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Biomedical subjects

B T Thach

Publications and source records attributed to B T Thach.

At least 55 records · Page 3Linked to original sources
PubMed1989

Effect of maturation on spontaneous recovery from hypoxic apnea by gasping.

The effect of maturation and motor activity on recovery from hypoxic apnea by gasping (autoresuscitation) was investigated in Swiss Webster mice and a SW-related strain. Apnea was induced with 97% N2-3% CO2 and air given at its onset. In 69 Swiss Webster-related strain mice that gasped for at least 5s, the survival rates were 100% in 1- to 16-day-old mice, 19% in 17- to 23-day-old mice, and 90% in adults. The rates in 81 Swiss Webster mice were 100, 70, and 95%, respectively. The decrease in survival in 17- to 23-day-old mice was statistically significant for both strains, as was the difference in survival in 17- to 23-day-old mice between the two strains. Gasping was absent or of brief (less than 5s) duration in 15 Swiss Webster mice that failed to recover. Seizure-like activity was observed during hypoxic apnea and gasping but was not seen significantly more often in mice that died. Simultaneous recordings of limb movement (electromyogram) and breathing pattern indicated that movement did not interfere with the occurrence of gasping. We conclude that in adults and very young mice autoresuscitation is a powerful mechanism for recovery from hypoxia, but at an intermediate age, it is frequently unsuccessful.

Aging↗
PubMed1989

Influence of upper airway negative pressure reflex on response to airway occlusion in sleeping infants.

Artificially produced upper airway suction inhibits the diaphragm in animals and infants; however, the effects of spontaneously generated suction in humans are unknown. We studied nine tracheostomized infants because separation of the upper from the lower airway allowed us to channel suction created by an occluded inspiratory effort to both upper and lower airways (upper + lower airway occlusions) or to the lower airway only (lower airway occlusion). The tracheostomy airway was briefly occluded at end expiration during quiet sleep. In upper + lower airway occlusions, peak airway pressure of the first occluded breath was less negative and rate of pressure decrease slower than that of lower airway occlusions, indicating that upper airway suction inhibits thoracic inspiratory muscles. The threshold for this response was less than or equal to 4 cmH2O suction pressure. The effect on inspiratory time was variable. A decrease in slope of the inspiratory pressure waveform occurring at approximately 0.12 s after inspiration onset was more marked in upper + lower airway occlusions. We conclude that infants have an upper airway reflex response to inspiratory pressure that alters not only the peak and slope but also the shape of the inspiratory pressure waveform.

Airway Obstruction↗
PubMed1989

Characteristics of upper airway chemoreflex prolonged apnea in human infants.

Water instilled into the pharynx of sleeping human infants elicits a range of chemoreflex responses that, occasionally, includes prolonged apnea (defined as absence of ventilation for at least 20 s, or for a shorter period if accompanied by bradycardia of less than or equal to 100 b.p.m. or cyanosis). To learn more about airway receptors mediating this prolonged apneic response and factors determining its occurrence, we examined the importance of stimulus location and associations between prolonged apnea, bradycardia, and upper airway responses. A total of 29 episodes of prolonged apnea were recorded after water stimulation in 12 infants. Bradycardia (HR less than 100) followed stimulus delivery but was always preceded by apnea and did not appear as an independent chemoreflex response. Behavioral arousal and prolonged apnea were not mutually exclusive responses and recovery from prolonged apnea was not always closely linked with arousal. Occurrence of prolonged apnea was greater after pharyngeal than nasal stimulation, and was frequently associated with coughing, but not with sneezing, suggesting that prolonged apnea is elicited from a sensory site close to, or the same as, one mediating cough. We conclude, using this water stimulus method, that the predominant receptors for chemoreflex-prolonged apnea are located in the pharynx or larynx rather than in the nose.

Apnea↗
PubMed1988

Upper airway chemoreflex responses to saline and water in preterm infants.

Laryngeal chemoreflex (LCR) responses elicited by fluid irrigation of the larynx have been described repeatedly in animals, whereas evidence for a similar reflex in human infants is extremely limited. Using nasopharyngeal catheters to instill small volumes of warm saline or water into the pharynx, we examined the incidence and characteristics of such a reflex in nine premature infants. Saline and water elicited the same pattern of responses, which frequently included swallows, central apnea, and airway obstruction and less commonly featured coughs, prolonged apnea, and arousal. With the exception of arousal, the incidence of these responses was significantly greater after delivery of water stimuli than after saline bolus administration. We therefore deduce chemoreceptor involvement in generation of these reflex responses and propose a laryngeal site for this sensory system, as in animals. Since greater potency of water compared with saline was demonstrable in all the infants studied, we further conclude that most preterm infants possess an upper airway chemoreflex.

Apnea↗
PubMed1988

Effects of mass loading on the upper airway.

To learn how increased cervical adipose tissue might affect upper airway function, we studied effects of mass loading on upper airway dimensions, stability, and resistance. Eight rabbits were studied (anesthetized and postmortem) using lard-filled bags to simulate cervical fat accumulation. Additionally, a handheld device was used to apply measured loads at localized sites along the airway. Upper airway resistance and closing pressure (a reflection of airway stability) were determined before and after loading. Endoscopy revealed concentric narrowing of the pharynx during loading in anesthetized and postmortem preparations. Upper airway resistance was increased by mass loads, with larger loads having greater effects. Loading caused decreased airway stability as reflected by closing pressures. The area over the thyrohyoid membrane was more vulnerable to mass loading than adjacent areas. Because mass loading of the upper airway causes changes in its configuration and function similar to those seen in obstructive sleep apnea syndrome (OSA), we speculate that such loading may contribute to the pathogenesis of OSA associated with obesity.

Adipose Tissue↗
PubMed1988

Prolonged apnea associated with upper airway protective reflexes in apnea of prematurity.

We examined the hypothesis that most idiopathic prolonged apneic spells in preterm infants originate as a form of upper airway protective reflex. We elicited a previously described upper airway reflex in 10 sleeping preterm infants with apnea of prematurity by instilling physiologic saline into the oropharynx and recording respiration, heart rate, and swallowing. Immediately after a small pharyngeal bolus of warm saline, one or more of several characteristic airway reflex responses occurred with most stimulus trials. These responses included brief central apnea, swallowing, and obstructed inspiratory efforts. Intermittently, this response pattern was more extensive and resulted in prolonged apnea. We compared the frequency of prolonged apnea immediately after saline boluses with spontaneously occurring prolonged apnea and found that apnea frequency after saline was greater than spontaneous apnea frequency in all 10 infants (p less than 0.0025). Also, the incidence of prolonged apnea occurring immediately after saline stimuli was significantly greater than that after randomly delivered control stimuli (p less than 0.005). These observations indicate that saline can induce prolonged apnea. Swallowing, a distinctive feature of the chemoreflex, occurred during 76% of spontaneous prolonged apnea spells. Spontaneous and postsaline prolonged apnea were nearly identical with respect to general features and frequency of specific airway protective responses. We conclude that the majority of spontaneous prolonged apneic spells in preterm infants have multiple features that are characteristic of an exaggerated airway protective response to an upper airway fluid stimulus. Circumstantial evidence suggests that endogenous upper airway secretions may be an important source of stimuli-inducing apneic spells in preterm infants.

Apnea↗
PubMed1986

Control of respiratory activity of the genioglossus muscle in micrognathic infants.

The genioglossus (GG) muscle activity of four infants with micrognathia and obstructive sleep apnea was recorded to assess the role of this tongue muscle in upper airway maintenance. Respiratory air flow, esophageal pressure, and intramuscular GG electromyograms (EMG) were recorded during wakefulness and sleep. Both tonic and phasic inspiratory GG-EMG activity was recorded in each of the infants. On occasion, no phasic GG activity could be recorded; these silent periods were unassociated with respiratory embarrassment. GG activity increased during sigh breaths. GG activity also increased when the infants spontaneously changed from oral to nasal breathing and, in two infants, with neck flexion associated with complete upper airway obstruction, suggesting that GG-EMG activity is influenced by sudden changes in upper airway resistance. During sleep, the GG-EMG activity significantly increased with 5% CO2 breathing (P less than or equal to 0.001). With nasal airway occlusion during sleep, the GG-EMG activity increased with the first occluded breath and progressively increased during the subsequent occluded breaths, indicating mechanoreceptor and suggesting chemoreceptor modulation. During nasal occlusion trials, there was a progressive increase in phasic inspiratory activity of the GG-EMG that was greater than that of the diaphragm activity (as reflected by esophageal pressure excursions). When pharyngeal airway closure occurred during a nasal occlusion trial, the negative pressure at which the pharyngeal airway closed (upper airway closing pressure) correlated with the GG-EMG activity at the time of closure, suggesting that the GG muscle contributes to maintaining pharyngeal airway patency in the micrognathic infant.

Cleft Palate↗
PubMed1985

Observations made on severe apneic spells in two infants at risk for sudden death.

We have studied the sequence of events during prolonged apneic spells in two infants who, by virtue of multiple risk factors or ultimate death, were at risk for sudden death. Using techniques to quantitate airflow and pharyngeal airway pressures, we have reported many descriptive aspects of the apnea in these infants. Specifically, we have noted that severe apneic spells in these infants were accompanied by a wide spectrum of lengths and types of shorter apneas. In the case of infant A, the most severe spells occurred in clusters. Physiologic phenomena usually considered normal (sighs, swallows, etc.) often occurred in association with the apneic spells. In addition, we found that: 1) mixed apnea was the clinically significant form of apnea; 2) in mixed apnea, the site of obstruction was in the pharynx; 3) obstruction often occurred at the beginning of the spells; and 4) this obstruction occurred progressively over a series of breaths prior to the spell and resolved in a similar fashion. Theophylline, in one infant, decreased the incidence of apnea.

Airway Resistance↗
PubMed1985

Apnea associated with regurgitation in infants.

To determine whether regurgitation might be a factor in the pathogenesis of apnea in certain infants, we compared the frequency of short and prolonged apnea immediately following regurgitation to that during control periods. Ten infants (nine preterm and one term) with histories of frequent regurgitation and also apneic spells were studied for 2 to 3 hours by monitoring nasal airflow, abdominal respiratory movements, electrocardiogram, pharyngeal pH, and pharyngeal pressure. In six of these infants additional observations were made without the intrapharyngeal recording devices. Fourty-four episodes of regurgitation were observed. Both prolonged apnea (P less than 0.05) and short apnea (P less than 0.01) occurred much more frequently during regurgitation than during the control period; however, the majority of prolonged apneic spells observed were unassociated with regurgitation. The increased frequency of apnea during regurgitation was not related to the presence of intrapharyngeal recording devices. Although nasal regurgitation was frequently associated with short apnea, no prolonged apnea was observed during the seven episodes of nasal regurgitation observed. The 14-fold increase in prolonged apnea frequency immediately following regurgitation supports the hypothesis for a causal relationship between apnea and regurgitation.

Apnea↗
PubMed1985

Sleep apnea in infancy and childhood.

Episodic apnea leading to asphyxia is a relatively common disorder of young children. Important apnea syndromes include apnea of prematurity, "narrow upper airway syndrome," congenital hypoventilation syndrome, breath-holding spells, and "near-miss" sudden infant death syndrome. More recently described syndromes include apnea associated with feedings, regurgitation or gastroesophageal reflux, and apnea initiated by epileptic seizures. Apnea occurring during wakefulness is common and may be related to that occurring during sleep. Knowledge of the clinical features and pathophysiology of these various kinds of apnea is important in their management.

Apnea↗
PubMed1985

Assessment of pharyngeal airway stability in normal and micrognathic infants.

A current hypothesis for obstructive sleep apnea states that 1) negative airway pressure during inspiration can collapse the pharyngeal airway, and 2) neural control of pharyngeal airway-dilating muscles is important in preventing this collapse. To test this hypothesis we performed nasal mask occlusions to increase negative pharyngeal airway pressures during inspiration in eight normal and five micrognathic infants. Both groups developed midinspiratory pharyngeal obstruction, but obstruction was more frequent in micrognathic infants and varied in some infants with sleep state. The airway usually reopened with the subsequent expiration. The occasional failure to reopen was presumably due to pharyngeal wall adhesion. If airway obstruction occurred in sequential breaths during multiple-breath nasal mask occlusions in normal infants, there was a breath-by-breath change in the airway pressure associated with airway closure (airway closing pressure); the airway closing pressure became progressively more negative. Micrognathic infants showed less ability to improve the airway closing pressure, but this ability increased with age. These findings suggest that nasal mask occlusion can test the competence of the neuromuscular mechanisms that maintain pharyngeal airway patency in infants. Micrognathic infants had spontaneous midinspiratory pharyngeal airway obstructions during snoring. Their episodes of obstructive apnea began with midinspiratory pharyngeal obstruction similar to that seen during snoring and nasal mask occlusions. These findings imply a similar pathophysiology for snoring, spontaneous airway obstruction, and obstruction during snoring.

Airway Obstruction↗
PubMed1985

Factors influencing regional patency and configuration of the human infant upper airway.

To study factors influencing patency and configuration of the upper airway, we studied 11 infant cadavers using endoscopy and photography. In most cases, studies were performed shortly after death. The naso-, oro-, and hypopharynx and the larynx were studied. The upper airway was sealed at the nose and mouth so that transmural airway pressure could be raised or lowered. As pressure was lowered airway closure was seen in each of the four regions studied. With respect to closing pressure, the oropharynx was the most compliant region and the larynx the least compliant. In the naso-, oro-, and hypopharynx, lowering the transmural pressure was associated with inward movement of the anterior, posterior, and lateral airway walls. In the larynx, closure occurred by vocal cord opposition in the midline. Tension applied to the genioglossus and geniohyoid tongue muscles had an effect opposite to that of airway suction, causing a more or less symmetrical dilation of the naso- and oropharynx. When the airway was closed, additional tension was needed to produce airway reopening, suggesting that adhesion forces act to maintain airway closure. Neck flexion caused pharyngeal closure, and neck extension caused pharyngeal dilation. Secretions adherent to the walls of the airway visibly narrowed its lumen. The relevance of these findings for the obstructive sleep apnea and laryngomalacia syndromes is discussed.

Airway Resistance↗
PubMed1985

Airway protective and abdominal expulsive mechanisms in infantile regurgitation.

To define the expulsive and airway protective mechanisms involved in infantile regurgitation, we studied 15 infants (9 premature and 6 mature infants) with histories of frequent postfeeding regurgitation. In 13 infants we recorded pharyngeal pressure, pH, nasal and oral airflow, and abdominal respiratory movements. In two additional infants we recorded gastric pressure. In eight infants observations were made without intrapharyngeal recording devices. Distinctive abdominal regurgitation movements (RMs) immediately preceded 84% of regurgitation episodes. These RMs were characterized by one or more large brief increases in abdominal girth. In the two infants with gastric pressure recordings, large increases in gastric pressure, with duration and frequency characteristics similar to the RMs, immediately preceded regurgitation episodes. Thus, in contrast to the generally accepted concept that flow of gastric contents out of the stomach is passive during infantile regurgitation, we documented an active expulsive mechanism similar to that of vomiting in the adult. In all regurgitation episodes, upper airway closure occurred at the onset of the regurgitation movement. One or more swallows occurred immediately following RMs and prior to airway reopening in 97% of regurgitation episodes. Brief respiratory pauses occurred during regurgitation in all premature infants and occasionally in mature infants. Nasal regurgitation, coughing, and sneezing occasionally accompanied regurgitation episodes. Thus upper airway closure and swallowing prior to airway reopening were the most frequently observed airway protective mechanisms during regurgitation. Brief respiratory pauses, sneezing, and coughing may be secondary airway protective mechanisms. Nasal regurgitation likely represents immaturity of airway protective mechanisms.

Abdomen↗
PubMed1985

Pulmonary protective mechanisms in human infants.

The flow of oral and nasal secretions into the pharynx is a recognized source of pulmonary aspiration during sleep in infants and adults alike. Such aspiration probably accounts for many cases of bacterial pneumonia. In infants, swallowing occurs frequently in sleep, and the rate of swallowing appears to be far greater than that of the sleeping adult. Such swallowing during sleep appears to be the major mechanism whereby oral and nasal secretions are transported to the digestive tract. Certain aspects of those swallows, such as "swallow-breaths" and their coordination with the respiratory cycle, have been documented. A brief period of airway closure is always seen during such nonfeeding swallows. Nonfeeding swallows are usually seen during mixed and obstructive apneic spells in preterm infants. The underlying reflex mechanisms responsible for such swallows during apnea are unclear. During ordinary postfeeding regurgitation, upper airway closure and swallowing occur in close temporal sequence. These events appear to be major mechanisms of airway protection during regurgitation. In certain infants, regurgitation and prolonged apnea often occur simultaneously. The mechanism underlying their association is unclear but may involve laryngeal chemoreceptors.

Apnea↗
PubMed1984

Benzyl alcohol toxicity in a neonatal intensive care unit. Incidence, symptomatology, and mortality.

We unknowingly "screened" all NICU infants for elevated levels of serum benzyl alcohol (Bz-OH) over a three-month period. The fortuitous "screening" procedure resulted from the interference by Bz-OH with a routine blood CO2 assay used for all infants; validity was proved by (1) replication of the interference pattern with Bz-OH or benzoic acid; (2) confirmation of elevated benzoic acid levels in serum in two of the four screening positive infants tested but not in control infants, and (3) disappearance of the interference patterns when Bz-OH solutions were discontinued in affected infants. Screened Bz-OH-positive infants were compared to screened negative control infants, matched for weight (less than 1000 g), severity of RDS (on respirators), exposure to Bz-OH, and survival for longer than 48 hours. Intraventricular hemorrhage (IVH), metabolic acidosis appearing prior to IVH, and mortality were increased in Bz-OH-positive infants (P less than .05 in each case); hyperbilirubinemia and thrombocytopenia were not. Gasping respirations were not a major symptom. It is concluded that Bz-OH poisoning was a major cause of morbidity and mortality in NICU infants weighing less than 1000 g at birth during the three-month screening period. A retrospective review of patient records covering a 16-month period showed significant improvement in the survival rate of infants weighing less than 1000 g following the discontinuation of Bz-OH solutions.

Acidosis, Respiratory↗
PubMed1984

Pharyngeal airway-stabilizing function of sternohyoid and sternothyroid muscles in the rabbit.

The upper airway is vulnerable to collapse from negative intraluminal pressures during inspiration. The tongue muscles, the genioglossi and geniohyoids, by contracting during inspiration, appear to function to resist this collapse. This study supports the hypothesis that two cervical strap muscles, the sternohyoid and sternothyroid, have a similar function. First, phasic inspiratory electromyographic activity was recorded from the sternohyoid and sternothyroid muscles of nine anesthetized rabbits during tidal breathing. Furthermore, each muscle showed a progressive increase in electromyographic activity with airway occlusion. Second, in eight rabbits, by determining the amount of negative pressure required to collapse the upper airway (airway closing pressure determination), it was shown that upper airway stability improved with electrical stimulation of either the paired sternohyoid or sternothyroid muscles. In addition, in 12 freshly killed rabbits, mechanical tension, mimicking the contraction of either the sternohyoid or sternothyroid, improved airway stability. Finally, observations of the pharyngeal lumen utilizing a fiber-optic endoscope, revealed concentric narrowing of the oro- and nasopharynx when airway pressure was lowered and concentric widening when tension was increased in the sternohyoid or sternothyroid muscles. These findings support the hypothesis that phasic inspiratory contraction of the sternohyoid and sternothyroid muscles functions to resist pharyngeal airway collapse due to negative intraluminal pressures.

Animals↗