-time monitors like the ExSpiron can detect “post-operative apneic events resulting from partial or complete airway collapse due to the residual effects of narcotic administration and volatile and/or intravenous anesthetics” [11].

With the advent of highly accurate tidal volume and minute ventilation monitoring, it appears we are on the cusp of a significant leap in patient safety (Figure 2).

Figure 2: ExSpiron matches spirometry values MV, TV, RR.

We will now be able to utilize these two objective criteria in unintubated patients to assess the adequacy of ventilation in intermediate and high-risk patients and to allow us to preemptively intervene prior to a respiratory crisis or adverse event occurring. This is especially important as the device appears sufficiently sensitive to detect respiratory depression in the setting of opioid-induced, propofol-induced and respiratory muscle weakness [12]. This high degree of sensitivity will likely drive the technology to become pervasive in the post-anesthesia care unit, intensive care units and floors, where patients may be on patient controlled analgesia devices (PCAs). The latter potentially has the greatest safety impact, as a significant number of respiratory depression events occur in patients on PCAs [13]. According to Overdyk, respiratory failure after surgery is the “third most common cause patient safety-related adverse events affecting the Medicare population in U.S. hospitals, accounting for 113 events per 1,000 at-risk patient admissions, and (results) in death or anoxic brain injury in the majority of cases” [14]. As respiratory depression is a prelude to respiratory failure, early detection of this condition is crucial to improving patient safety.