Immediate ventilatory response to added inspiratory elastic loads in cats

Altered breathing pattern after prenatal nicotine exposure in the young lamb

Maternal smoking during pregnancy is a risk factor for sudden fetal and infant death as well as obstructive airway disease in childhood. Fetal nicotine exposure affects organ development. The aim of the present study was to investigate effects of fetal nicotine exposure on lung function in young lambs. Nine unanesthetized, awake, prenatally nicotine-exposed lambs (N) (approximate maternal dose: 0.5 mg/kg) and 12 nonexposed control lambs (C) were studied repeatedly for 5 weeks after birth using a pneumotachograph and a computerized method for breath-by-breath determinations. N and C lambs had similar minute ventilation but a markedly different breathing pattern. At both 5 and 21 days, average age, N lambs had significantly lower tidal volumes and higher respiratory rates than C lambs. Inspiratory drive (P(0.1)) and effective impedance were significantly higher in N lambs compared with C lambs only at 5 days. Prenatal nicotine exposure appears to have long-term effects on the postnatal breathing pattern, suggesting altered lung function, e.g., increased airway resistance, decreased lung compliance, or both. The increased inspiratory drive is most likely secondary to increased impedance of the respiratory system. These changes are most marked close to birth but persist during the initial postnatal period.

Influence of external loading and assisted ventilation on chest wall mechanical properties

Instantaneous respiratory effective elastance (Eteff) and tissue resistance (Rt) may be measured by studying the relationship between flow at the airway opening and at the chest during forced oscillations; using that method it has been shown that Rt varies little during breathing while Eteff presents much larger phasic variations than can be explained by the curvature of the static pressure-volume curve (Tomalak et a1., 1997). The aim of this study was to test the hypothesis that the large variations of Eteff were related to the activity of respiratory muscles. For this, we studied in six healthy subjects the changes in Eteff and Rt induced by inspiratory and expiratory elastic loading (IEL and EEL, respectively), resistive loading (REL) and assisted ventilation (ASV) while keeping lung volume, ventilation and breathing pattern as constant as possible. IEL and EEL predominantly increased Eteff during the inspiratory and expiratory phases, respectively; in contrast, ASV decreased Eteff during inspiration and almost completely abolished its positive volume dependence. The changes of Rt usually paralleled those of Eteff. We conclude that respiratory muscle activity is responsible for most of the variations of Eteff during spontaneous breathing.

Respiratory tissue properties derived from flow transfer function in healthy humans

Assuming homogeneity of alveolar pressure, the relationship between airway flow and flow at the chest during forced oscillation at the airway opening [flow transfer function (FTF)] is related to lung and chest wall tissue impedance (Zti): FTF = 1 + Zti/Zg, where Zg is alveolar gas impedance, which is inversely proportional to thoracic gas volume. By using a flow-type body plethysmograph to obtain flow rate at body surface, FTF has been measured at oscillation frequencies (f(os)) of 10, 20, 30 and 40 Hz in eight healthy subjects during both quiet and deep breathing. The data were corrected for the flow shunted through upper airway walls and analyzed in terms of tissue resistance (Rti) and effective elastance (Eti,eff) by using plethysmographically measured thoracic gas volume values. In most subjects, Rti was seen to decrease with increasing f(os) and Eti,eff to vary curvilinearly with f(os)2, which is suggestive of mechanical inhomogeneity. Rti presented a weak volume dependence during breathing, variable in sign according to f(os) and among subjects. In contrast, Eti,eff usually exhibited a U-shaped pattern with a minimum located a little above or below functional residual capacity and a steep increase with decreasing or increasing volume (30-80 hPa/l2) on either side. These variations are in excess of those expected from the sigmoid shape of the static pressure-volume curve and may reflect the effect of respiratory muscle activity. We conclude that FTF measurement is an interesting tool to study Rti and Eti,eff and that these parameters have probably different physiological determinants.

Similarities between behavior of respiratory muscles in breath-holding and in elastic loading

Breath-holding subjects often exhibit involuntary contractions of respiratory muscles which are much stronger and faster than the efforts they would make during unrestricted breathing at the same level of CO2 and O2. To gain a better understanding of the genesis of these contractions, we compared them with the respiratory response to external elastic loading. Normal men rebreathed a mixture of 8% CO2 in oxygen against no load, elastic loads of 25 and 75 cm H2O/L, and held their breath, equivalent to an elastic load of 226 cm H2O/L. At iso-CO2, increasing loads led to progressively smaller tidal volumes, inspiratory flow rates and ventilation. However, respiratory muscles were progressively activated by the loads, as indicated by increasing occlusion pressure, so that inspiratory flow rate and ventilation were defended much better than could be expected if no neural compensation occurred. The pattern of respiratory muscle activity in breath-holding was qualitatively similar to that in elastic loading, and seemed quantitatively to be an extreme form of reaction to a large load. The reduction in inspiratory time and therefore of peak inspiratory pressure and ratio of inspiratory to total time with very large loads could be viewed as an adaptive response to limit respiratory muscle fatigue.

Effects of induced hypotension on breathing pattern in halothane-anaesthetized man

The effects of hypotension induced by trimetaphan on ventilatory control were assessed in sixteen normal subjects under halothane anaesthesia. The breathing pattern, mouth occlusion pressures, lung mechanics, acid-base balance, and arterial blood gases were analysed before and during trimetaphan infusion. During induced hypotension, the only significant change in the ventilatory variables was an increase in the ratio of inspiratory duration to total cycle duration from 0.39 +/- 0.05 (SD) to 0.42 +/- 0.03; P less than 0.01. The average minute ventilation remained unchanged. No modification in lung mechanics was observed, but all subjects developed a slight but significant hypocapnic alkalosis: PaCO2 was reduced from 5.5 +/- 0.4 to 5.2 +/- 0.4 kPa (P less than 0.001) and pH increased from 7.34 to 7.36 (P less than 0.05), without change in standard bicarbonate concentration. Our data indicate that the reduction in sympathetic nervous system activity induced by trimetaphan infusion in spontaneously breathing man causes only a minor alveolar hyperventilation. The weak respiratory response to hypotension suggests that changing peripheral afferent activity has little influence on the typically rapid breathing pattern induced by halothane.

Immersion vs pressure-breathing and diaphragmatic function in the upright position

In seven anesthetized dogs, immersion in the upright position to mid-neck level (IM) was compared to pressure breathing (PB) under dry conditions during constant-stimulus diaphragmatic contraction (EPS). The comparison was in terms of EPS-induced changes in alveolar pressure under static condition (Pmus); EPS-induced tidal volume (VT); and the VT/Pmus ratio (C'). It was found that at iso-lung volume (V): (a) Pmus was greater in IM than in PB, the difference increased at higher V; (b) VT was greater in IM than in PB, but the VT difference (deltaVT) did not parallel that in Pmus; VT was maximum at a V equal to approximately 90% of FRC in air (FRCd) and decreased below and above this volume; (c) during both IM and PB, the VT-V relationship reflected a biphasic relationship of C' to V and appeared to be inherent to the upright position.

Effects of uneven elastic loads on breathing pattern of anesthetized and conscious men

In anesthetized subjects rib cage strapping (RCS) did not change tidal volume (VT) and increased ventilation (V), whereas abdomen strapping (AS) markedly decreased VT and V. Both kinds of strapping decreased expiratory duration (TE), but did not change inspiratory duration (TI) and breathing rate. RCS and AS decreased lung volume by about 200 ml and increased the elastance of the repiratory system by 12 cm H2O/1 and 9 CM H20/l, repectively. The changes produced are mainly due to mechanical factors, although reflexes also seem to be operating in some cases. In conscious subjects RCS decreased VT, TI, TE and did not change V, whereas AS did not change these parameters. The different changes in conscious and anesthetized subjects show the effects of cortical influences, which also partly explain the differen effects elicited in conscious subjects by RCS and AS. The effects produced by RCS are mainly due to the sensation of hindrance to rib cage expansion, rather than to that of rib cage squeezing, as shown by experiments of RCS without reduction of rib cage volume.

Ventilation and sleep states in newborn infants

Recent studies have shown that ventilation in the newborn period is affected by sleep state. We investigated various measures affecting ventilation using the single breath airway occlusion technique in ten healthy, full-term newborn infants. There was a significant increase in respiratory rate and in minute ventilation in rapid eye movement sleep compared to non-REM sleep, and there was no significant change in tidal volume between the two sleep states. The variability of ventilation was increased in REM sleep, and inspiratory pressure at one-tenth of a second following airway occlusion was significantly increased in REM sleep. Effective elastance was similar in both sleep states. Measures that reflect activity of the Hering-Breuer reflex were significantly increased in non-REM sleep as compared with REM sleep. These results document the interdependence of sleep state and respiratory control mechanisms in full-term infants.

Respiratory regulation after elastic loading and CO2 rebreathing in normal term infants

Studies of airway pressure, tidal volume, respiratory duration, and total breath duration before and after elastic loading airway occlusions were carried out on ten full-term, normal infants on two occassions in the first week of life. Using these noninvasive techniques we infer that static compliance and the Hering-Breuer reflex are unchanged during the first week; that infants may increase sensitivity to chemical drive toward the end of the first week; and that there may be a volume related threshold for vagal inhibition of inspiration in some infants.

The progressive response of the newborn infant to added respiratory loads

The responses of 10 normal full-term infants to 10-sec airway occlusions at functional residual capacity were studied. The responses were quantified by measuring the pressure generated on successive inspiratory efforts following occlusions in room air and 100% oxygen. The response after oxygen breathing was 30-40% less than after air breathing, indicating that hypoxia accounted for part of the response. There was an increase in the response in air from the first to the fourth day of life. Endtidal carbon dioxide tension was shown to increase during the occlusion, but the response remaining after hypoxia was eliminated may not have been due entirely to hypercapnea. We conclude that the compensatory response of the infant to added respiratory loads is in part due to hypoxia, and that the hypoxic response increases over the first week of life.

Abnormal breathing patterns

In health, breathing is regular and the respiratory rate is sufficiency constant to be useful as a vital sign of health and disease. This regularity depends on a complex interplay of chemical and neural control systems that operate automatically to reset the rate and depth of breathing as changes occur in posture and activity, to adjust the level of ventilation so that changes in gas tensions and pH in the blood and in the brain intersitial fluid are exceedingly modest despite wide swings in metabolic rate and in environmental conditions, and to coordinate ventilation and circulation so that the requirements of individual tissues for O2 delivery and CO2 removal are satisfied. Two broad categories of disorders can result from malfunction of these systems (Table 1): (1) disproportionate ventilation (too high or too low) for the level of metabolic activity, thereby producing severe abnormalities in blood gas tensions or in acid-base balance, and (2) an irregular breathing pattern without eliciting gross changes in blood gas tensions or in acid-base balance. Because of the complexity of the control system, each of these categories represents a final common pathway that can be produced in different ways. In this presentation, we will attempt to describe the general features that characterize the operation of the control system and some new technics that make it possible to trouble-shoot the malfunctioning system in order to identify the mechanism(s) responsible for the abnormality in breathing pattern.

Occlusion pressure as a measure of respiratory center output in conscious man

The output of the "respiratory centers" has been estimated by measuring ventilation, inspiratory muscle power, EMG of the diaphragm, and by various other means, each of which has serious disadvantages. The static pressure generated by the inspiratory muscles at FRC against an obstructed airway is here suggested as a useful alternative. Ten conscious, normal, sitting human subjects were subjected to CO2 rebreathing (Read, 1967) and their airways were occluded at end-expiration at intervals without the subjects being aware in advance. The inspiratory pressure waves so generated were found to be distorted by conscious or unconscious responses to the occlusion which had a minimum latency of 0.15 sec. The pressure generated at 0.1 sec after the onset of inspiration (P0.1) was nevertheless easy to measure and was reproducible in each subject. The CO2 response obtained by plotting P0.1 against PCO2, was curvilinear, the P0.1 increasing more rapidly at high PCO2. The P0.1 is independent of pulmonary mechanics. Since it measures the rate of rise of inspiratory activity and not the peak activity it is also independent of mechanisms that alter the respiratory pattern by affecting inspiratory duration, in particular the vagal volume-related inspiratory-inhibitory reflex. It is concluded that measurements of P0.1 represent a useful index of the output of the respiratory centers.