Reversal of Partial Neuromuscular Block and the Ventilatory Response to Hypoxia: A Randomized Controlled Trial in Healthy Volunteers

Residual neuromuscular blockade, defined by a train-of-four ratio less than 0.9, is associated with impaired function of respiratory and pharyngeal muscles with an increased risk of hypoventilation, hypoxia, upper-airway obstruction, and pulmonary aspiration.^1–5  Importantly, Eikermann et al.²  demonstrated that even when the train-of-four ratio fully recovered to unity, respiratory function tests (e.g., forced vital capacity) may still be depressed in some patients. There is now robust evidence that nondepolarizing neuromuscular blocking agents additionally influence ventilatory control by acting within the peripheral chemoreflex loop at the carotid bodies.^6–11  The carotid bodies are located at the bifurcation of the common carotid artery and are important sensors involved in maintaining respiratory homeostasis.¹²  For example, in case of hypoxia, carotid body activation results in hyperventilation aimed at increasing pulmonary oxygen uptake. The carotid body response to hypoxia (the acute hypoxic ventilatory response) is a life-saving reflex that is impaired by various drugs used in the perioperative phase, including opioids and anesthetics.^13,14  In two pivotal studies in the early 1990s, Eriksson et al.^6,7  showed in humans that the nondepolarizing neuromuscular blocking agent vecuronium at a train-of-four ratio of 0.7 blunts the acute hypoxic ventilatory response by 15 to 60%. Because the acute hypoxic ventilatory response was significantly more depressed than the hyperoxic ventilatory response to carbon dioxide (which is a central chemoreflex response), an effect of vecuronium on the peripheral chemoreflex loop seems most likely. Animal studies give further proof for a selective effect of neuromuscular blocking agents at neuronal nicotinic acetylcholine receptors located postsynaptically on the afferent nerve that connects the oxygen sensing or glomus cells of the carotid bodies to the brainstem.^9–11 

Eriksson⁸  showed later similar results for atracurium and pancuronium and additionally demonstrated that spontaneous return of the train-of-four ratio to values greater than 0.9 resulted in the recovery of acute hypoxic ventilatory response to values not different from control, although some subjects showed persistent depression of the hypoxic response. The picture that emerges from human and animal data from the Eriksson research group is that nondepolarizing neuromuscular blocking agents, irrespective of their chemical structure or receptor affinity, significantly impair the peripheral chemoreflex at the carotid bodies at train-of-four ratios less than 0.9. So far, no other laboratories replicated Eriksson’s human studies. The aim of our study was twofold. We replicated the concept of the human studies originally performed by Eriksson et al.^6–8  with rocuronium to confirm their results for a nondepolarizing neuromuscular blocking agent not yet studied. Additionally, we measured the acute hypoxic ventilatory response, the hyperoxic hypercapnic ventilatory response after recovery of the partial neuromuscular block (as measured at the thumb), following reversal with neostigmine and sugammadex versus placebo. We hypothesized that the carotid body–mediated hypoxic ventilatory response is fully restored following the return to a train-of-four ratio of 1, irrespective of reversal strategy.

This single-center, double-blind, parallel, randomized controlled trial was performed at the Anesthesia and Pain Research Unit of the Department of Anesthesiology at Leiden University Medical Center (Leiden, The Netherlands) from May 2017 to September 2018. The protocol was approved by the local institutional review board (Commissie Medische Ethiek, Leiden, The Netherlands) and the Central Committee on Research Involving Human Subjects in The Hague, The Netherlands. The study was registered at the trial register of the Dutch Cochrane Center (Amsterdam, The Netherlands) under identifier 6427 (May 4, 2017). Before enrollment and after being informed about the study, all participants gave written informed consent. All study procedures were conducted according to good clinical practice guidelines and adhered to the tenets of the Declaration of Helsinki.

Healthy male volunteers aged 18 yr or older and a body mass index less than 30 kg/m² were eligible to participate in the study. Exclusion criteria were (1) known or suspected neuromuscular disorders impairing neuromuscular function; (2) suspected allergies to muscle relaxants, anesthetics, or narcotics; (3) a history (self or family) of malignant hyperthermia or any other muscle disease; (4) any medical, neurologic, or psychiatric illness (including a history of anxiety); (5) inability to give informed consent; and (6) signs (or history) of a possible difficult intubation. Subjects were asked not to eat or drink for at least 8 h before dosing with rocuronium. The subjects were not allowed to participate more than once in the study. The subjects were recruited and enrolled in the study by the study team.

This study had a randomized, double-blind, placebo-controlled parallel design. Upon arrival in the research unit, all subjects received an intravenous line for administration of study drugs. Participants were randomized to receive placebo (2 ml of normal saline), 1 mg of intravenous neostigmine (combined with 0.5 mg of atropine), or 2 mg/kg sugammadex, following a continuous rocuronium infusion for 90 to 120 min aimed at a train-of-four ratio of 0.7. Before and during the rocuronium infusion, acute hypoxic and hyperoxic hypercapnic tests were performed. After the rocuronium infusion was stopped and a reversal agent was administrated, another set of ventilatory tests was performed. Throughout the study the subjects were monitored by electrocardiogram and oxygen saturation via a finger probe.

Initially, we had set out to obtain respiratory responses at two levels of neuromuscular blockade with train-of-four ratios 0.6 and 0.8. Especially at the deeper level of relaxation, we observed frequent upper-airway obstructions that, although short-lived and not hazardous, interfered with the control of end-tidal carbon dioxide and oxygen concentrations. We therefore decided to change the protocol to studying just one level of blockade in between the original targets. Additionally, we originally intended to study repetitive hypoxic exposures following reversal to measure the acute hypoxic ventilatory response at increasing train-of-four ratio values (from 0.7 to 1). However, we observed that the train-of-four ratio returned rather rapidly toward 1 and therefore decided to obtain one measurement of acute hypoxic ventilatory response at the time point at which the train-of-four ratio was first equal to 1. All changes were made after consultation with the data safety committee, were approved by the institutional review board, and documented in the trial registry.

During ventilatory measurements, subjects were in the semirecumbent position. The ventilatory responses to hypoxia and hypercapnia were obtained using the dynamic end-tidal forcing technique. The technique is described in detail elsewhere.^15,16  In brief, subjects breathed through a facemask that was attached to a pneumotachograph and pressure transducer system (catalog no. 4813; Hans Rudolph Inc., USA) and to a set of mass flow controllers (Bronkhorst High Tech, The Netherlands) for the delivery of oxygen, carbon dioxide, and nitrogen. The mass flow controllers were controlled by a computer running software (RESREG/ACQ, Leiden University Medical Center, The Netherlands) that steers end-tidal gas concentrations (by varying the inspired concentration) and collects respiratory variables. The inspired and expired oxygen and carbon dioxide partial pressures were measured at the mouth using a capnograph (Datex Capnomac, Finland); heart rate and arterial oxygen saturation were measured by pulse oximetry (Masimo Corporation, USA). The following variables were collected on a breath-to-breath basis and averaged over 1 min for further analysis: minute ventilation (V_E), end-tidal carbon dioxide concentration (ETco₂), end-tidal oxygen concentration (ETo₂), and arterial oxygen saturation (Spo₂).

To obtain the isocapnic hypoxic ventilatory response, we performed steps into hypoxia by lowering the ETo₂ to 52 mmHg such that the Spo₂ dropped to 80 ± 2%. The hypoxic test took about 7 to 9 min, i.e., 2 to 4 min of normoxia (ETo₂ 100 mmHg) followed by 5 min of hypoxia. Throughout the experiment the ETco₂ was kept constant at 1 to 2 mmHg above resting values. To obtain the hyperoxic hypercapnic ventilatory response, we applied three 5- to 7-min steps in ETco₂ with step sizes of 7.5, 10, and 15 mmHg. To suppress the contribution of the carotid bodies to the hypercapnic response, all hypercapnic tests were performed in hyperoxia (inspired oxygen fraction, 0.5).¹⁵  The hypoxic ventilatory sensitivity was calculated by linear regression of the Spo₂–V_E data using the last 2 min of normoxia and hypoxia. The hypercapnic ventilatory sensitivity was calculated by linear regression of the ETco₂–V_E data using the last 2 min of each hypercapnic step. This analysis provided the slope of the hypercapnic ventilatory response (S) and the extrapolated x-axis intercept (apneic threshold, B). Next, we calculated ventilation at an extrapolated ETco₂ of 55 mmHg (V_E55) using the following equation V_E55 = S × (55 − B). Ventilation at an extrapolated ETco₂ of 55 mmHg takes the slope and the position of the hypercapnic ventilatory response into account and hence gives a reliable reflection of the effect of the intervention on hypercapnic ventilatory control (see fig. 2 of van der Scrier et al.).¹⁷  To ensure that responses were unaffected by previous responses, we allowed rest periods between measurements. Additionally, minute ventilation was assessed in real time on-screen, and only when ventilation had returned to baseline levels was the next ventilatory measurement initiated.

Experiments were performed at baseline (before any drug administration), at a stable neuromuscular block with train-of-four ratio equals 0.7, and after recovery to a train-of-four ratio equals 1. The sequence of hypoxic and hypercapnic tests was randomized at baseline and during rocuronium infusion; however, following return of the train-of-four ratio to 1 after reversal, first one hypoxic test was obtained followed by the hypercapnic test.

All drugs were given intravenously. For rocuronium (Esmeron, MSD BV, The Netherlands), the dosing was dependent on the measured train-of-four ratio. In all subjects the initial bolus dose was 5 mg, after which a rocuronium continuous infusion was started at 0.42 mg/min. Additional bolus doses of 1 to 5 mg were given, and/or the infusion rate was modified when the train-of-four ratio remained above the target. In case of an overshoot with train-of-four ratios of less than 0.7, the infusion was lowered until the target was reached. At the desired train-of-four ratio target, we waited 10 to 15 min, and when the train-of-four ratio remained stable, the first respiratory test was performed. Otherwise the infusion rate was further adapted until the target was reached. On average, 55 ± 15 mg rocuronium was given throughout the experiment. The rocuronium dosing was based on simulations using the pharmacokinetic data set of Kleijn et al.¹⁸  The reversal agents 1 mg of neostigmine (Hameln Pharmaceuticals Ltd., United Kingdom), 2 mg/kg sugammadex (Bridion, MSD BV), and 2 ml of placebo (NaCl 0.9%) were given as a bolus infusion when the rocuronium infusion was stopped. In case of an upper-airway obstruction or severe respiratory depression (with Spo₂ less than 70%) due to a more intense neuromuscular block than intended, the subject received 2 mg/kg sugammadex, and the experiment was ended.

Neuromuscular block was measured by electromyography at the adductor pollicis muscle, using the CARESCAPE B450 monitor combined with the electromyography–neuromuscular transmission module (both General Electric, Finland). After degreasing the skin with alcohol, the electrodes were placed at the wrist according to the guidelines of the manufacturer (in the arm opposite to the arm with the intravenous line). Before administration of rocuronium, a series of measurements were obtained at a stimulus strength of 30 mA. Stable recordings were verified and defined as a difference in train-of-four ratio of less than 5% in three consecutive measurements. All subsequent measurements were obtained at 1-min intervals. Skin temperature was maintained throughout the study by keeping a constant room temperature.

Randomization (placebo:neostigmine:sugammadex equals 1:1:1) was performed by an independent third party (research nurse not involved in the study) using a computer-generated randomization list. On the day of the experiment, each subject was allocated to treatment, and all study medication was delivered to the laboratory by the same nurse in unmarked sequentially numbered syringes of equal size and volume. In case of neostigmine administration, the syringe additionally contained 0.5 mg of atropine. The study was independently monitored, ensuring all Good Clinical Practices requirements were met.

No formal sample size analysis was performed because we based our samples size on the previous studies of Eriksson et al.^6–8  We defined the acute hypoxic response (AHR), slope of the hypercapnic ventilatory response (HCVR), and ventilation at an extrapolated ETco₂ of 55 mmHg (V_E55) obtained at baseline as AHR₁, HCVR₁, and V_E55₁, respectively. Similarly, at a train-of-four ratio of 0.7, the responses are denoted AHR₂, HCVR₂, and V_E55₂, and after recovery of the neuromuscular block, the responses are denoted AHR₃, HCVR₃, and V_E55₃. We calculated the ratio of acute hypoxic response and ventilation at an extrapolated ETco₂ of 55 mmHg relative to their baseline values, with AHR_2R = AHR₂/AHR₁, AHR_3R = AHR₃/AHR₁, V_E55_2R = V_E55₂/V_E55₁, and V_E55_3R = V_E55₃/V_E55₁. Next, we calculated the ratios F₂ = AHR_2R/V_E55_2R and F₃ = AHR_3R/V_E55_3R as carotid body index (i.e., markers of hypoxic chemosensitivity).⁷  In some participants V_E55₃ values exceeded baseline ventilation at an extrapolated ETco₂ of 55 mmHg values with consequently V_E55_3R values of more than 100%. We consider this excitatory phenomenon a procedural effect because the hypercapnic test after reversal was always performed after the hypoxic test. Because this may have underestimated the effect of reversal treatment on F₃, we calculated corrected V_E55_3R and F₃ values by constraining V_E55_3R to 100% in case values of more than 100% were observed.

All data were checked for normality by evaluation of their empirical distribution (i.e., by histogram). The effects of treatment on acute hypoxic response, slope of the hypercapnic ventilatory response, and ventilation at an extrapolated ETco₂ of 55 mmHg were analyzed by one-way ANOVA (factor: treatment) with post hoc Holm–Sidak multiple comparison test to compare treatment effect (either measurement 2 or 3) to control (baseline) data. The hypoxic and hypercapnic ratios F were compared to 1 using two-tailed paired t tests. These analyses were performed (using a two-tailed approach) on the complete data set and on the three distinct reversal treatments, allowing within-group comparison. To test our hypothesis that at a train-of-four ratio of 1 the responses fully returned to baseline levels irrespective of the reversal strategy, an analysis of covariance was performed on AHR₃, HCVR₃, V_E55₃, with the baseline as covariate and treatment (placebo, neostigmine, sugammadex) as fixed effect. To compare the F₃ ratios among treatments, a one-way ANOVA was performed. If a significant main effect was observed, protected post hoc tests were performed. The statistical analysis was performed using GraphPad Prism version 7 for Mac OS X (GraphPad Software, USA) and SPSS Statistics for Windows (IBM, USA), version 23.0. P values of less than 0.05 were considered significant. All values are means ± SD unless otherwise stated; the data in the figures are means ± 95% CI.

In the amended protocol 40 subjects were randomized (fig. 1). All subjects completed the protocol without serious adverse events. Four subjects developed upper-airway obstruction: three during the administration of rocuronium and one during recovery following placebo reversal. All four were treated with sugammadex, after which they fully recovered; they were taken out of the study, and each was replaced by another subject. The characteristics of the 36 subjects that completed the study are given in table 1. Data from two subjects (one in the neostigmine group and the other in the sugammadex group) were unreliable due to lack of calibration. Consequently, the data from 34 subjects were analyzed. Apart from upper-airway obstruction, adverse effects included diplopia (80%), difficulty swallowing (40%), and ptosis (10%). After reversal, all subjects recovered fully. None of the subjects reported the occurrence of distress or anxiety during relaxation.

Upon the administration of rocuronium, the train-of-four ratio decreased slowly and reached a steady state of 0.68 ± 0.01 (fig. 2) within 45 min, after which the respiratory tests were performed. During the hypoxic tests, the ETco₂ averaged to 41.1 ± 2.5 mmHg in control studies, 41.5 ± 2.6 mmHg in rocuronium studies, and 41.4 ± 2.6 mmHg in reversal studies. Hypoxic ventilator sensitivities during control, during relaxation, and after reversal were 0.55 ± 0.22 (AHR₁), 0.31 ± 0.20 (AHR₂), and 0.45 ± 0.16 (AHR₃) l · min⁻¹ · %⁻¹ (AHR₂vs. AHR₁, P < 0.001; AHR₃vs. AHR₁, P < 0.001; table 2), respectively. AHR₂ was depressed by 42%, and V_E55₂ was depressed by 11% (tables 2 and 3). Consequently, the effect of low-dose rocuronium on AHR₂ may be attributed largely to carotid body impairment with carotid body index F₂ = 0.67 ± 0.32 (P < 0.001; table 4). Following reversal, AHR₃ was still depressed by 18%, but V_E55₃ was on average 15% greater than the value measured at baseline; V_E55₃ exceeded baseline values in 29 subjects. After adjustment for this excitatory effect, the corrected carotid body index F₃ value averaged 0.89 ± 0.34 (P = 0.076).

Subject characteristics were similar among the three treatment groups (table 1). The train-of-four ratio recovery profiles for the three treatments are given in figure 2B. The times from reversal until the start of the hypoxic studies were 2.5 ± 0.7 (range 1 to 3) min, 8.2 ± 3.2 (4 to 16) min, and 15.1 ± 4.6 (12 to 25) min, for reversal with sugammadex, neostigmine, and placebo, respectively (arrows in fig. 3B). Although the magnitude of the control hypoxic responses among the three treatment groups varied (AHR₁ in table 2), these variations were not significantly different (P = 0.175), and partial relaxation by low-dose rocuronium resulted in a similar reduction by 38 to 46% (AHR₂) in the three treatment groups.

When considering the complete study population, reversal to a train-of-four ratio equals 1 led to an AHR₃ of 0.45 ± 0.16 l · min⁻¹ · %⁻¹ (P < 0.001 vs. AHR₁) or a residual 18% depression compared to baseline. Within-group comparisons are given in table 2. The treatment effect (between-group comparison) did not reach the level of significance (analysis of covariance main effect P = 0.299), indicating that the three reversal strategies had a similar effect on the acute hypoxic ventilatory response. Relative to control levels, all treatments produced a similar increase in the hypercapnic ventilatory response (analysis of covariance main effect P = 0.938) and ventilation at an extrapolated ETco₂ of 55 mmHg (analysis of covariance main effect P = 0.679; tables 2 and 3). Both uncorrected and corrected AHR_3R values and corresponding F₃ values are given in table 4 and figure 3. The uncorrected and corrected F₃ values in the complete population were 0.78 ± 0.35 (P = 0.001) and 0.89 ± 0.34 (P = 0.076), respectively. Neither for the corrected nor for the uncorrected F ratios a significant treatment effect was observed (F₃ uncorrected, ANOVA main effect P = 0.231; F₃ uncorrected ANOVA main effect P = 0.232). Corrected F₃ values less than 0.95 were observed in 10 subjects in the placebo group, 7 in the neostigmine group, and 5 in the sugammadex group.

The main outcomes of our experimental study are summarized as follows:

• (1)

  Rocuronium-induced partial neuromuscular blockade (train-of-four ratio ≈ 0.7) blunts the isocapnic hypoxic response by 42%, whereas hypercapnic ventilation was reduced by just 11% (table 3). As estimated from the carotid body index (F₂ = 0.67), the depression of the hypoxic response is primarily due to an effect on the peripheral chemoreflex loop. The additional depression of the hypoxic response is most probably related to mild respiratory muscle weakness.

• (2)

  Reversal of the neuromuscular block to a train-of-four ratio of 1 did not result in a complete return of the acute hypoxic response to baseline values (AHR₃ = 86 ± 25% of baseline, P = 0.002). We therefore reject the null hypothesis that full reversal of the neuromuscular block at the thumb results in a complete return of the acute hypoxic ventilatory response. Interestingly, this residual effect was independent of reversal strategy with 62% of subjects that still had AHR₃ values less than 95% of baseline (placebo n = 10, neostigmine n = 7, and sugammadex n = 5).

In the first part of our study, we convincingly replicate the findings of Eriksson et al.^6–8  and show that partial neuromuscular block reduces the acute hypoxic response primarily via an effect at the carotid bodies. As proposed by Eriksson et al.,⁷  the separation between carotid body and muscle effects on ventilatory control was performed by calculating the carotid body index F₂, which is the ratio AHR_2R/V_E55_2R or the ratio of relative changes in hypoxic response and ventilation at an extrapolated ETco₂ of 55 mmHg. Because ventilation at an extrapolated ETco₂ of 55 mmHg was obtained at hyperoxia, ventilation at an extrapolated ETco₂ of 55 mmHg is minimally influenced by carotid body activity. Hyperoxia silences the contribution of the carotid bodies to hypercapnic ventilation, which is 10 to 20% under normoxic conditions versus 0 to 5% under hyperoxic conditions.¹⁵  Impairment of the hypoxic response via the respiratory muscles rather than carotid bodies would have resulted in carotid body index values not different from 1.⁷  The rocuronium F₂ value of 0.66 (95% CI, 0.55 to 0.77) is smaller than the F value earlier observed for vecuronium (F = 0.84).^7,8  This suggests a greater potency of rocuronium at impairing the carotid bodies compared with vecuronium.

Our data indicate a small non–carotid body–related effect on the hypoxic response during low-dose rocuronium infusion. Although this is most probably due to respiratory muscle weakness (illustrated by the decrease in V_E55₂), we cannot exclude other causes. Theoretically, one such cause could be a decrease in arousal level from deafferentation related to reduced muscle spindle input to central sites. There is ample evidence that peripheral deafferentation from spinal or epidural anesthesia changes the brain sensory and arousal states.^19,20  A reduction in arousal level will decrease hypoxic response.¹⁴  Because we did not detect any obvious changes in arousal during partial muscle relaxation, we conclude that the non–carotid body–related effect of low-dose rocuronium on hypoxic response was related to mild respiratory muscle weakness. Although in line with the findings of Eikermann et al.²  on rocuronium, our data differ from those of Eriksson et al.,^6–8  who observed no effect of partial neuromuscular block on the slope of the hypercapnic ventilatory response. These differences may be attributed to differences in protocol or differences in drug sensitivity with greater rocuronium sensitivity in relaxation of respiratory muscles compared with other relaxants.

The between-group comparison indicated that despite full reversal of partial neuromuscular block as evidenced by the measurement of the train-of-four ratio of 1 and a fully restored ventilation at an extrapolated ETco₂ of 55 mmHg, impairment of the peripheral chemoreflex persisted in the majority of subjects, irrespective of the reversal strategy. It is further of interest to discuss the within-group comparisons.

Following placebo reversal and recovery of the train-of-four ratio to values more than 0.90 as measured at the thumb, the hypoxic response was just 76% of control, whereas ventilation at an extrapolated ETco₂ of 55 mmHg was fully restored. The resultant reduced carotid body index indicates that despite the return of muscle function at the thumb, carotid body function remained suboptimal. A possible explanation is a difference in rocuronium affinity for muscle versus neuronal nicotinic acetylcholine receptors. In vitro experiments give evidence for the distinct pharmacologic properties of rocuronium at various human muscle and neuronal nicotinic acetylcholine receptor subtypes expressed on Xenopus oocytes.²¹  However, functional affinity of nondepolarizing muscle relaxants was higher for muscle type receptors, with half-maximum inhibitory concentrations in the nanomolar concentration range for muscle nicotinic acetylcholine receptors versus micromolar range for neuronal nicotinic acetylcholine receptors. This seems to contradict our findings. Still, reversal of rocuronium-impaired carotid body function is a dynamic process that is influenced by several factors, such as acetylcholine receptors receptor kinetics at pre- and postsynaptic sites (including involvement of specific receptor subtypes), local blood flow, local acetylcholine and acetylcholinesterase concentrations, interaction with other (including muscarinic acetylcholine, dopamine, purinergic) receptors, receptor (de)sensitization, neuronal dynamics, etc. Currently, little is known about this complex process, and we postulate that reversal of the rocuronium effect at carotid bodies is slower than at peripheral muscles. This might not be so for the other nondepolarizing neuromuscular blockers, because Eriksson⁸  showed full recovery of hypoxic responses and F values following spontaneous return of the train-of-four ratio to values of more than 0.9 after partial muscle relaxation induced by vecuronium, atracurium, and pancuronium.⁸ 

Seven subjects (64%) had corrected F₃ values less than 0.95 following reversal. Consequently, we argue that although neostigmine was effective in restoring muscle function at the thumb, it did not concurrently restore the hypoxic response in all subjects. We cannot exclude a possible role for atropine in this suboptimal reversal. Animal data indicate that atropine, a muscarinic acetylcholine receptor antagonist, significantly attenuates the carotid body response to hypoxia.^22,23  Because data derived from human carotid bodies detected expression of just neuronal nicotinic acetylcholine receptors,²⁴  a possible inhibitory effect of atropine on the hypoxic response in our sample seems unlikely. What remains is the possibility that the neostigmine dose (13 µg · kg⁻¹) was sufficient to reverse respiratory muscle impairment but insufficient for full reversal of carotid body function in some subjects. Dose-response studies are needed to determine the dose required to restore carotid body function in all individuals following rocuronium relaxation.

Five subjects (45%) had corrects F₃ values less than 0.95. Consequently, even with sugammadex reversal and restoration of muscle function at the thumb, the hypoxic response may not be restored in all patients.

Our data indicate that reversal to a train-of-four ratio of 1 does not fully reverse blunting of the acute hypoxic response in most subjects. Because our protocol was unable to determine the timing at which the response returned to baseline values (i.e., AHR₃ > 0.95 × AHR₁), further studies are urgently needed to establish the dynamics of the return of the hypoxic response toward baseline following reversal strategies (neostigmine/atropine, sugammadex) or spontaneous recovery.

Although we randomized the hypoxic and hypercapnic tests during relaxation, we refrained from randomization following reversal. Although this was done to ensure that the hypoxic tests were performed at similar stages of reversal, this may have affected the hypercapnic test results. Indeed, ventilation at an extrapolated ETco₂ of 55 mmHg values following reversal were on average 15% larger than control values. This observation made us decide to correct for this excitatory effect by constraining the upper limit of V_E55_3R to 100% to prevent overestimation of the effect of the three reversal agents. Uncorrected and corrected F₃ values differed by about 10%. However, because 22 of 34 subjects had a corrected F₃ value of less than 0.95, we do not believe that this correction affected the clinical interpretation of the data.

Our method of measuring muscle relaxation may have been suboptimal. Although mechanomyography is considered the gold standard in neuromuscular monitoring, electromyography is generally regarded a good and accurate alternative to mechanomyography²⁵  and lacks the staircase effect that troubles acceleromyography or mechanomyography.²⁶  However, measurements may have been influenced by the fact that our subjects were awake, which may have caused occasional (unnoticed) thumb movements disturbing the measurements with possibly some overestimation of the train-of-four ratio. In addition, supramaximal stimulation was not used to avoid a possible confounding effect of discomfort on respiratory measurements. Finally, train-of-four ratios were not corrected for baseline train-of-four ratio, which may have caused overestimation of the recovery of the neuromuscular block at the time of measurement. However, this effect was relatively small (1 to 2%).

In conclusion, we successfully replicated the original study by Eriksson et al.⁶  showing an inhibitory effect of rocuronium at the carotid bodies. Our reversal data point toward persistence of carotid body impairment despite recovery of the train-of-four ratio to values of more than 0.9 at the thumb. These are important and clinically relevant observations. However, given the complexity of this experimental study, we highlight the need for further investigations. We encourage others to replicate our study and address the effect of spontaneous return of neuromuscular function following rocuronium relaxation on carotid body function. Finally, we would like to stress that in clinical practice, residual anesthetic and muscle relaxant levels synergistically affect breathing postoperatively. Further research should investigate such interactions with special emphasis on mechanisms and sites of action.

Supported in part by MSD Nederland BV (Haarlem, The Netherlands) and by institutional and departmental funds.

Drs. Boon, Martini, and Dahan received speaker and/or consultancy fees from MSD Nederland BV (Haarlem, The Netherlands). MSD was not involved in the design and conduct of the study, data analysis, or writing of the manuscript. The other authors declare no competing interests.

Full protocol available at: a.dahan@lumc.nl. Raw data available at: a.dahan@lumc.nl.