Saliva production and surface tension: influences on patency of the passive upper airway.

Pharyngeal patency is influenced by the surface tension (gamma) of the upper airway lining liquid (UAL), of which saliva is a major component. We investigated the influences of saliva production on gamma of the UAL, and upper airway re-opening and closing pressures. In 10 supine, male, anaesthetized, tracheostomised, mechanically ventilated New Zealand White rabbits, we measured re-opening and closing of the passive isolated upper airway at baseline and following graded (cumulative) doses of methacholine or atropine. Upper airway liquid volume index (UALVI) was assessed using a standardized suction procedure (secretion weight obtained per second) expressed as the natural logarithm (LnUALVI). The gamma of UAL samples were measured using the 'pull-off' force technique. Across all animals, baseline values were: LnUALVI -6.2 (-8.6 to -5.4) median (interquartile range), gamma of UAL 58.9 (56.6-59.9) mN m(-1), re-opening 8.6 (6.9-11.1) cmH(2)O, and closing pressures 3.2 (1.8-5.7) cmH(2)O. LnUALVI increased by approximately 0.17 per microg kg(-1) methacholine and decreased by approximately 0.14 per 100 microg kg(-1) atropine (both P < 0.03, linear mixed effects modelling). Surface tension was unchanged by methacholine but increased by approximately 0.6 mN m(-1) per 100 microg kg(-1) atropine (P < 0.004). When data were analysed across all animals, both re-opening and closing pressures increased as surface tension increased (by approximately 0.4 cmH(2)O mN(-1) and by approximately 0.7 cmH(2)O mN(-1), respectively; both P < 0.05). We conclude that saliva production influences upper airway mechanical properties partly via alterations in gamma of UAL. We speculate that in obstructive sleep apnoea, altered autonomic activity may reduce saliva production and increase surface tension of the upper airway lining liquid, thus increasing the likelihood of upper airway obstruction.

Route of breathing in patients with asthma.

STUDY OBJECTIVES: To measure route of breathing in chronic asthmatic patients during and after an acute severe exacerbation. PATIENTS OR PARTICIPANTS: Thirteen asthmatic patients were studied during hospital admission for acute asthma and, in 9 patients, again when asymptomatic. Nine healthy subjects were also studied. INTERVENTIONS: Spontaneous route of breathing was qualitatively assessed using oral and nasal thermistor probes, and was then quantified using a dual compartment face mask with attached pneumotachographs. MEASUREMENTS AND RESULTS: All asthmatic patients had severe bronchoconstriction initially (FEV(1), 46 +/- 3% of predicted) that had resolved at follow-up (FEV(1), 91 +/- 6% of predicted). No healthy subject had evidence of bronchoconstriction (FEV(1), 102 +/- 5% of predicted). During acute asthma, 11 asthmatics were spontaneously breathing oronasally, as assessed using thermistor probes, while all 13 breathed oronasally via face mask. When assessed using thermistor probes, seven of nine asymptomatic asthmatic patients studied were breathing exclusively via the nose; however, all breathed oronasally via face mask. In contrast, while eight of nine healthy subjects were also breathing exclusively via the nose when assessed using thermistor probes, all breathed nasally only via face mask. CONCLUSIONS: Thus, when asymptomatic and at rest, asthmatic patients breathe exclusively via the nose. However, during acute exacerbations of asthma, these patients switch to oronasal breathing. Unlike healthy subjects, chronic asthmatic patients also switch to oronasal breathing when wearing a face mask, irrespective of the degree of bronchoconstriction. We speculate that asthmatics may have an increased tendency to switch to oral breathing, a factor that may contribute to the pathogenesis of their asthma.

The influence of endogenous mineralocorticoids on the composition of fetal urine.

Experiments were carried out in 10 chronically catheterised fetal sheep aged 121-128 days. In 3 animals infusion of aldosterone (5 micrograms/h) caused a fall in fetal urinary Na/K ratio; an effect that was reversed by spironolactone 2.5 mg/kg followed by an infusion of 100 micrograms/h per kg. In 9 fetal sheep which had no previous treatment the same doses of spironolactone had no effect on fractional sodium excretion although the fractional excretion of potassium decreased (P less than 0.05) and the urinary sodium potassium (Na/K) ratio rose (P less than 0.05). Amiloride had a variable effect on sodium excretion but the fractional excretion of potassium decreased markedly (P less than 0.05). Thus in chronically catheterised fetal sheep, endogenous mineralocorticoid activity altered urinary potassium excretion and the urinary Na/K ratio. However this activity was low, as distal blockade with amiloride further decreased the fractional excretion of potassium and increased the urinary Na/K ratio.