Effects of vagotomy on respiratory mechanics in newborn and adult pigs.

We have examined breathing patterns and respiratory mechanics in anesthetized tracheostomized newborn piglets and adult pigs and the changes determined by cervical bilateral vagotomy. Piglets had a respiratory system compliance and resistance, on a per kilogram basis, respectively, higher and smaller than the adults. After vagotomy neither variable changed in the newborn, but resistance dropped in the adult. This may suggest that efferent vagal control of bronchomotor tone is more pronounced in the adult. Respiratory system time constant was longer in newborns both before and after vagotomy. The distortion of the chest wall, examined as the ratio between the volume inhaled spontaneously and the passive volume for the same abdominal motion, was more marked in newborns, reflecting their higher chest wall compliance. The work per minute, computed from the pressure and volume changes, was larger in piglets. After vagotomy the external work per minute was not different; however, the larger tidal volumes were accompanied by a larger chest distortion. This may indicate that vagal control of the breathing pattern, by limiting the depth of inspiration and hence the amount of chest distortion, has implications on the energetics of breathing.

Effects of PGF2 alpha on the EMG of costal and crural parts of the diaphragm of the newborn pig.

We investigated the effects of PGF2 alpha on the breathing patterns and electric activity of costal and crural parts of the diaphragm in 9 anesthetized newborn pigs. The change in diaphragmatic tension was evaluated as the change in transdiaphragmatic pressure. Because PGF2 alpha induces bronchoconstriction and an increase in respiratory resistances, the changes induced by prostaglandin were evaluated as differences between bronchoconstriction after PGF2 alpha and resistive load obtained by applying gradual occlusion to the inspiratory line of the breathing circuit. Our results show that PGF2 alpha decreased respiratory frequency with lengthening of expiratory time, while the resistive load increased both respiratory phases. The changes in breathing pattern were associated with different electrical activities of the diaphragm. While resistive load did not significantly change the EMG power spectrum, PGF2 alpha recruited new motor units. Furthermore, resistive load induced synchronization of the inspiratory time discharge of the costal and crural parts of the diaphragm, while after PGF2 alpha infusion there was an early inspiratory discharge of the crural part.

PGF2 alpha and breathing pattern in newborn pigs.

The effect of PGF2 alpha has been evaluated in 11 unanaesthetized unrestrained piglets and in 3 anaesthetized piglets (2-3 days old) using a barometric-plethysmographic technique. PGF2 alpha (mg 0.25/pig) was administered as aerosol for 5 min. In 3 of the unanaesthetized newborn pigs the effect of PGF2 alpha aerosol has been evaluated after indomethacin (mg 1/Kg i.v.). The vagal dependent activity of the prostaglandin was also evaluated after atropine (mg 0.08/Kg i.m.). Our results show that PGF2 alpha in newborn pigs causes hypoventilation due to a decrease in respiratory rate and to a lengthening in TE. The changes in TE are due to an increase in the incidence and duration of apneic events characterizing the respiratory activity at birth. After indomethacin PGF2 alpha does not change the breathing pattern. Atropine only partially reduces the effects of PGF2 alpha while, after anaesthesia, prostaglandin does not change the breathing pattern. Consequently our results show that PGF2 alpha in newborn animals similar to other prostaglandins acts as a depressant of respiratory activity.

PAF and the role of the vagus nerve in the breathing pattern of the pig.

In 14 anesthetized, spontaneously breathing pigs we examined the changes in breathing pattern, in respiratory mechanics and in systemic and pulmonary vascular parameters after i.v. PAF administration. In another 3 pigs, the effects of PAF were also examined after bilateral vagotomy. In intact pigs, PAF induces apnea, bronchoconstriction, pulmonary hypertension and systemic hypotension. Our results also show that administration of PAF alters the phasic vagal activity, modifying the slope of VT vs TI and TE vs TI relationships and the TI0/TI ratio. These effects and apnea are vagus-dependent. The central excitatory timing effect of PAF on inspiratory duration (TI0) was correlated with a decrease in passive compliance but not with active compliance. We postulate that the activation by vagal input strengthens the mechanisms that counteract the bronchoconstrictor effect of PAF.

Inspiratory timing regulation of PGF2 alpha in newborn pigs.

In intact and vagotomized anesthetized, spontaneously breathing piglets, we investigated the regulation of inspiratory timing evoked by i.v. administration of prostaglandin (PG) F2 alpha. The inspiratory time was evaluated from the flow trace as an index of mechanical inspiratory time (Ti) and from costal and crural diaphragmatic EMG (TiEMG) as an index of neural inspiratory time. Our results under control conditions showed that TiEMG was shorter than Ti. Vagotomy abolished the difference, inducing a change in the power spectrum without modifying the centroid frequency (Cf). PGF2 alpha lengthened TiEMG, causing a postinspiratory diaphragmatic discharge to appear, while mechanical inspiratory time decreased significantly. Postvagotomy i.v. administration of PGF2 alpha did not cause any significant changes in inspiratory time and did not evoke the postinspiratory discharge. The i.v. administration of PGF2 alpha before and after vagotomy did not change the centroid frequency in spite of recruitment of new motor units synchronized with those that are active under control conditions.

Effects of warm ischemia on valve endothelium.

BACKGROUND: This study investigates the time-dependent resistance of the endothelium of porcine aortic and pulmonary valves to different periods of warm ischemia (WIT). METHODS: Twenty-five 9-month-old swine were divided after death into five groups of WIT (0, 6, 12, 24, and 36 hours). Aortic and pulmonary valves were removed and a total of 15 aortic and 15 pulmonary valve specimens were obtained for each WIT interval. Valves were then examined for (1) their viability rate by the trypan blue dye exclusion method at light microscopy (percent of viability compared with 0 hours of WIT); (2) ultrastructural signs of irreversible or reversible ischemic damage by transmission electron microscopy (cell disruption, dilation of endoplasmic reticulum, cytoplasmic edema, nuclear and mitochondrial changes); (3) endothelial function by pharmacologic evaluation of both the endothelial-releasing capacity of prostacyclin and the endothelial-dependent dynamic responses to relaxing (acetylcholine from 1 x 10(-10) mol/L to 1 x 10(-4) mol/L) in aortic and pulmonary valve segments precontracted with norepinephrine (1 x 10(-6) mol/L) and contracting (NG-monomethyl-L-arginine, 1 x 10(-4) mol/L) drugs. RESULTS: Our results showed an endothelial progressive time-dependent ischemic injury, which reached significance after 12 hours of exposure. Viability and functional data indicated that 6 hours of WIT only provoked slight endothelial damage (p > 0.05 respect to time 0 hours), with signs at transmission electron microscopy consistent with a reversible injury. At 12 hours of exposure, we observed a significant reduction (p < 0.05) with respect to time 0 of the viability rate of prostacyclin production and of the endothelium-dependent dynamic responses to acetylcholine and NG-monomethyl-L-arginine. These functional impairments, although significant, were not consistent, however, with a complete loss of viability. Transmission electron microscopic observations confirmed the appearance of signs of irreversible injury; nevertheless, some elements were found to be well preserved or presented reversible damage. After 24 hours of WIT, ultrastructural and functional data were consistent with a dramatic decrease compared with controls in endothelial viability and functions (p < 0.01). Finally, after 36 hours of WIT, there was a subtotal loss of viability, of functions (p < 0.001) and, at transmission electron microscopic observations, of the endothelial layer of the valves. CONCLUSIONS: Our data show that the endothelial cells are resistant to short periods of WIT (up to 6 hours), and suggest that these cells can endure longer exposures, up to 12 hours of warm ischemia. Periods of 24 and 36 hours of WIT provoke progressive irreversible damage.

Unusual response of anaesthetised pigs to asphyxia.

The effects of vagosympathectomy, asphyxia, hypoxia and hypercapnia on the breathing of anaesthetised pigs are described. Vagosympathectomy caused changes in cardiovascular variables and in the pattern of breathing characteristic of the loss of stretch receptor activity. After vagosympathectomy the linear relationship between tI and tE was abolished. Hypoxia in intact animals produced changes in minute ventilation by peripheral chemoreceptor drive. When this drive was removed by vagosympathectomy the central depressing effects of hypoxia were revealed as a slowing of breathing and reduction in minute volume. The central depressing effect of hypoxia on respiration was very potent in the pig and very clearly seen in asphyxia. Vagosympathectomy caused a reduction in frequency of breathing and respiratory arrest occurred when a dead space of only moderate size was used. Breathing slowed from the moment of connection of the dead space to produce respiratory arrest within 2 min. The pig lung has been considered similar to the human lung on morphometric and physiological grounds but these results show that there are very important species differences in response to asphyxia.

The effects of glibenclamide, a blocker of K+ ATP-sensitive potassium channels, on diaphragmatic fatigue during endotoxaemia in pigs.

An in vivo porcine model of endotoxaemia was used to study the effects of glibenclamide, a K+ ATP-sensitive potassium channel blocker. Escherichia coli lipopolysaccharides (LPS, 70 micrograms/kg, i.v., as a bolus) were infused into anaesthetized, mechanically ventilated, indomethacin-treated pigs. After 120 min of endotoxaemia, glibenclamide was administered (10 mg/kg, i.v., over 5 min) to half the pigs. The strength at different frequencies of stimulation (10, 20, 30, 50 Hz, 20 V,) 1 s) and the endurance capacity (10 Hz, 20 V, 30 s) of the diaphragm were evaluated after 120 min of endotoxaemia and 5, 10, 20 and 30 min after drug infusion. Glibenclamide transiently increased the blood pressure without changing the decreased cardiac output and at the same time further impaired the diaphragmatic activity. The reduced ability of the diaphragm to generate force in response to different electrical stimulations was shown by a significant reduction in strength. The endurance index decreased 5 min after glibenclamide infusion, returning to the pre-glibenclamide values by 150 min. These results indicate that glibenclamide modifies the activity of vascular smooth muscle and of the diaphragm.

Cardiovascular responses to glibenclamide during endotoxaemia in the pig.

The effects of blockading the ATP-sensitive potassium channel (K+ATP channels) on endotoxin-induced vascular derangements was studied. Escherichia coli endotoxin was infused (20 micrograms/kg per h) intravenously for 180 min into anaesthetized, mechanically ventilated, indomethacin-treated pigs. After 150 min of endotoxaemia, glibenclamide (a K+ ATP channel blocker) was infused intravenously at 2 mg/kg per min for 5 min. The cardiovascular parameters were recorded before (control), every 30 min up to 150 min during endotoxaemia, and then at 5, 15 and 30 min after administration of glibenclamide. Infusion of endotoxin reduced the systemic arterial pressure to 60.6% +/- 3.7% (p < 0.01) and increased the pulmonary arterial pressure by 75.9% +/- 11.0% (p < 0.01) of the control values. Within 5 min, infusion of glibenclamide transiently but significantly reversed the systemic hypotension by raising the systemic vascular resistance, whereas the increased pulmonary arterial pressure was further augmented. Glibenclamide infusion did not influence the cardiac output. Within 30 min, the cardiovascular parameters had returned to the values induced by endotoxin, except for the systemic vascular resistance. Infusion of glibenclamide into normal pigs did not change the systemic pressure or resistance, but the pulmonary pressure and resistance were augmented transiently. These data suggest that, in pigs, the ATP-sensitive K+ channels may be one factor playing a role in the vascular changes due to endotoxaemia, especially in the systemic circulation.

Effects of nitric oxide on diaphragmatic muscle endurance and strength in pigs.

The aim of the study was to evaluate the effects of nitric oxide (NO) on diaphragmatic fatigue in fifteen anaesthetized, mechanically ventilated pigs, divided into three groups. The animals were pre-treated with indomethacin (3 mg kg-1, i.v.) to block the cyclo-oxygenase pathway. To group 1 pigs (n = 6) NG-nitro-L-arginine methyl ester (L-NAME, 5 mg kg-1 i.v.) was administered as a bolus to block endogenous NO production, while group 2 pigs (n = 6) were infused with sodium nitroprusside (SNP, 0.023 mg kg-1, i.v.), a donor of NO. Group 3 pigs (n = 3) were used as the controls. We evaluated diaphragmatic strength by measuring the transdiaphragmatic pressure (P di) generated during bilateral phrenic nerve stimulation at 10, 20, 30 and 50 Hz, 15 V, while the diaphragmatic endurance was assessed by a 30s stimulation at 10 Hz, 15 V. Diaphragmatic index was assessed as the ratio of peak force between single twitches performed before and after the 30 s stimulation west. We also evaluated mean systemic (MAP) and pulmonary (MPAP) arterial pressures, pulmonary wedge pressure (PW), systemic (SVR) and pulmonary vascular resistance (PVR) and cardiac output (CO). L-NAME increased MAP, MPAP, PW, SVR and PVR, but decreased CO. SNP caused a decrease in MAP, MPAP, PW and SVR, while PVR and CO did not change. The main finding of this study was that diaphragmatic strength was not significantly weakened after L-NAME administration, except at 10 Hz, while it did not change after SNP infusion. However, both L-NAME and SNP caused significant decreases in diaphragmatic endurance capacity. The fatigue appearing after L-NAME is probably correlated with a decline in diaphragmatic blood flow, as evidenced by the increase in SVR and the decrease in CO, and consequently in oxygen supply. In contrast, the decrease in endurance capacity after SNP infusion can be attributed to a direct action of NO on skeletal muscle.

Respiratory modulation by vagal afferents in pigs.

We have studied the role of pulmonary vagal afferents and baroreceptors of the aortic arch on control of breathing in anaesthetized pigs. As in other species (dog, cats and rabbits) the depth and rate of breathing was found to be controlled by a volume-related vagal feed-back loop from pulmonary stretch receptors. Rapid shallow breathing following histamine aerosol and phenyldiguanide i.v. (which are likely to stimulate pulmonary vagal irratant and J receptors) occurred through a decrease in the activation threshold of the inspiratory cut-off mechanism without altering the matching of inspiratory to expiratory time. These results are also similar to those found in cats and rabbits. Histamine caused an increase in pulmonary resistance and a decrease in compliance: these effects were partially vagally mediated and partially due to a direct stimulation of smooth muscles by the drug. It seems, from relating the respiratory to the vascular systemic effect of histamine administration, that pigs have a greater respiratory response than cats or rabbits. A stepwise decrease in blood pressure through haemorrhage under iso-PCO2 and iso-PO2 conditions was found to affect the matching of the expiratory to the inspiratory time without affecting the activation threshold of the inspiratory cut-off mechanism.

The effects of PGF2 alpha on respiratory and cardiovascular variables in pig.

PGF2 alpha was injected into different parts of the circulatory system (intravenously by femoral and jugular veins, into the pulmonary artery, into the ascending or descending aorta and into the internal carotid artery at a level above the sinus and carotid body) of anaesthetized spontaneously breathing pigs. It caused changes in respiratory and cardiovascular variables. In vagosympathectomized animals the changes in pattern of breathing produced by PGF2 alpha were greatly reduced and total lung resistance was increased. This may indicates more than one site of action of PGF2 alpha, lung resistance being influenced by a direct action of PGF2 alpha and by an immediate reflex while the pattern of breathing being modulated by vagal efferents from the lungs.

Work of breathing in dog during exercise.

In six dogs trained to wear a mask and to swallow an esophageal balloon, the dynamic work of breathing (Wdyn) was measured while the animals ran on a treadmill at different intensities (7-13 km.h-1,+10%). Wydn (kg.m.min-1) increased with ventilation (VE, 1.min-1) according to Wdyn = 0.308.10(-2) VE2 + 0.0098.10(-2).VE3. However, if the exercise was prolonged so that the body temperature rose above approximately 39 degrees C, Wdyn, for a given ventilation, decreased; and hence Wdyn = 0.253.10(-2).VE2 -- 0.0011.10(-2).VE3. Similar observations have been made on another dog heated from an external source. From this finding it seems that during exercise, when the temperature rises and the ventilation increases to dissipate heat, the airway size, at least in some portion of the respiratory tract, increases markedly and therefore the cost of breathing is greatly diminished. This mechanism would save oxygen for the exercising limb muscles when exercise has to be continued for an extended time.

Glibenclamide, a blocker of ATP-sensitive potassium channels, reverses endotoxin-induced hypotension in pig.

In anaesthetized, mechanically ventilated, indomethacin-treated pigs, we infused E. coli endotoxin (LPS, 20 micrograms kg-1 h-1, i.v.). After 150 min of endotoxaemia, 10 mg kg-1 glibenclamide (an ATP-sensitive K+ channel antagonist) was administered i.v. over 5 min. Vascular variables were recorded before (control), after 150 min of endotoxaemia and 5, 15 and 30 min after glibenclamide infusion. Glibenclamide transiently (within 5 min) increased systemic arterial pressure, reduced by LPS administration, without an effect on cardiac output. Our data indicate that ATP-sensitive K+ channels may play a partial role in the vascular changes due to endotoxaemia.

Lung reflexes affecting the larynx in the pig, and the effect of pulmonary microembolism.

Laryngeal airflow resistance was measured in anaesthetized pigs during stimulation of lung vagal reflexes by injection of phenylbiguanide and of capsaicin, before and during pulmonary microembolism due to intravenous injection of hardened red blood cells; the microembolism caused pulmonary hypertension. The Breuer-Hering reflex was also assessed before and after pulmonary microembolism. Phenylbiguanide and capsaicin caused apnoea followed by rapid shallow breathing, hypertension, bradycardia and laryngeal constriction in inspiration and expiration. Most of these effects were abolished by bilateral cervical vagotomy. Pulmonary microembolism caused only small changes in breathing pattern, mainly a decrease in inspiratory time. After microembolism the Breuer-Hering reflex was enhanced, and injections of phenylbiguanide and capsaicin caused longer apnoeas by a vagal reflex. The changes in pattern of breathing and in lung reflexes after lung microembolism and during the associated pulmonary hypertension are consistent with an enhancement of pulmonary stretch receptor activity in this condition, but do not indicate any important role for C fibre reflexes.

The effect of level of contraction on the electromyographic power spectrum of the diaphragm in pigs.

We investigated the relationship between the frequency components of myoelectric power spectra of the diaphragm and the level of diaphragmatic contraction in seven anaesthetized spontaneously breathing pigs. Electromyographic activity of the costal and crural portions of the diaphragm were recorded with fish-hook electrodes and the frequency-power spectra during inspiration were computed and expressed in terms of centroid frequency (fc). Diaphragmatic force was indirectly assessed as transdiaphragmatic pressure (Pdi) which was measured with balloon-catheter systems placed in the abdomen and oesophagus. The relationships between Pdi and costal and crural fc were assessed during brief (2 min) and incremental increases in diaphragmatic contraction, achieved by gradual occlusion of the inspiratory line of the breathing circuit. When Pdi was increased to 128, 191, 287 and 421% of the value measured during unobstructed breathing, costal and crural fc rose significantly in all animals because of an increase in the power of high-frequency components and a decline in the power of low-frequency components. Both costal and crural fc returned to control values within 5 min of the release of inspiratory occlusion. Our results indicate that the level of contraction is an important determinant of the diaphragmatic myoelectric power spectrum and should be taken into consideration when using power spectral analysis to diagnose diaphragmatic mechanical failure.

Role of hypotension induced by PGI2 on the control of depth and frequency of breathing.

We studied the respiratory response to infusion of prostacyclin (PGI2) in 7 anaesthetized pigs in an attempt to understand whether the changes in pattern of breathing were due to a direct action of the substance or to the concomitant hypotensive effect. The depth and frequency of breathing were analyzed in terms of the threshold-inhibition curve for termination of inspiration (VT/TI relationship) and of bulbo-pontine rhythm, estimated from inspiratory and expiratory time during occlusion of the airways at the end expiratory level (TE/TI relationship). This provide the central respiratory rhythm when the phasic lung volume receives no input from pulmonary stretch receptors. The hypotension induced by PGI2 increased pulmonary ventilation mainly through a change in VT and caused a slight rightward displacement of VT/TI relationship without modifying the slope of the curve. This effect seems to be vagally mediated. PGI2 also changed the bulbo-pontine rhythm. Our results show that PGI2 modifies the vagal discharge and bulbo-pontine rhythm by two opposite mechanisms: hypotension and an apparent direct action on carotid and aortic baroreceptors.