Apnea of prematurity: pathogenesis and management strategies

Apnea of prematurity (AOP) is a significant clinical problem manifested by an unstable respiratory rhythm reflecting the immaturity of respiratory control systems. This review will address the pathogenesis of and treatment strategies for AOP. Although the neuronal mechanisms leading to apnea are still not well understood, recent decades have provided better insight into the generation of the respiratory rhythm and its modulation in the neonate. Ventilatory responses to hypoxia and hypercarbia are impaired and inhibitory reflexes are exaggerated in the neonate. These unique vulnerabilities predispose the neonate to the development of apnea. Treatment strategies attempt to stabilize the respiratory rhythm. Caffeine remains the primary pharmacological treatment modality and is presumed to work through blockade of adenosine receptors A(1) and A(2). Recent evidences suggest that A(2A) receptors may have a greater role than previously thought. AOP typically resolves with maturation suggesting increased myelination of the brainstem.

Spectrum of Haemoglobinopathies in a Tertiary Care Hospital of Armed Forces

BACKGROUND

Thalassaemia and other structural haemoglobinopathies are the major genetic disorders prevalent in certain parts of the world including India. This study presents the pattern of haemoglobinopathies amongst the referred patients of anaemia in a two-year period.

METHODS

A total of 1032 patients were studied during a two-year period for anaemia investigation. Haematological indices, sickling test and haemoglobin electrophoresis with quantification of the bands was done in all cases.

RESULT

Out of 1032 cases, 774 (75%) were normal and 258 (25%) cases had abnormal haemoglobin pattern. Of the 258 abnormal cases, 136 (53%) were males and 122 (47%) were females. Of all cases of anaemia 370 (36%) were microcytic hypochromic, 237 (23%) macrocytic, 151 (15%) were dimorphic and the rest (26%) had normocytic normochromic picture. 82% of microcytic hypochromic anaemias had reduced serum iron and elevated total iron binding capacity (TIBC), whereas 85% had decreased serum ferritin levels. Spectrum of haemoglobinopathies prevalent were β-Thalassemia trait (17%), followed by sickle cell trait (2.3%). Other haemoglobinopathies in descending order of frequency were sickle cell disease (1.7%), Hb D trait (1%), Hb E trait (0.8%), sickle cell - β thalassemia, Hb E disease, E - β thalassemia (0.6% each) and thalassemia major (0.4%).

CONCLUSION

This study provides a comprehensive database on the spectrum of haemoglobinopathies in the Armed Forces. It is suggested that detection of HbA2 should be carried out in all the high-risk groups with anaemia.

A case of hereditary protein S deficiency presenting with cerebral sinus venous thrombosis and deep vein thrombosis at high altitude

A 35-year-old healthy male with no history of any past medical illness developed severe headache, vomiting and drowsiness while at high altitude (4,572 m) in the eastern Himalayan ranges. He was evacuated to a tertiary-care hospital where he was diagnosed to have cerebral sinus venous thrombosis (CSVT) on magnetic resonance imaging, with deep vein thrombosis (DVT) of his right popliteo-femoral vein on color Doppler study. Investigation for thrombophilia revealed protein S (PS) deficiency in this patient. Family screening revealed low levels of PS in two elder brothers. One brother had a history of 'stroke in young' at the age of 20 years with the other being asymptomatic. This established the hereditary nature of PS deficiency. We are not aware of any previously published report on hereditary PS deficiency combined with CSVT and DVT occurring at high altitude. However, 1 case of protein C deficiency with CSVT has been reported previously.

WPW and preexcitation syndromes

Wolff-Parkinson-White syndrome is a disorder characterized by presence of an accessory pathway which predisposes patients to tachyarrhythmias and sudden death. Among patients with WPW syndrome, atrioventricular reentrant tachycardia (AVRT) is the most common arrhythmia, accounting for 95% of re-entrant tachycardias. It has been estimated that one-third of patients with WPW syndrome have atrial fibrillation (AF). AF is a potentially life-threatening arrhythmia. If an accessory pathway has a short anterograde refractory period, then rapid repetitive conduction to the ventricles during AF can result in a rapid ventricular response with subsequent degeneration to ventricular fibrillation (VF). The accessory pathway may be located anywhere along the atrioventricular valve Most of the patients are young and do not have structural heart disease hence it is important to risk stratify these patients so as to prevent the sudden death. Management of asymptomatic patients with WPW syndrome has always remained controversial Catheter ablation of accessory pathways has become an established mode of therapy for symptomatic patients and asymptomatic patients employed in high-risk professions.

Non-sinus bradyarrhythmias in very low birth weight infants

Transient bradycardias occur frequently in preterm infants with apnea of prematurity. However, occurrence of non-sinus bradyarrhythmias in very low birth weight (VLBW) infants has received very little attention. This case series documents transient bradyarrhythmias in VLBW infants. Most frequently observed arrhythmia was a non-sinus atrial rhythm followed by junctional escape rhythm. Ventricular rhythm with wide QRS complexes occurred rarely. The majority of these episodes occurred in the absence of oxygen desaturation. Increase in vagal tone is presumed to mediate this response.

Angioplasty for chronic total coronary occlusions: safety and efficacy

OBJECTIVE

To determine the short term results and safety of angioplasty in chronic coronary occlusions.

METHODS

Eighty consecutive patients undergoing percutaneous transluminal coronary angioplasty (PTCA) for chronic coronary occlusions were prospectively analyzed for acute success rate and safety of the procedure.

RESULTS

The mean age was 46.7 years (range 30-78 years). There were 72 males and eight females. Clinical presentation was recent myocardial infarction (MI) in four cases (5%), unstable angina in 20 (25%), chronic stable angina in 24 (30%) and past history of MI in 32 (40%) cases. Vessel distribution was left anterior descending artery (LAD) in 40 (50%), left circumflex artery (LCx) in 12 (15%) and right coronary artery (RCA) in 28 (35%) cases. Lesion length varied from 8 mm to 37 mm with a mean of 16.7 mm. Acute success rate was 70% (56/80). Twenty four cases (30%) had unsuccessful result due to failure to cross with wire (18 cases) or inability to cross with the balloon (six cases). One major complication in the form of type III coronary perforation was encountered which was successfully managed surgically.

CONCLUSION

Percutaneous transluminal coronary angioplasty (PTCA) in chronic total occlusion has a reasonable success rate and very low complication rate.

Does supine positioning increase apnea, bradycardia, and desaturation in preterm infants?

OBJECTIVE

The purpose of this study was to determine the effects of prone and supine positioning on the cardiorespiratory stability of preterm infants with apnea and bradycardia.

METHODS

A total of 22 preterm infants with symptomatic apnea and bradycardia (gestational age of 26.9 +/- 1.8 weeks and birth weight of 865 +/- 235 gm) were monitored for 24 hours (in four sequential 6-hour blocks) for apnea, bradycardia, and oxygen desaturation in alternating positions (prone or supine) following randomization. Postconceptional age at the time of study was 31.9 +/- 3.0 weeks. Respiratory rate, heart rate, and transcutaneous oxygen saturation were continuously monitored. All episodes of apnea (> or = 10 seconds), bradycardia (< 100 beats per minute), and oxygen desaturation (< 90%) were recorded on an event monitor. Episodes of apnea, bradycardia, and oxygen desaturation were defined as clinically significant if the following criteria were met: apnea, > or = 15 seconds; bradycardia, < 90 beats per minute; and oxygen desaturation, < 80%. All other recorded episodes were considered mild. The episodes were analyzed for statistical significance using the paired t-test.

RESULTS

No significant differences (p > 0.05) in the incidence of clinically significant apnea, bradycardia, or desaturation between supine and prone positions were seen in these preterm infants.

CONCLUSION

Our results suggest that the cardiorespiratory stability of preterm infants is not significantly compromised by supine positioning.

Effects of transient intrathoracic pressure changes (hiccups) on systemic arterial pressure

The purpose of the study was to determine the effect of transient changes in intrathoracic pressure on systemic arterial pressure by utilizing hiccups as a tool. Values of systolic and diastolic pressures before, during, and after hiccups were determined in 10 intubated preterm infants. Early-systolic hiccups decreased systolic blood pressure significantly (P < 0.05) compared with control (39.38 +/- 2.72 vs. 46.46 +/- 3.41 mmHg) and posthiccups values, whereas no significant change in systolic blood pressure occurred during late-systolic hiccups. Diastolic pressure immediately after the hiccups remained unchanged during both early- and late-systolic hiccups. In contrast, diastolic pressure decreased significantly (P < 0.05) when hiccups occurred during diastole (both early and late). Systolic pressures of the succeeding cardiac cycle remained unchanged after early-diastolic hiccups, whereas they decreased after late-diastolic hiccups. These results indicate that transient decreases in intrathoracic pressure reduce systemic arterial pressure primarily through an increase in the volume of the thoracic aorta. A reduction in stroke volume appears to contribute to the reduction in systolic pressure.

Activation of water-responsive laryngeal afferents: role of epithelial ion transport

The role of epithelial ion transport in the activation of water-responsive laryngeal afferent was investigated in anesthetized, spontaneously breathing cats. Single-fiber recordings from the peripheral cut-end of the superior laryngeal nerve were carried out to identify water-responsive laryngeal afferent. Substitution of chloride ions (Cl-) of the Krebs solution with gluconate activated the water-responsive endings when the gluconate concentration was > or = 50 mM. Amiloride (10(-4), 10(-3) and 10(-2) M), an inhibitor of epithelial sodium channels, reduced the water-responsiveness of these afferents, whereas EIPA (5 x 10(-5) M), an amiloride analogue which inhibits Na+/H+ exchange, had no effect. Both ouabain (10(-4) M), an inhibitor of Na+/K+ ATPase, and bumetanide (10(-4) M), an inhibitor of Na(+)-K(+)-2Cl- cotransport, reduced the water response, but no significant reduction in the response was observed with DIDS and DPC, two chloride channel inhibitors. These findings suggest that the epithelium modulates the water-responsiveness of laryngeal afferent but is not the primary determinant of the response.

Airway protective reflexes elicited by laryngeal ammonia: role of C-fiber afferents

Intralaryngeal ammonia induces an airway protective reflex in rats characterized by apnea, bradycardia and increased laryngeal resistance. Since ammonia is known to stimulate both the myelinated and the nonmyelinated afferents, the role of nonmyelinated afferents in eliciting the above response is unclear. The present study was designed to investigate this by utilizing two techniques that selectively block C-fiber afferents: intravenous ruthenium red and perineural application of capsaicin. Ammonia vapor was introduced into the functionally isolated larynx of anesthetized rats for 10 sec at a flow rate of 5 ml/sec. Changes in expiratory duration were expressed as the ratio between test and control expiratory time (TEmax/control). Perineural application of capsaicin (n = 8) to SLNs attenuated the responses of both intralaryngeal ammonia (TEmax/control: 12.32 +/- 1.67 to 6.89 +/- 1.62; P < 0.05) and capsaicin (8.86 +/- 2.02 to 3.35 +/- 2.17; P < 0.05). Pretreatment with ruthenium red (1 mg/kg, iv., n = 10) significantly reduced the effects of ammonia (20.43 +/- 4.38 vs. 11.66 +/- 3.21; P < 0.05) and nebulized capsaicin (16.28 +/- 4.58 to 3.90 +/- 0.83; P < 0.05). These results suggest that the C-fiber endings of the SLN play an important role in eliciting the airway protective reflexes by irritants such as ammonia.

Epithelial modulation of afferent nerve endings: differential effects of amiloride on afferent subtypes

Mechanisms underlying the differing chemosensitivity of laryngeal afferents have not been defined. The role of airway epithelium in transducing the chemical stimuli to neural signals was investigated by using Na(+)- and Cl(-)-channel inhibitors in anesthetized spontaneously breathing cats. Single-fiber action potentials were recorded from the peripheral cut end of the superior laryngeal nerve. Luminal application of amiloride (10(-4) M), an inhibitor of epithelial Na+ channels, reduced the responsiveness of non-respiratory-modulated endings (n = 25) to distilled water (65.76 +/- 5.77 vs. 50.67 +/- 5.13 Hz; P < 0.01). Water responsiveness of these endings was unaffected by 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid and diphenylamine-2-carboxylate, two Cl(-)-channel blockers. Respiratory-modulated endings (water responsive, n = 8; water nonresponsive, n = 9) were unaffected by Na(+)- and Cl(-)-channel blockers. These results suggest that epithelial Na+ channels play a role in the modulation of non-respiratory-modulated laryngeal endings. The lack of an effect by amiloride on other subtypes may be due to differences in location or intrinsic properties of nerve endings. Cl- channels do not appear to play an important role in the modulation of laryngeal afferents targeted in this study.

Influence of intralaryngeal CO2 on the response of laryngeal afferents to upper airway negative pressure

Effects of intralaryngeal CO2 on the response of superior laryngeal afferents to negative pressure were investigated in 20 anesthetized spontaneously breathing adult cats. Single-fiber action potentials were recorded from the peripheral cut end of the superior laryngeal nerve. The larynx was exposed to negative pressure during inspiration when the animal breathed against an occluded upper airway. Among the 99 receptors evaluated, 54 were respiratory modulated and 45 were nonmodulated endings. The effect of intralaryngeal CO2 on the response of 39 receptors responding to negative pressure was determined by exposure of the larynx to CO2 or air for 1 min followed immediately by upper airway occlusion. The mean discharge frequency of 22 fibers inhibited by negative pressure was 32.4 +/- 2.6 Hz during air trials compared with 29.9 +/- 2.6 Hz during CO2 trials (P < 0.005). During occlusion of the upper airway after the warm humidified air trial, the discharge frequency of these endings decreased to 24.2 +/- 2.3 Hz compared with 17.5 +/- 2.2 Hz after CO2 trial (P < 0.001). The mean discharge frequencies of 17 fibers stimulated by negative pressure were 3.7 +/- 2.6 and 4.4 +/- 1.8 Hz, respectively, during air and CO2 trials. The mean frequencies increased to 14.7 +/- 3.5 Hz (air) and 18.6 +/- 4.0 Hz (CO2) during upper airway occlusions (P < 0.01). We conclude that intralaryngeal CO2 can alter the response of pressure-sensitive laryngeal afferents, thereby having a role in the maintenance of upper airway patency.(ABSTRACT TRUNCATED AT 250 WORDS)

Respiratory and cardiovascular effects of inhaled and intravenous bradykinin, PGE2, and PGF2 alpha in dogs

Prostaglandins (PGs) and bradykinin act as potent respiratory irritants in both normal and asthmatic subjects, but their sites of action are unknown. We compared the cardiorespiratory effects of bradykinin, PGE2, and PGF2 alpha nebulized into the isolated "in situ" larynx, inhaled into the tracheobronchial tree, and injected intravenously in anesthetized spontaneously breathing dogs. Laryngeal administration only resulted in a brief burst of rapid shallow breaths produced by bradykinin (1,000 micrograms/ml) in one of five dogs. Tracheobronchial administration of bradykinin (1,000 micrograms/ml) increased breathing rate and tidal volume (VT) in four of seven dogs without changing cardiovascular parameters, whereas PGE2 (500 micrograms/ml) caused similar effects in two of six dogs. Lower concentrations of both agents were essentially without effect. PGF2 alpha (50-500 micrograms/ml) inhaled into the lower airway increased breathing rate, reduced VT, and caused a concentration-dependent bronchoconstriction that was significantly reduced by atropine. Inhaled PGF2 alpha only slightly increased arterial blood pressure (5.8 +/- 2.8%) and heart rate (12.0 +/- 6.4%). Intravenous PGF2 alpha (5 micrograms/kg) increased upper and lower airway resistances, which were accompanied by a decrease in breathing rate and VT, hypertension, and bradycardia. Bradykinin (1 micrograms/kg) and PGE2 (1 and 3 micrograms/kg) produced apnea followed by rapid shallow breathing, bradycardia, and hypotension. These results indicate that the tracheobronchial tree is considerably more responsive to aerosolized bradykinin, PGE2, and PGF2 alpha than the laryngeal region. Moreover, the stronger effects produced by intravascular administration suggest a greater accessibility of rapidly adapting stretch receptors and C-fiber endings from the vascular bed than from the airway lumen.

Decrease in lung volume-related feedback enhances laryngeal reflexes to negative pressure

Negative pressure applied to the upper airway has an excitatory effect on the activity of upper airway muscles and an inhibitory effect on thoracic inspiratory muscles. The role of lung volume feedback in this response was investigated in 10 anesthetized spontaneously breathing adult rabbits. To alter lung volume feedback, the lower airway was exposed to SO2 (250 ppm for 15 min), thereby blocking slowly adapting receptors (SARs). Negative pressure pulses (5, 10, and 20 cmH2O, 300-ms duration) were applied to the functionally isolated upper airway before and after SAR blockade. Tracheal airflow and electromyogram (EMG) of the genioglossus and alae nasi were recorded. Peak EMG, peak inspiratory flow, tidal volume, and respiratory timing of control breaths (3 breaths immediately preceding test) and test breaths were determined. Analysis of variance was used to determine the significance of the effects. Negative pressure pulses increased peak EMG of genioglossus and alae nasi and inspiratory duration and decreased peak inspiratory flow. These effects were larger after SAR blockade. We conclude that a decrease in volume feedback from the lung augments the response to upper airway pressure change.

Sucking patterns of neonates during bottle feeding: comparison of different nipple units

The present study was designed to compare the sucking pattern of term and preterm infants during bottle feeding with different types of nipple units (Enfamil single-hole nipple units for term and preterm infants and SMA Nuk nipple units). In addition, the sucking pattern of term neonates during a feeding regimen commonly used in many feeding studies was evaluated (reservoir nipple system). In this system milk flows from a reservoir through a tube and depends on the sucking pressure generated by the infant. Only the Enfamil single-hole nipple units for term and preterm infants were compared in preterm infants. No significant difference in sucking frequency was observed in term neonates with different types of nipple units. Although the mean sucking pressures generated tended to be less among nipple units with higher flow, these differences were not statistically significant. Similarly, no significant difference in total sucking or feeding time was observed among the three nipple units tested. Sucking pressures generated by term infants were significantly less when milk flow was increased markedly utilizing the reservoir system. In preterm infants no differences in sucking frequency, sucking pressure, mean flow, or total feeding time were observed when sucking patterns with term and preterm nipple units were compared. Implications of these findings in feeding neonates are discussed.

Response of laryngeal mechanoreceptors to high-frequency pressure oscillation

High-frequency pressure oscillations (HFPO) in the upper airway induce arousal, activation of genioglossus muscle, and bronchoconstriction. The present study was designed to determine the response of superior laryngeal nerve afferent fibers to HFPO. In 10 anesthetized dogs spontaneously breathing through a tracheal cannula, the upper airway was converted to a closed system. The activity of thin bundles separated from the peripheral cut end of the superior laryngeal nerve was monitored. Of 104 mechanoreceptors identified, 87 were classified as respiratory modulated and 17 as non-respiratory modulated on the basis of their response to transmural pressure change and muscle activity. The responses of these fibers to HFPO of +/- 2.5 cmH2O at 10, 20, and 30 Hz were determined. Among the respiratory-modulated receptors, 86 of 87 increased their activity in response to HFPO. Of the 17 non-respiratory-modulated receptors, 12 receptors showing a random or tonic activity did not respond to HFPO, whereas the 5 that were silent during control condition responded exclusively to HFPO. Our results show that HFPO of similar frequency but much less magnitude than snoring is capable of activating the vast majority of laryngeal mechanoreceptors. Pressure-sensitive respiratory-modulated endings appear to mediate the arousal and genioglossal response, whereas non-respiratory-modulated receptors responding to HFPO presumably mediate the bronchoconstrictive response.

Breathing patterns of preterm infants during bottle feeding: role of milk flow

Milk flow achieved during feeding may contribute to the ventilatory depression observed during nipple feeding. One of the important determinants of milk flow is the size of the feeding hole. In the first phase of the study, investigators compared the breathing patterns of 10 preterm infants during bottle feeding with two types of commercially available (Enfamil) single-hole nipples: one type designed for term infants and the other for preterm infants. Reductions in ventilation, tidal volume, and breathing frequency, compared with prefeeding control values, were observed with both nipple types during continuous and intermittent sucking phases; no significant differences were observed for any of the variables. Unlike the commercially available, mechanically drilled nipples, laser-cut nipple units showed a markedly lower coefficient of variation in milk flow. In the second phase of the study, two sizes of laser-cut nipple units, low and high flow, were used to feed nine preterm infants. Significantly lower sucking pressures were observed with high-flow nipples as compared with low-flow nipples. Decreases in minute ventilation and breathing frequency were also significantly greater with high-flow nipples. These results suggest that milk flow contributes to the observed reduction in ventilation during nipple feeding and that preterm infants have limited ability to self-regulate milk flow.

Afferent activity in the external branch of the superior laryngeal and recurrent laryngeal nerves

We investigated the presence of respiratory-modulated receptors in the recurrent laryngeal nerve (RLN) and the external branch of the superior laryngeal nerve (ExtSLN) in anesthetized, spontaneously breathing dogs. Of 39 receptors recorded from the ExtSLN, the vast majority responded with a slowly adapting discharge to compression of the cricothyroid muscle, and only 1 responded to probing of the laryngeal mucosa. Ten receptors showed a respiratory modulation. All 30 receptors recorded from the RLN responded to probing of the laryngeal lumen, most of them (60%) with a rapidly adapting response. Seven of the slowly adapting receptors exhibited a respiratory modulation; 38% of the receptors tested were stimulated by water, and only 15% by smoke. No receptors stimulated by laryngeal cooling were identified in either nerve. Our study indicates that in the RLN and the ExtSLN there are relatively few afferents responding to changes in transmural pressure and mechanical irritation, as compared to the internal branch of the SLN. The relative scarcity of receptors responding to transmural pressure and irritant stimuli is consistent with previous observations in dogs that indicate a preponderant role for afferents in the internal branch of the SLN in the reflex responses to laryngeal stimulation.

Motor activity and apnea in preterm infants. Is there a causal relationship?

It is generally believed that all apneic episodes in preterm infants occur during sleep. Because occurrence of apnea during wakefulness has been documented in these infants, we investigated its frequency and characteristics in 10 premature infants using simultaneous polygraphic and video recordings. Behavioral arousal with motor activity preceded the onset of apnea in 202 episodes. Approximately 60% of episodes began within 15 s of arousal. Whereas most of the episodes were short and asymptomatic, hypoxia and/or bradycardia developed during 18 apneic spells, and these episodes accounted for a third of all apneic episodes that resulted in bradycardia or hypoxia in these infants studied. Essentially, all movement-related apneas (17 of 18) began within 15 s of arousal; motor activity continued throughout apnea in 13, whereas apnea resolved after cessation of motor activity in the remaining five. The main finding of the present study is that movement-related apnea is far more frequent than previously recognized. This finding is important in the clinical management of preterm infants with apnea.

Behavioral arousal in newborn infants and its association with termination of apnea

Arousal is an important protective mechanism that aids in the resolution of obstructive sleep apnea in adults and children, but its role in neonatal apnea has not been investigated. The primary aim of the present study was to determine the role of arousal in the termination of apnea in preterm infants. Videorecording was used to identify spontaneous behavioral arousal in a group of healthy full-term (n = 7) and preterm (n = 10) infants before and during polygraphic monitoring of cardiorespiratory variables and in a group of preterm infants with apnea (n = 10) during similar polygraphic monitoring. Spontaneous arousal rates (mean +/- SE) in full-term infants before and during polygraphic monitoring were 0.18 +/- 0.03 and 0.23 +/- 0.07 episodes/min, respectively. Corresponding values in nonapneic preterm infants were 0.24 +/- 0.03 and 0.24 +/- 0.02 episodes/min. In apneic preterm infants, mean spontaneous arousal rate during polygraphic recording was 0.26 +/- 0.02, but it was considerably higher during apneic sleep periods (0.59 +/- 0.17) than during nonapneic sleep periods (0.25 +/- 0.01). The frequency of occurrence of arousal was significantly higher (P less than 0.005) in long vs. short apnea, mixed vs. central apnea, and severe vs. mild apnea. Although a clear association between arousal and apneic resolution was observed in preterm infants, lack of arousal responses in a large number of apneic episodes suggests that behavioral arousal is not essential for the termination of apnea in these infants.

Response of laryngeal receptors to water solutions of different osmolality and ionic composition

Water-responsive laryngeal receptors with fibres in the superior laryngeal nerve were studied to characterize the specific physicochemical properties of aqueous solutions that activate these endings. The responses to water (37 degrees C) of 141 receptors were studied in 39 anaesthetized dogs breathing through a tracheostomy with the larynx functionally isolated. Of the 89 receptors stimulated by water, 53 were also challenged with isosmotic (275-315 mOsm) solutions of dextrose and sodium gluconate at 37 degrees C. Receptors that only responded to water (n = 31) with a long delay, long duration discharge were generally respiratory modulated. On the other hand, laryngeal receptors that responded to all test solutions (n = 22) with a short delay, short duration discharge were generally not respiratory modulated. We conclude that the former type of receptor responds to lower osmolality, whereas the latter responds to the lack of chloride ions in the test solutions. These two types of receptor may be responsible for the cough and bronchoconstriction induced by inhaled aerosols of different osmolalities and ionic compositions.

Cooling mediates the ventilatory depression associated with airflow through the larynx

Although constant airflow through the upper airway has been shown to induce ventilatory depression in anesthetized newborn animals, the role of laryngeal temperature in this response has not been studied. Experiments were performed in fourteen 1-5 day-old anesthetized puppies breathing through a tracheostomy. Tidal volume and laryngeal temperature were recorded while a constant stream of air (15-25 ml/sec) at room temperature was passed in the expiratory direction for 20 sec through the isolated upper airway. Warm (35-37 degrees C), humidified air at the same flow served as control. When laryngeal temperature was decreased by 7.5 +/- 0.9 degrees C, a marked change in breathing pattern was observed (VT = 54 +/- 5, TI = 187 +/- 33, TE = 636 +/- 179, VT/TI = 45 +/- 10% of control; n = 9). Warm air at the same flow induced no significant changes. Superior laryngeal nerve section abolished the effects of cooling on breathing pattern. In 5 puppies we compared the effect of 'fast' and 'slow' laryngeal cooling. Fast trials altered breathing pattern earlier than slow trials. We conclude that the depressant effect of airflow through the upper airway is entirely due to a decrease in laryngeal temperature and is mediated by superior laryngeal nerve afferents.

Carbon dioxide-responsive laryngeal receptors in the dog

The purpose of this study was to relate the carbon dioxide (CO2) response of laryngeal receptors to their behavior during the breathing cycle (i.e. their response to transmural pressure changes, laryngeal movement or decreases in temperature) or during exposure to irritant stimuli (water or cigarette smoke). In 9 anesthetized mongrel dogs breathing spontaneously through a tracheostomy, unit activity from the superior laryngeal nerve was recorded while warmed and humidified gas mixtures (air or 10% CO2 in O2) were passed, for 1 min, through the functionally isolated upper airway in the expiratory direction. None of the 10 cold receptors studied were affected by CO2. Eleven of 20 laryngeal non-modulated mechano-receptors were stimulated (from 0.3 to 1.6 imp/sec) by exposure to CO2. These CO2-responsive receptors were also stimulated by known irritant stimuli (cigarette smoke, water), although not all receptors which responded to these irritants were stimulated by CO2. Twelve of 33 respiratory-modulated receptors were affected by CO2; 4 were stimulated and 8 inhibited. Receptors inhibited by CO2 were also inhibited by negative pressure while receptors stimulated by CO2 were also stimulated by negative pressure. These results show that CO2-responsive laryngeal receptors are not specialized endings. Although it is not clear to what extent each separate group of laryngeal receptors is involved, each may contribute to the reflex bradypnea which has been observed during exposure of the upper airway to elevated levels of CO2. However, the importance of CO2-responsive laryngeal receptors in physiological conditions remains unclear.

Determinants of milk flow through nipple units. Role of hole size and nipple thickness

The aim of the present study was to elucidate the role of hole size and thickness in determining milk flow through nipple units during bottle feeding. Commonly used standard nipple units (SMA single-hole, Enfamil single-hole, and Twist-on) for term and preterm infants, as well as Nuk-type nipple units (SMA Nuk, Enfamil Natural, and Nuk) were tested. The size of the nipple hole and wall thickness were determined for each nipple unit. Airflow was measured by forcing pressurized air through the feed hole. Simulated sucks were used to measure the milk flow. A marked variability in airflow and milk flow was observed within and among the various types of nipple units studied. Within each type of nipple unit, both milk flow and airflow measurements correlated well with hole size. The thickness of the nipple units contributed minimally to the observed variability. We conclude that differences in hole size primarily account for the observed variability in milk flow. This finding may be clinically important in that rapid milk flow can lead to apnea and bradycardia in some preterm infants. The above observations imply that design changes are necessary to reduce the variability of milk flow within each nipple type. Moreover, milk-flow measurements made using a simple mechanical system and airflow measurements used by the industry are equally sensitive to evaluate nipple flow.

Water-responsive laryngeal receptors in the dog are not specialized endings

The primary purpose of this study was to ascertain whether laryngeal receptors activated by water are specialized endings or whether they also respond to other stimuli, such as pressure, temperature and laryngeal motion as they occur during the breathing cycle. In 35 anesthetized mongrel dogs, breathing spontaneously through a lower cervical tracheostomy, water and other test solutions at approximately 37 degrees C were injected into the functionally isolated larynx with a small catheter. Of the 130 receptors studied, none of the cold receptors (N = 13) responded to water, whereas approximately 60% of all laryngeal mechanoreceptors (72 of 117) responded with either a short delay, short duration or a long delay, long duration response. In general the former pattern of response was exhibited by nonrespiratory-modulated receptors, whereas the latter was typical of respiratory-modulated receptors. The specific nature of the stimulus (hypotonicity or lack of chloride ion) of the water response was further studied in 53 receptors with isoosmotic solutions of dextrose and sodium gluconate. The long delay, long duration response was dependent on a decreased osmolality, while the short delay, short duration response was dependent on the lack of chloride ion of the test solutions. All water-responsive receptors tested (N = 17) were blocked within 50 sec by topically applied 2% lidocaine and thus presumed to be superficial. However, 10 receptors which did not respond to water were also blocked within 50 sec, suggesting that not all superficial receptors are stimulated by water. Based on these observations, we propose that changes in osmolality or ionic composition of the laryngeal surface liquid could play an important role in modifying reflexes involved in the maintenance of upper airway patency.

Reflex responses to capsaicin: intravenous, aerosol, and intratracheal administration

Intravenous capsaicin elicits the "pulmonary chemoreflex" (apnea, bradycardia, and hypotension) presumably through the stimulation of "pulmonary C-fibers." The present study was designed to ascertain whether tracheobronchial C-fibers play a role in the above reflex response. We compared the effects of capsaicin injected intravenously, administered as an aerosol, and administered topically into the intrathoracic trachea in anesthetized dogs (n = 17) and rats (n = 17). We measured esophageal, subglottic, and arterial pressures together with abdominal muscle electromyogram. Changes in expiratory duration [(TE), measured as the ratio TEtest to TEcontrol, mean +/- SD] due to capsaicin were similar with all three routes of administration in both dogs (intravenous, 7.9 +/- 4.6; aerosol, 5.5 +/- 3.1; topically into intrathoracic trachea, 7.1 +/- 4.8) and rats (intravenous, 22.6 +/- 10.3; aerosol, 11.1 +/- 8.2; topically into intrathoracic trachea, 21.6 +/- 4.6). An increase in laryngeal resistance was a constant finding in the rat, but it was less frequent in the dog. Cardiovascular responses consisting of bradycardia and hypotension occurred with all three routes of administration but had longer delays than the respiratory responses. Capsaicin instillation into the extrathoracic trachea in dogs (n = 7) also induced qualitatively similar cardiorespiratory responses. We conclude that 1) capsaicin-sensitive receptors are accessible from both the pulmonary circulation and the airway lumen and 2) afferents, even in the extrapulmonary portion of the tracheobronchial tree, can play a role in the reflex responses to intraluminal capsaicin.

Time of application of negative pressure pulses and upper airway muscle activity

Effect of upper airway pressure changes on thoracic inspiratory muscles has been shown to depend on the time of application during the breathing cycle. The present study was designed to investigate the importance of the time of application of upper airway negative pressure pulses on upper airway muscles. The upper airway was functionally isolated into a closed system in 24 anesthetized spontaneously breathing rabbits. Negative pressure pulses were applied in early (within the first 200 ms) and late (greater than or equal to 200 ms) inspiration, while electromyograms (EMG) of the diaphragm (Dia), genioglossus (GG), alae nasi (AN), and/or posterior cricoarytenoid (PCA) muscles were simultaneously monitored. When negative pressure pulse was applied in early inspiration, the increase in GG activity was greater [0.49 +/- 0.37 to 4.24 +/- 3.71 arbitrary units (AU)] than when negative pressure was applied in late inspiration (0.44 +/- 0.29 to 2.64 +/- 3.05 AU). Similarly, increased activation of AN (2.63 +/- 1.01 to 4.26 +/- 1.69 AU) and PCA (3.46 +/- 1.16 to 6.18 +/- 2.93 AU) was also observed with early inspiratory application of negative pressure pulses; minimal effects were seen in these muscles with late application. An inhibitory effect on respiratory timing consisting of a prolongation in inspiration (TI) and a decrease in peak Dia EMG/TI was observed as previously reported. These results indicate that the time of application of negative pressure during the breathing cycle is an important variable in determining the magnitude of the response of upper airway muscles.(ABSTRACT TRUNCATED AT 250 WORDS)

Sucking and breathing patterns during breast- and bottle-feeding in term neonates. Effects of nutrient delivery and composition

We attempted to determine whether differences in milk composition or flow rate are the primary determinants in altering breathing pattern during nipple-feeding. In the first phase of the study, 15 neonates were studied during breast-feeding and bottle-feeding; in the second phase, 15 were evaluated during bottle-feeding of expressed human milk and formula. A reduction in inspiratory duration was observed with all feeding regimens. Moreover, significant prolongation in expiratory duration and reduction in breathing frequency were observed during bottle-feeding of formula and expressed human milk (compared with control); these effects were greater with formula feeding. Higher sucking frequency was observed during breast-feeding compared with bottle-feeding; no difference in suckling frequency or sucking pressure was observed between bottle-feeding of expressed human milk and formula. We conclude that most of the differences in sucking and breathing patterns between breast- and bottle-feeding can be attributed to nutrient delivery rather than nutrient composition.

Specificity of esophageal electrode recordings of posterior cricoarytenoid muscle activity

Esophageal electrodes have been used for recording the electromyographic (EMG) activity of the posterior cricoarytenoid muscle (PCA). To determine the specificity of this EMG technique, esophageal electrode recordings were compared with intramuscular recordings in eight anesthetized mongrel dogs. Intramuscular wire electrodes were placed in the right and left PCA, and the esophageal electrode was introduced through the nose or mouth and advanced into the upper esophagus. On direct visualization of the upper airway, the unshielded catheter electrode entered the esophagus on the right or left side. Cold block of the recurrent laryngeal nerve (RLN) ipsilateral to the esophageal electrode was associated with a marked decrease in recorded activity, whereas cold block of the contralateral RLN resulted only in a small reduction in activity. After supplemental doses of anesthesia were administered, bilateral RLN cold block essentially abolished the activity recorded with the intramuscular electrodes as well as that recorded with the esophageal electrode. Before supplemental doses of anesthesia were given, especially after vagotomy, the esophageal electrode, and in some cases the intramuscular electrodes, recorded phasic inspiratory activity not originating from the PCA. Therefore, one should be cautious in interpreting the activity recorded from esophageal electrodes as originating from the PCA, especially in conditions associated with increased respiratory efforts.

Respiratory activity of the cricothyroid muscle

Single motor units of the cricothyroid muscle (CT), the contralateral CT electromyogram, and the posterior cricoarytenoid muscle (PCA) electromyogram were recorded in anesthetized, spontaneously breathing dogs. In quiet breathing the CT was active predominantly in inspiration, and distinct phasic expiratory activity was observed at lighter levels of anesthesia. Both the CT and PCA increased their inspiratory and expiratory activity with hypercapnia, whereas only their inspiratory activity increased in response to negative pressure and/or absence of volume feedback. Cold blockade of either the recurrent laryngeal nerves or the external branch of the superior laryngeal nerves did not modify CT or PCA activity. In general, activity of CT motor units reflected the behavior of the whole muscle, but different units were recruited at different levels of CT activity. Even though the majority exhibited similar thresholds for inspiration and expiration, some units showed a lower threshold for either one, suggesting some degree of specialization. However, for a few units with high threshold for inspiration the expiratory threshold could not be determined, since a comparable level of CT activity was not achieved in expiration.

Laryngeal paralysis on receptor and reflex responses to negative pressure in the upper airway

Mechanoreceptors affected by changes in transmural pressure and/or contraction of intrinsic muscles are present in the larynx. This study is designed to test the hypothesis that laryngeal paralysis alters laryngeal mechanoreceptor and reflex responses to collapsing pressure. Experiments were carried out on anesthetized, spontaneously breathing dogs. The activity of 65 mechanoreceptors was recorded from the superior laryngeal nerve during upper airway occlusion before and during laryngeal paralysis induced by cold block of both recurrent laryngeal nerves (RLN). Esophageal and upper airway pressures were also recorded. Thirty-three laryngeal mechanoreceptors stimulated by negative pressure decreased their inspiratory activity during upper airway occlusion from 48.1 to 30.4 imp/sec when the RLNs were blocked. In contrast, 21 inspiratory modulated mechanoreceptors inhibited by negative pressure and 11 responding only to negative pressure did not change their activity during RLN block. The effect of laryngeal paralysis on the cricothyroid muscle response to negative pressure was assessed in 7 dogs and found to be minimal. These results suggest that 'drive' receptors stimulated by negative pressure do not play a significant role in the reflex activation of upper airway dilating muscles.

Control of Upper Airway Muscles

The upper airway serves several functions for both the alimentary and respiratory systems. Whereas some of the functions are mutually exclusive, others can be performed simulatneously, generally with some reciprocal interference. Compromising and establishing priorities are the main features of upper muscle control.

Interaction between upper airway negative pressure pulses and CO2 on breathing pattern

The interaction between CO2 and negative pressure pulses on breathing pattern was investigated in 10 anesthetized, spontaneously breathing rabbits. The upper airway was functionally isolated into a closed system. A servo-respirator triggered by the inspiratory activity of the diaphragm was used to apply pressure pulses of -15 cmH2O to the isolated upper airway in early inspiration while the animal was breathing room air, 100% O2, 6% CO2 in O2, or 9% CO2 in O2. The negative pressure pulses produced a reversible inhibition of inspiration in most trials with resultant increase in inspiratory duration (TI); no change was observed in peak diaphragmatic electromyogram (Dia EMG) or expiratory duration, whereas a decrease was seen in mean inspiratory drive (peak Dia EMG/TI). This prolongation of inspiratory duration and decrease in mean inspiratory drive with negative pressure pulses persisted at higher levels of CO2; the slopes of the test breaths were not significantly different from that of control breaths. These results suggest that upper airway negative pressure pulses are equally effective in altering the breathing pattern at all levels of CO2.

Nipple units for newborn infants: a functional comparison

Milk flow characteristics of nipple units commonly used in the neonatal period were compared in the laboratory using a mechanical system. The number of simulated sucks required to empty 120 mL of formula was determined for each nipple unit. In general, the number of simulated sucks required to empty the bottle decreased when the applied negative pressure was increased from -60 to -120 cm of H2O except for SMA nipple units for premature infants. The Nuk type required less sucks (ie, higher flow) than standard nipple units. Among the Nuk-type nipple units, the SMA nipple had the highest mean flow and Enfamil Natural the lowest mean flow; among the standard nipple units, SMA single-hole had the highest flow and Ross Twist-on had the lowest flow. However, wide variability in performance was observed not only between different types of nipple units but also within the same type. Flow characteristics of nipple units for preterm infants overlapped markedly, with that for term neonates with Enfamil nipples exhibiting the highest flow. Clinical relevance of these differences in flow characteristics among the nipple units is discussed.

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.

Characteristics of laryngeal cold receptors

This study was designed to further characterize the properties of previously described laryngeal cold receptors (Respir. Physiol. 59:35, 1985). Single unit action potentials were recorded from the internal branch of the superior laryngeal nerve (SLN) in anesthetized, spontaneously breathing dogs. The nervous conduction of fibers originating from 12 laryngeal cold receptors was blocked at a mean (+/- SE) temperature of 18.8 +/- 0.7 degrees C. Twelve receptors were localized on the edge of the vocal folds in correspondence of the vocal process of the arytenoid cartilage. Topical anesthesia (2% lidocaine) blocked their activity within 4-18 sec, suggesting a superficial location. Paralysis of the vocal folds during spontaneous breathing through the upper airway did not alter the activity of 9 of 13 cold receptors. On the other hand, 7 of 12 cold receptors tested with constant flow showed respiratory modulation and laryngeal paralysis abolished the modulation of 3 of these tested with a constant flow of air. During progressive cooling in a stepwise fashion, as in frigid air breathing, laryngeal cold receptors maintained a phasic discharge. Our results indicate that these endings are particularly suited for detecting changes in temperature.

Respiratory control during nipple feeding in preterm infants

The present study was designed to investigate the development of respiratory control during feeding in premature infants. Cardiorespiratory disturbances during feeding were evaluated with polygraphic monitoring in 24 premature infants within 1 week of beginning nipple feeds. During the initial study, 15 infants exhibited one or more episodes of short apnea (greater than or equal to 10 sec) and three infants exhibited prolonged apnea (greater than or equal to 20 sec). Bradycardia developed in seven infants; apnea and decreases in oxygen saturation invariably preceded the development of bradycardia. In contrast, short apnea occurred during sleep in five infants and associated bradycardia developed in four infants. Occurrence of apnea during sleep was significantly lower than that observed during feeding (P less than 0.05). Subsequently, 18 of these infants were reevaluated 7-10 days later. Seven infants developed one or more episodes of short apnea, five developed prolonged apnea, and four developed bradycardia. These occurrences were not significantly different from those observed during the initial study (P greater than 0.05). Short apnea persisted during sleep in four infants during reevaluation. Most of the apneic episodes in both studies were mixed apnea. The high frequency of cardiorespiratory disturbances during the first 2 weeks of nipple feeding indicates that in most preterm infants respiratory control during feeding is still immature at the postconceptional age of 35-36 weeks.

Relative contribution of ribcage and abdomen during augmented breaths in infants

Changes in the relative contribution of ribcage and abdomen during augmented breaths were assessed in two groups of infants (less than 2 weeks and 4-12 weeks old). In both groups ribcage and abdominal movements increased during augmented breaths in quiet sleep. In the older infants the relative increase was greater for the ribcage (P less than 0.05) when expressed either as a ribcage to abdominal movement ratio or as a percent of the respective movements during control breaths. In contrast, no significant change of the relative contribution was observed in the younger age group. During active sleep paradoxical movement of the ribcage, observed during tidal breaths, continued during the first phase of the augmented breath; however, during the second phase of inspiration, the ribcage moved outward in all but one infant. Our results show that marked increases in tidal volume are associated with greater increases in ribcage contribution in infants beyond the neonatal period. This may reflect differences in recruitment patterns of intercostal muscles during development.

Respiratory activity in the superior laryngeal nerve of the rabbit

We studied the respiratory modulation of laryngeal afferents and their response to transmural pressure in 24 anesthetized, spontaneously breathing rabbits. Laryngeal afferent activity has a predominant inspiratory augmentation during tracheal breathing or tracheal occlusion that can be accounted for by the respiratory movement transmitted to the larynx through the trachea. During upper airway breathing or upper airway occlusion SLN afferent activity increases in expiration and decreases in inspiration. This respiratory modulation is due to changes in upper airway pressure (Pua). In fact, positive pressure stimulates SLN afferent activity, while negative pressure inhibits it. Mechanical restriction of epiglottal movement reduced the response to Pua changes during upper airway occlusion and application of maintained positive (0.1-0.5 kPa) and negative (-0.1 to -0.5 kPa) pressures (P less than 0.005). Furthermore, surgical removal of epiglottis decreased the baseline activity of SLN to 16.5% of control. These experiments suggest that in the rabbit the epiglottis is the main source of SLN afferent activity and that its displacement, due to changes in Pua, is the most important factor for modulating SLN activity. Most of the laryngeal receptors showed an inspiratory augmentation with tracheal breathing and occlusion, were stimulated by positive pressure and inhibited by negative pressure, reflecting the behavior observed in the whole nerve.

Acute effect of cigarette smoke on laryngeal receptors

Upper airway exposure to cigarette smoke elicits reflex changes in breathing pattern. To examine whether laryngeal afferents are affected by cigarette smoke, neural activity was recorded from the peripheral cut end of superior laryngeal nerve in anesthetized dogs. A box-balloon system, connected to the breathing circuit, allowed smoke to be inhaled spontaneously through the isolated upper airway while preserving its normal respiratory flow and pressure. Our results showed the following. Inhalation of cigarette smoke (25-50% concentration, 300-400 ml) caused a marked increase in activity of laryngeal irritant receptors which were either silent or randomly discharging during control breathing [their activity increased from a control value of 1.67 +/- 0.50 (mean +/- SE; n = 21) to a peak of 5.03 +/- 0.85 impulses/s in 11-15 s]. The activity of laryngeal cold receptors was reduced to 77.3 and 63.8% of control (n = 9) during the two breaths of smoke inhalation, respectively. After returning toward the base-line activity, a more pronounced inhibition (26.3% of control) occurred at three to nine breaths after the smoke inhalation. A small but significant decrease (88.5% of control) in the inspiratory discharge of laryngeal mechanoreceptors was observed during the first test breath. These effects were independent of the CO2 content of the smoke. Furthermore, there was no difference between the responses of these laryngeal afferents to high- and low-nicotine cigarette smoke.

Effects of airway cooling on tracheal stretch receptors

We studied the effect of cold air on tracheal slowly adapting stretch receptors (SAR) in 6 anesthetized, spontaneously breathing dogs. Air at constant flow and two different temperatures was passed through an isolated segment of the extrathoracic trachea. We recorded SAR action potentials, esophageal pressure, tracheal pressure and temperature. With a reduction in tracheal temperature of approximately 10 degrees C the steady state response of 30 SARs to a distending pressure of 1.0 kPa decreased to 75% of control (P less than 0.001). At lower distending pressure the inhibitory effect of tracheal cooling decreased: 87% of control at 0.5 kPa (P less than 0.05, n = 8) and 96% of control at 0.2 kPa (P greater than 0.05, n = 8). The response of 13 tracheal SARs to sinusoidal pressure oscillations (0.15 kPa) superimposed on a bias pressure (0.5 kPa) was reduced (P less than 0.01) by local cooling to the same extent at the 2 pressure extremes ('peak' value = 71% of control; 'valley' = 67% of control), resulting in a similar change in receptor discharge within the oscillatory cycle. The inhibitory effect of airway cooling on stretch receptors may play a role in cold-induced bronchoconstriction.

Effect of upper airway pressure pulses on breathing pattern

Importance of the time of application of upper airway pressure pulses on breathing pattern was investigated in 19 anesthetized, spontaneously breathing rabbits. The upper airway was functionally isolated into a closed system. A servo-respirator, triggered by the inspiratory activity of the diaphragm, was used to apply pressure pulses to the isolated upper airway. Negative pressure pulses of -5, -10, and -15 cm H2O when applied in early inspiration (within the first half) produced a reversible inhibition of inspiration in most trails (86.2%). This resulted in a prolongation of inspiratory duration (TI) and a decrease in mean inspiratory drive (P.Dia/TI) whereas peak diaphragm (P.Dia) activity and expiratory duration (TE) remained largely unaffected. In the remaining 13.8% of trials, an irreversible inhibition with short TI and reduced P.Dia activity was observed. In contrast, with late application of negative pressure pulses the only significant change was a shortening of TI. When positive pressure pulses were applied during expiration, no significant change in TE occurred with either early or late application. A significant prolongation of subsequent TI was seen irrespective of the time of positive pressure application. These results indicate that time of application during the respiratory cycle is an important variable in determining the response to upper airway pressure pulses.

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.

Effects of cooling on laryngeal reflexes in the dog

We investigated the reflex effects of laryngeal cooling on posterior cricoarytenoid (PCA) muscle activity, breathing pattern, arterial blood pressure and heart rate. We performed experiments on 9 anesthetized, spontaneously breathing dogs. Laryngeal temperature was decreased by passing cold air through the functionally isolated larynx while the dog was breathing through a tracheostomy. Inspiratory and expiratory durations, esophageal pressure, peak PCA activity, heart rate and blood pressure did not change significantly during laryngeal cooling. Upon interruption of cold airflow, while the laryngeal temperature was returning to control values, we assessed PCA response to upper airway occlusion. At laryngeal temperatures of 20-25 degrees C the peak PCA activity during upper airway occlusion was approximately 2/3 of that observed at control temperature (approximately equal to 33 degrees C). This difference was abolished by topically applied anesthetics or by superior laryngeal nerve section. In addition, we recorded from 4 laryngeal mechanoreceptors stimulated by negative pressure; their response to upper airway occlusion was reduced to 1/2 by laryngeal cooling. These results indicate that laryngeal cooling has a marked depressive effect on the PCA response to collapsing pressure in the larynx, thereby compromising the mechanism subserving upper airway patency.

Laryngeal receptors and their reflex responses

The afferent activity originating from the larynx shows a considerable respiratory modulation. Receptors responding to pressure changes, inspiratory airflow (cold), and laryngeal movements have been identified. In addition, other receptors without a respiratory modulation are also described. Possible reflex effects of these receptors on breathing pattern, upper airway patency, and defense mechanisms in both adults and newborns are discussed.