Influence of negative pressure applied to the upper airway on the breathing pattern in unanesthetized awake dogs

We examined the influence of changes in upper airway pressure on the breathing pattern in 5 unanesthetized awake dogs. The dogs breathed through an endotracheal tube or through a comfortably fitting fiberglass snout mask. With matched resistances and volume of the dead space, the inspiratory duration, tidal volume, and minute ventilation were higher during nasal breathing compared to tracheal breathing. Nasal and tracheal occlusion produced prolongation of inspiration in the first occluded breathing attempt, but the prolongation was more marked in nasal occlusion tests. Augmentation of genioglossus muscle activity occurred on the first occluded breath in nasal but not tracheal occlusion. In another series of experiments, negative pressure was applied to the isolated upper airway while the dog breathed through a tracheostomy tube. Negative pressure caused a prolongation of inspiratory duration which was proportional to the level of the applied pressure. However, the prolongation of inspiratory duration was significantly more marked when application of negative pressure was timed simultaneously with tracheal occlusion. Our results demonstrate that the upper airway has a powerful effect on the control of breathing, which becomes more evident during tracheal occlusion.     

Ventilatory responses to hypoxia nullify hypoxic tracheal constriction in awake dogs

Three awake dogs with chronic tracheostomies were used to study the effects of hypoxia (12% O2) on tracheal smooth muscle tone. Pressure changes within a water-filled cuff in an isolated portion of the cervical trachea reflected changes in tracheal tone. During spontaneous ventilation, hypoxia produced hyperventilation, but no significant change in tracheal tone. If hypocapnia was prevented with inspired CO2 during hypoxia, one of three dogs increased tracheal tone, and all dogs increased ventilation beyond that measured with hypoxia alone. When the awake dogs were ventilated mechanically to prevent changes in ventilation, hypoxia always increased tracheal tone. We made independent changes in ventilation and CO2 similar to the spontaneous responses to hypoxia to test these effects on tracheal tone. When the dogs were ventilated mechanically first with 2% CO2, and then with no CO2, the resulting drop in end-tidal CO2 always decreased tone. When the tidal volume on the ventilator was increased under hyperoxic, isocapnic conditions, tracheal tone always decreased. We conclude that the normal ventilatory response to hypoxia opposes the bronchoconstrictor effect of hypoxia, resulting in no net change in tracheal smooth muscle tone.     

Abdominal muscle activity in conscious dogs: effect of sleep and route of breathing

Abdominal muscle activity (EMGabd) was studied in 4 adult dogs during wakefulness and sleep. The dogs were previously prepared with a permanent side-hole tracheal stoma, and were trained to sleep with a tightly-fitted snout mask, hermetically sealed in place. They breathed either through a cuffed endotracheal tube inserted distally into the tracheal stoma (tracheal), or through the upper airway, with the tracheal stoma plugged (nasal). Sleep state was determined by behavioural, electroencephalographic and electromyographic criteria. EMGabd activity was measured using fine bipolar needles inserted into the abdominal muscle layers. Expiratory EMGabd augmented progressively from sleep onset to SWS regardless of route of breathing, and without major changes in the animal's ventilation. Maximal EMGabd occurred in SWS during nasal breathing; EMGabd increased from a mean of 16.6 +/- 0.3 mV awake, to 23.8 +/- 0.3 mV in SWS, representing an overall increase of 55.0 +/- 7.5% from the awake level. EMGabd increased similarly during tracheal breathing, with an overall increase of 62.0 +/- 15.4% in SWS. We conclude that the consistent augmentation of expiratory EMGabd activity in sleep indicates that expiration in the dog is an active process, which is enhanced during nasal breathing and NREM sleep.     

Late complications of tracheotomy

Complications of tracheotomy are largely preventable. Although some authors cite these complications as indications for prolonged endotracheal intubation to avoid tracheotomy, others believe that the laryngotracheal complications of prolonged endotracheal intubation warrant early tracheotomy. Obviously, unnecessary tracheotomies should not be performed, and the controversy regarding the timing of conversion of endotracheal intubation to tracheotomy is handled in an earlier article in this issue. We feel, however, that a properly performed tracheotomy has a low incidence of complications that are more easily managed than are the complex laryngotracheal complications of prolonged endotracheal intubation. Significant post-tracheotomy tracheal stenosis occurs in 8% of patients and is secondary to an overly large tracheotomy stoma or damage at the tracheostomy tube cuff site. Stoma stenosis can be minimized by not making an overly large tracheal stoma and by prevention of undue leverage on the tracheostomy tube. Cuff stenosis can be minimized by the use of the high-volume low-pressure cuffs with careful prevention of overdistention of the cuff. Bronchoscopic dilatation, laser resection, and Silastic T-tube placement provide control of the airway until definitive surgical resection and reconstruction can be performed safely. Tracheoesophageal fistula is an uncommon but life-threatening complication that can be prevented by avoiding overdistention of the tracheostomy tube cuff and by avoiding the concomitant use of a stiff nasogastric tube. These patients are best managed conservatively until they are able to be weaned from a ventilator. A single-stage repair of both the esophagus and the trachea should then be done. Tracheoinnominate artery fistula can be avoided by correct placement of the tracheostomy stoma through the second and third tracheal rings rather than lower in the trachea and by avoidance of overinflation of tracheostomy tube cuffs.     

Intracuff pressures in endotracheal and tracheostomy tubes. Related cuff physical characteristics

This study compared intracuff pressure (ICP) during mechanical ventilation in a variety of currently used endotracheal (ET) and tracheostomy (trach) tube cuffs and related cuff physical characteristics. Tracheostomy tube physical characteristics were also measured. Variation was observed to exist between "just-seal" inspiratory and end-expiratory intracuff pressure during mechanical ventilation. Cuff diameter, thickness, compliance, geometry (shape), resting volume, and just-seal volume also varied. ICP varied with cuff diameter, thickness, compliance, geometry (shape), and trachea size, as well as tube curve and cuff position in the trachea. Thin, large-diameter, compliant cuffs generally "just seal" with relatively low ICPs. We recommend use of tracheal airways (endotracheal and tracheostomy) fitted with cuffs that seal in patients with low intracuff pressures. We also recommend nonrigid (soft) thermolabile tracheostomy tubes.     

A chronic isolated tracheal segment to study airway reflexes in conscious dogs.

Airway reflexes are difficult to study in conscious animals because associated changes in ventilation alter intrathoracic airway dimensions. By studying an isolated segment of extrathoracic trachea, we have overcome this problem. In each of 2 dogs, we created surgically an isolated tracheal segment just below the larynx, sealed at one end and tapered at the other to a 3-mm opening via a skin fistula. A chronic tracheostomy was also created near the thoracic outlet. We monitored intraluminal pressure (Pseg) of the isolated segment to reflect changes in smooth muscle tone. During anesthesia, with pentobarbital, gentle mechanical stimulation of the carina, deflation of the lungs, and asphyxia for one min increased Pseg (+9 to +/- 16 cm H2O). Lung inflation and alveolar hyperventilation decreased Pseg (-9 to -16 cm H2O). Five breaths of 2 per cent histamine aerosol increased Pseg (+5 cm H2O) when resting tone was normal. We also coated lumen of the isolated segment with tantalum powder and documented roentgenologically changes in the size of the segment that reflected changes in smooth muscle tone; constriction and dilation in response to asphyxia and lung inflation, respectively, were demonstrated directly by this technique. In conscious dogs, lung inflation decreased Pseg, and carinal stimulation increased Pseg. Instillation of lidocaine hydrochloride (Xylocaine) into the isolated tracheal segment blocked cough caused by mechanical stimulation of the segment, but carinal stimulation still caused constriction of the segment under these conditions which indicated that afferent, but not effrent parasympathetic innervation of the segment had been blocked selectively. Conversely, instillation of atropine sulfate into the isolated tracheal segment blocked constriction of the segment caused by carinal stimulation, but mechanical stimulation of the segment still caused cough under these conditions, which indicated that efferent, but not afferent parasympathetic innervation of the segment had been blocked selectively. We conclude that an innervated extrathoracic tracheal segment constricts and dilates via cholinergic pathways and is suitable for the study of airway reflexes in conscious dogs.     

Reflex influences from the extrathoracic trachea during airway occlusion.

We have performed experiments in 26 dogs anesthetized with pentobarbital and fitted with an endotracheal tube. The inflatable cuff of this tube was positioned either at the level of the cricoid cartilage or at the thoracic inlet. In this latter situation the extrathoracic trachea (E.T.T.) is not subjected to any change in transmural pressure both during breathing and airway occlusion. We have compared the inspiratory output in term of the integrated phrenic discharge during airway occlusion at FRC with the tracheal tube positioned at either one of the two levels. In most of the experiments (16 out of 26) the inspiratory output during airway occlusion is significantly greater (157%) when the E.T.T. is not by-passed and this difference disappears after bilateral vagotomy. We interpret these results by the asymmetrical response of the tracheal stretch receptors to positive and negative transmural pressure (Pt); most of these receptors are active at FRC and decrease their activity at low negative Pt, as that attained in the first occluded breath. These results seem to suggest that the reflex influences from the extrathoracic tracheal receptors on the inspiratory output are similar to those originating from the intrathoracic airway stretch receptors.     

MDCT of the central airways: comparison with bronchoscopy in the evaluation of complications of endotracheal and tracheostomy tubes

OBJECTIVE: To evaluate the accuracy of multidetector row computed tomography (MDCT) compared with bronchoscopy in the assessment of airway complications related to endotracheal and tracheostomy tubes. MATERIALS AND METHODS: A review was performed of all consecutive patients undergoing computed tomography (CT) and bronchoscopy for symptomatic complications of endotracheal or tracheostomy tubes during an 18-month period. MDCT imaging was performed according to a standard protocol and interpreted by an experienced thoracic radiologist before bronchoscopy. After bronchoscopy by an experienced interventional pulmonologist, CT and bronchoscopic findings were reviewed jointly, and the accuracy of CT was determined using bronchoscopy as the "gold standard." RESULTS: The study population was comprised of 32 patients (range: 26 to 88 y, mean 55.6) with a total of 47 airway complications: tracheal stenosis (n=25), tracheomalacia (n=8), tracheal granulation tissue (n=8), tracheal cartilage fracture (n=4), tracheal perforation (n=1), and tracheostomy tube disruption (n=1). CT accurately diagnosed 42 of 47 airway complications (sensitivity 89.4%, specificity 95.2%, positive predictive value 85.7%, negative predictive value 96.5%). False negative findings at CT occurred in 5 (11.1%) of 47 cases. Contributing technical factors were identified in 3 (60.0%) of 5 false-negative cases, including the presence of tracheostomy tube during imaging and patient inability to complete the CT protocol. CONCLUSIONS: MDCT of the central airways is highly accurate for detecting symptomatic airway complications of endotracheal and tracheostomy tubes, particularly when technical limitations to the performance of CT are minimized.     

Tracheostomy tube enabling speech during mechanical ventilation

BACKGROUND: A voice tracheostomy tube (VTT) was developed to enable patients to speak during mechanical ventilation. METHODS: The VTT has slits cut in it and is covered on part of its side with an elastic cuff, enabling the cuff to expand with positive pressure from the ventilator on inspiration and to deflate on expiration. By this mechanism, inspired air from the ventilator goes to the lung with the cuff inflated, and some of the expired air passes out around the deflated cuff and discharges through the glottis, allowing sufficient ventilation and also enabling vocal fold vibration. An experiment using a model lung showed that there was little leakage on inspiration even for low lung compliance and high airway pressure, and that the leakage volume on expiration was approximately 40% of the ventilated volume, ie, the volume discharging through the vocal fold in clinical use. RESULTS: Sixteen patients who had been managed by ventilation via a conventional tracheostomy tube were switched to the VTT. All patients except one were able to speak after switching to the VTT without change in PaO(2) and PaCO(2). There were no complications associated with the use of the VTT. Bronchoscopy showed that the cuff of the VTT did not damage the tracheal mucosa. CONCLUSION: The VTT enables patients to speak during mechanical ventilation with sufficient ventilation and without aspiration and damage to the tracheal mucosa, even in patients with low lung compliance.     

Contraction of trachealis muscle and activity of tracheal stretch receptors

This study examined the relationship between tracheal slowly adapting stretch receptor discharge and smooth muscle activity in a preparation in which the efferent supply to the airway was essentially intact. In 7 anesthetized, paralyzed, artificially ventilated dogs, smooth muscle activity was assessed by measuring the pressure of a water-filled cuff placed in the extrathoracic trachea and action potentials originating from 19 extrathoracic tracheal stretch receptors were recorded from the superior laryngeal nerve. Challenges were: hypercapnia (FI = 0.05 and FI = 0.10), hypoxia (FI = 0.10 and FI = 0.05) and asphyxia. Concurrent increases in cuff pressure and receptor discharge were present in 18 of the endings studied in response to all the challenges presented. The remaining receptor increased its rate of discharge with 10% CO2 and asphyxia; neither receptor discharge or cuff pressure increased with 5% CO2 and hypoxia. Following block of the recurrent laryngeal nerves, baseline values of both cuff pressure and receptor discharge, as well as the responses to asphyxia, decreased; any residual response was eliminated by atropine. Of the 17 receptors whose location could be precisely ascertained, 14 were found in the proximal third of the extrathoracic trachea, and the remaining 3 in the middle third. The temperature at which the nervous conduction was blocked was determined for 3 slowly adapting receptors; it ranged from 4.5 to 12.5 degrees C. Of 5 extrathoracic tracheal rapidly adapting receptors encountered during the course of the experiments, 3 were tested with asphyxia and found to be unaffected. This study shows that tracheal slowly adapting stretch receptors are activated by smooth muscle contractions reflexly induced by chemoreceptor stimulation.     

The effect of a gas leak around the endotracheal tube on the mean tracheal pressure during mechanical ventilation

Infants requiring mechanical ventilation are usually intubated with uncuffed endotracheal tubes, which permit gas to leak between the tube and the trachea. This gas leak may alter the mean pressure transmitted to the trachea by changing the pattern of airway flow and modifying the resistive behavior of the endotracheal tube. To test this hypothesis, we measured mean tracheal pressure, gas flow through the endotracheal tube, and resistance of the tube in rabbits ventilated with and without a leak. We also studied the effect of the tube size and the pattern of ventilation on these measurements. We found that a leak reduced the mean tracheal pressure by 15 to 21% with respect to the mean proximal airway pressure. This reduction was caused by an increased difference between inspiratory and expiratory flow through the endotracheal tube, and by the mean expiratory resistance of the tube being lower than its mean inspiratory resistance. The rabbits with smaller tubes had lower mean tracheal pressures. A ventilatory pattern of short inspiratory times and high peak pressures was associated with a proportionally greater decrease in mean tracheal pressure caused by the leak. These findings suggest that the mean proximal airway pressure, measured at the ventilator, may overestimate the mean tracheal pressure in the presence of a gas leak around the tube. Furthermore, the decrease in mean tracheal pressure caused by the leak may decrease oxygenation despite a constant mean proximal airway pressure.     

Comparison of upper airway collapse during general anaesthesia and sleep

Measurement of the collapsibility of the upper airway while a patient is awake is not a good guide to such collapsibility during sleep, presumably because of differences in respiratory drive, muscle tone, and sensitivity of reflexes. To assess whether a relation existed between general anaesthesia and sleep, we measured collapsibility of the upper airway during general anaesthesia and severity of sleep-disordered breathing in 25 people who were having minor surgery on their limbs. Anaesthetised patients who needed positive pressure to maintain airway patency had more severe sleep-disordered breathing than did those whose airways remained patent at or below atmospheric pressure. Such an association was strongest during rapid-eye-movement (REM) sleep. Our findings suggest that sleep-disordered breathing should be considered in all patients with a pronounced tendency for upper airway obstruction during anaesthesia or during recovery from it.     

Respiratory activity of laryngeal muscles in awake and sleeping dogs

Experiments were conducted in adult dogs to determine the respiratory activity of laryngeal muscles during wakefulness and sleep. We studied the EMG activity of three laryngeal muscles in five trained dogs, two of which were completely intact, and three of which had a previously-formed side-hole tracheal stoma. Pairs of electrodes were implanted chronically into the posterior cricoarytenoid muscle (PCA), a laryngeal dilator, cricothyroid (CT), and thyroarytenoid (TA), a laryngeal adductor. EMG electrodes were also inserted into the costal portion of the diaphragm. In wakefulness (W), slow wave sleep (SWS) and rapid eye movement (REM) sleep the EMGs of the PCA and CT muscles increased in intensity during diaphragm activation, with varying levels of basal activity during expiration. However, the greatest levels of inspiratory activity in PCA and CT during sleep were found in REM sleep, usually in the absence of augmented diaphragm EMG activity. This laryngeal muscle activity was associated with laryngeal dilation. There were also marked state-related changes in the level of activity of CT during expiration, suggestive of changes in the degree of expiratory adduction of the larynx. The adductor muscles (TA) were not active during expiration, except during alert W. There were no consistent differences in respiratory activity of the laryngeal muscles between the two intact dogs and those with a tracheal stoma (whether or not an endotracheal tube was in place), nor was laryngeal muscle activity affected by the subsequent creation of a tracheal stoma in the two intact dogs. The findings indicate that sleep-wakefulness state exerts important influences on the respiratory activity of laryngeal muscles in the adult dog.     

Tracheal tear and tension pneumothorax complicating bronchoscopy-guided percutaneous tracheostomy

Percutaneous dilatational tracheostomy (PDT) is a frequently conducted procedure in critically ill patients. Bronchoscopic guidance of PDT is generally recommended to minimize the risk of unintentional tracheal injury. We present a case of tracheal tear and tension pneumothorax, a rare but potentially life-threatening complication, during continuously bronchoscopy-guided PDT. Sealing the large tracheal air fistula with the cuff of an endotracheal tube helped bridge time to definitive surgical repair in our patient. Bronchoscopic guidance may minimize, but cannot completely eliminate, the risk of tracheal injury during PDT.     

Ventilating with tracheal gas insufflation and periodic tracheal occlusion during sleep and wakefulness

INTRODUCTION: The current invasive and noninvasive methods for delivering long-term ventilatory support rely on cumbersome patient interfaces that may interfere with upper airway function. To overcome these limitations, a novel system was developed to ventilate conscious, spontaneously breathing dogs through a self-contained cuffed cannula that was used for tracheal gas insufflation (TGI) and periodic tracheal occlusion (PTO). We hypothesized that TGI + PTO would provide greater ventilatory support than would TGI alone and that its effect would be more pronounced during sleep than wakefulness. METHODS: Chronically tracheostomized dogs were monitored for sleep (ie, EEG, electro- oculogram, and nuchal electromyogram) and breathing (ie, tracheal pressure [Ptr] and upper airway flow via snout mask). A thin transtracheal cannula housed within a cuffed tracheostomy tube was used for TGI and PTO monitoring. E, gas exchange, and breathing patterns were examined during sleep and wakefulness at baseline (ie, no TGI) and during the application of TGI alone (at 5, 10, and 15 L/min) and the application of TGI + PTO. RESULTS: Compared to baseline breathing without TGI, TGI at 5, 10, and 15 L/min decreased minute ventilation without influencing PaCO(2). In contrast, TGI + PTO led to progressive increases in ventilation, positive Ptr swings, and decreases in PaCO(2) as the flow rate was increased during sleep and wakefulness. Moreover, spontaneous breathing efforts ceased during TGI + PTO at flow rates of 10 and 15 L/min during wakefulness, and at all flow rates during sleep. CONCLUSIONS: The findings indicate that TGI + PTO can fully support ventilation in a spontaneously breathing canine model during sleep and wakefulness. Its streamlined interface could ultimately prove to be clinically significant, once technical concerns are addressed.     

Tracheostomy tube ignition during microlaryngeal surgery using diode laser: a case report

Ignition of the tracheal tube during laser microlaryngeal surgery under general anesthesia is an uncommon complication with potentially serious consequences. We present here a case of a patient with glottic stenosis following endotracheal intubation, who experienced this potentially catastrophic combustion during endoscopic arytenoidectomy, using a diode laser under general anesthesia via 60% FiO2, with an airway fire occurring at the tracheostomy tube and causing tubal damage and obstruction. The anesthetic connecting tube was immediately disconnected and the tracheostomy tube replaced. No adverse consequences to this patient's upper airway were noted during follow-up visits. Higher oxygen concentrations, the presence of combustibles, and the narrowness of the surgical field during endolaryngeal diode laser surgery are risk factors for airway fires.     

Reflex effects of isocapnic changes in ventilation on tracheal tone in awake dogs

We studied the effects of passive, isocapnic changes in ventilation on tracheal smooth muscle tone in 3 awake and 6 anesthetized dogs. Ventilation was altered using mechanical ventilation under hyperoxic conditions, and tracheal tone was measured using the pressure within a water-filled balloon in an isolated segment of trachea. Hypocapnia was prevented at increased VA by increasing FICO2. When f and VT were changed reciprocally, keeping VA constant, tracheal tone did not change. However, when either for VT was changed independently, tracheal tone decreased with an increase in VA. This effect was abolished following thoracic vagotomy. We conclude that tracheal tone is reflexly decreased during an increase in VA.     

Local mechanisms drive genioglossus activation in obstructive sleep apnea

Individuals with obstructive sleep apnea (OSA) require increased pharyngeal muscle dilator activation during wakefulness to maintain upper airway patency. Negative pressure is one potential stimulus for this neuromuscular compensation. Individuals with OSA who have previously undergone tracheostomy provide an opportunity to study upper airway physiology in both the presence and absence of upper airway respiratory stimuli. If negative pressure (or another local airway stimulus) were important in driving pharyngeal dilator muscle activation, one would predict that during nasal breathing, the pharynx of a tracheostomized patient would be exposed to negative pressure, and that high levels of muscle activation would therefore be measured. Conversely, during breathing by the patient through the tracheal stoma, one would expect low levels of muscle activation in the absence of local stimuli. We measured a number of respiratory variables, including genioglossus activation under both nasal and tracheal stomal breathing conditions, in five patients. In all five patients there was a significant and substantial decrease in both peak phasic (100 +/- 0 to 53.4 +/- 9.2 arbitrary units [mean +/- SEM], p < 0.01) and tonic genioglossus activation (36.3 +/- 5.3 to 20.7 +/- 3.9 arbitrary units, p < 0.05) during stomal breathing as compared with nasal breathing. We conclude that local upper airway respiratory stimuli, possibly negative pressure, are important in mediating the increased pharyngeal dilator muscle activation seen in sleep apnea patients during wakefulness.     

Automatic regulation of the cuff pressure in endotracheally-intubated patients.

To avoid tracheal wall damage or inadvertent falls of the endotracheal tube cuff pressure (Pcuff) in intubated and mechanically-ventilated patients, the authors devised a simple procedure for automatic and continuous regulation of Pcuff. The procedure, only requiring a simple aquarium air pump and conventional tubing, was first tested at the bench when applied to an intubated and ventilated lung model, including an artificial trachea with an externally-variable section. The clinical performance of the procedure was tested in eight intubated patients, in whom the endotracheal tube cuff was connected to the designed Pcuff regulator during 24 h. The bench test showed that the procedure was able to maintain Pcuff constant, regardless of the changes imposed in the tracheal section. It was also effective in maintaining Pcuff during routine mechanical ventilation. Actual Pcuff recorded over the 24-h period always coincided with the target value within +/-2 cmH2O in all the patients. The procedure devised to maintain endotracheal tube cuff pressure is readily implemented, cheap, easy to operate and can be used regardless of the specific ventilator or tube used. Routine implementation of this procedure may be useful for protecting the trachea from tissue damage and for reducing the risk of ventilator-associated pneumonia.     

Effects of electrical stimulation of the genioglossus on upper airway resistance in anesthetized dogs

We examined the relationship between the frequency of stimulation of the genioglossus and upper airway resistance in six anesthetized dogs in the supine position. The upper airway was isolated from the lower airway by transecting the cervical trachea, and the pressure flow relationship of the upper airway was obtained by applying constant negative pressure (5, 10, and 20 cm H2O) to the proximal cut end of the trachea. Electrical stimulation of the genioglossus was performed at a constant voltage (10 to 20 V) and at various frequencies (as high as 100 Hz). Upper airway resistance (Rua) during both inspiration and expiration increased with an increase in tracheal negative pressure, and at each tracheal negative pressure Rua was significantly reduced by stimulation of the genioglossus. The effects of genioglossal muscle stimulation were nonlinearly dependent on the stimulating frequency. Below 50 Hz, Rua decreased markedly as the stimulating frequency was increased, but above 50 Hz, Rua plateaued at a minimum value. These findings suggest that at a stimulating frequency of more than 50 Hz, upper airway patency is stably maintained in anesthetized dogs.