Immediate ventilatory response to elastic loads and positive pressure in man

Immediate response to inspiratory resistive loading in anesthetized patients with kyphoscoliosis: spirometric and neural effects

In kyphoscoliosis (KS), lung volumes are reduced, respiratory elastance and resistance are increased, and breathing pattern is rapid and shallow, attributes that may contribute to defense of tidal volume (VT) in the face of inspiratory resistive loading. The control of ventilation of 12 anesthetized patients about to undergo corrective spinal surgery was compared to that of 11 anesthetized patients free of cardiothoracic disease during quiet breathing and the first breath through one of three linear resistors. Mean forced vital capacity (FVC) of the KS group was 48% that of the controls (C). Passive elastance (Ers) and active elastance and resistance (E'rs and R'rs, respectively) were computed according to previously described techniques (Behrakis PK, Higgs BD, Baydur A, Zin WA, Milic-Emili J (1983) Active inspiratory impedance in halothane-anesthetized humans. J Appl Physiol 54: 1477-1481). Baseline tidal volume VT, inspiratory duration Tl, expiratory duration TE, duration of total breathing cycle TT, and inspiratory duty cycle TI/TT were significantly reduced, while VE was slightly decreased in the KS. Ers, E'rs, and R'rs, were, respectively, 72, 69, and 89% greater in the KS. Driving pressure (Pmus) was derived from the equation of motion, using active values of respiratory elastance. With resistive loading, there was greater prolongation of TI in the C, while percent reduction in VT and minute ventilation VE was less in KS. Compensation in both groups was achieved through three changes in the Pmus waveform. (1) Peak amplitude increased. (2) The duration of the rising phase increased. (3) The rising Pmus curve became more concave to the time axis. These changes were most marked with application of the highest resistance in both groups. Peak driving pressure and mean rate of rise of Pmus were greater in the KS. Increased intrinsic impedance, Pmus, and differences in changes in neural timing in anesthetized kyphoscoliotics contribute to modestly greater VT defense, compared to that of anesthetized subjects free of cardiorespiratory disease.

Effects of extrinsic positive end-expiratory pressure on mechanically ventilated patients with chronic obstructive pulmonary disease and dynamic hyperinflation

OBJECTIVE

To examine the circulatory and respiratory effects of extrinsic positive end-expiratory pressure (PEEPe) in patients with chronic obstructive pulmonary disease (COPD) and dynamic hyperinflation during controlled mechanical ventilation.

DESIGN

Different levels of PEEPe were applied randomly in mechanically ventilated patients with COPD and dynamic hyperinflation.

SETTING

Respiratory Intensive Care Unit of a University Hospital.

PATIENTS

9 patients with acute respiratory failure and dynamic hyperinflation due to acute exacerbation of COPD.

INTERVENTIONS

PEEPe 35%, 58% and 86% of intrinsic PEEP (PEEPi) were applied.

MEASUREMENTS AND RESULTS

Using flow-directed pulmonary artery catheters hemodynamic measurements were obtained, while simultaneously lung volumes, airflows and airway pressures were recorded. In order to estimate alveolar pressures (Palv), rapid airway occlusions during passive expiration were also performed. At no level of PEEPe were significant changes in cardiac output, gas exchange variables, dead space, airways inflation resistances and respiratory system static end-inspiratory compliance observed. At high level of PEEPe central venous, mean pulmonary arterial and pulmonary capillary wedge pressures were increased significantly. All but one patient were flow-limited during passive expiration. PEEPe 86% of PEEPi caused a significant increase in end-expiratory lung volume and total PEEP. Iso-volume pressure-flow curves showed volume-dependence expiratory flow limitation in 2 patients, while in 8 patients volume-dependence of critical driving pressure (Palv-mouth pressure) that decreased flows was also observed.

CONCLUSIONS

The effects of PEEPe on iso-volume flow and hence on lung mechanics and hemodynamics, depend on many factors, such as airways resistances, lung volumes and airway characteristics, making the patient response to PEEPe unpredictable.

Effect of nasal pressure support ventilation and external PEEP on diaphragmatic activity in patients with severe stable COPD

Nasal pressure support ventilation (NPSV) has been shown to be useful in the treatment of acute and chronic pulmonary failure. However, little is known about respiratory muscle activity during NPSV in stable patients with COPD. The aim of this study was to test the effect of two levels (10 and 20 cm H2O) of NPSV on diaphragmatic activity, in a group of seven stable, severe COPD patients (FEV1 20 percent +/- 7 of pred, FEV1/FVC 35 percent) with hypercapnic respiratory insufficiency. Since these patients had an intrinsic PEEP (PEEPi) of 2.6 +/- 1.3 cm H2O, we also investigated the effect of adding 5 cm H2O external PEEP (PEEPe) during NPSV. Blood gases, ventilatory pattern by inductive plethysmography, integrated electromyogram of the diaphragm (Edi), transdiaphragmatic pressure (Pdi), and the diaphragmatic pressure time product (PTPdi) were recorded during randomized 15-min runs of both levels of NPSV with and without the addition of PEEPe. Minute ventilation did not change with the application of NPSV, but a significant decrease in breathing frequency with a parallel increase in tidal volume was observed, so that blood gas determinations improved at the higher levels of support. A marked statistically significant reduction in diaphragmatic activity, as assessed by a decrease in Pdi swings, PTPdi, and Edi, was detected at the levels of 10 and 20 cm H2O; a further significant decrease in these values was observed when PEEPe was added. PEEPi decreased significantly only with the application of PEEPe, resulting in a small increase in end-expiratory lung volume. We conclude that NPSV improves diaphragmatic function in patients with severe stable COPD; this effect may be enhanced by the applications of external PEEP.

Effect on breathing of raising end-expiratory lung volume in sleeping laryngectomized man

In animals, tonic vagal activity from lung receptors provides a means by which changes in end-expiratory lung volume can influence respiratory timing. We wished to examine whether increasing the end-expiratory lung volume within the tidal volume range had a similar effect in man. In order to minimize behavioral influences on breathing, the study was performed in subjects during deep non-rapid eye movement sleep. Five laryngectomized subjects were chosen for the study since their permanent tracheal stomata allow simple, airtight connection to respiratory apparatus and avoided problems with glottic closure. During EEG-documented sleep, end-expiratory volume was increased by up to 350 ml with the addition of expiratory threshold loads of 1 to 10 cm H2O. End-expiratory volume increased linearly with expiratory pressure. Inspiratory and expiratory times (TI and TE) were not affected by increases in end-expiratory volume. Tidal volume (VT) was decreased such that end-inspiratory volume remained unchanged. The decrease in VT may result from a reduction in inspiratory muscle efficiency at a higher lung volume. The results of the study provide no evidence that tonic vagal afferent information from the lungs is important in controlling respiratory rhythm within the tidal volume range in man when behavioral control of breathing is minimized.

Continuous positive airway pressure reduces work of breathing and dyspnea during weaning from mechanical ventilation in severe chronic obstructive pulmonary disease

Dynamic hyperinflation and the development of intrinsic positive end-expiratory pressure (PEEPi) are commonly observed in patients with severe chronic obstructive pulmonary disease (COPD) and acute respiratory failure. The presence of intrinsic PEEP acts as an inspiratory threshold load, and contributes significantly to the observed increase in work and oxygen cost of breathing. The present study examined the effects of continuous positive airway pressure (CPAP) (at 5, 10, and 15 cm H2O) and its ability to reduce the mechanical load imposed by PEEPi on breathing pattern, work of breathing, and dyspnea in seven patients with severe COPD during weaning from mechanical ventilation. Tidal volume remained stable at all levels of applied pressure. Breathing frequency was also stable except for a small (12%) decrease during CPAP of 15 cm H2O. Inspiratory pulmonary resistance and elastance were unaltered by the application of CPAP. There were progressive reductions in the inspiratory work of breathing as the level of CPAP increased. At the highest level of CPAP, the amount of inspiratory work performed per minute and per liter of ventilation decreased by 49.8 and 41.8%, respectively. Similar progressive reductions were also obtained in the pressure-time product for the inspiratory muscles and the diaphragm, which amounted to decreases of 42.9 and 42.2%, respectively, at the highest level of CPAP. End-expiratory lung volume remained stable at the lowest level of CPAP, with only modest increases occurring at the higher levels. In addition, all patients reported a reduction in dyspnea during the administration of CPAP.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of slow wave sleep on ventilatory compensation to inspiratory elastic loading

We determined the effects of slow wave sleep on ventilatory compensation to inspiratory elastic loads (18 cm H2O/L). Multiple loading trials of variable duration were applied in three healthy adult humans in wakefulness and during NREM sleep. During wakefulness, ventilatory response over 5 loaded breaths were highly variable. Tidal volume (VT), mean inspiratory flow (VT/TI), and minute ventilation (VE) were preserved or increased in 2 of the 3 subjects in whom mouth occlusion pressure (P0.1) was augmented in the immediate (second breath) response to the load. In the third subject who showed no change in P0.1, VE was not preserved during loading. During NREM sleep, the loading response was highly consistent in all trials and in all 3 subjects. P0.1 on the second loaded breath was not increased; thus VE, VT and VT/TI were reduced over five loaded breaths. This absence of immediate load compensation during NREM sleep was similar during normoxia, hyperoxia, and hypercapnia. During sustained loading in NREM sleep VE and VT returned toward control levels coincident with an increase in end tidal CO2. We conclude that augmentation of inspiratory neural drive sufficient for immediate compensation to elastic loads requires wakefulness. Compensatory responses to loading do not occur during NREM sleep until inspiratory effort is augmented by chemical stimuli.

Pattern of breathing during exercise in patients with interstitial lung disease

The responses to exercise were studied in 41 patients with pulmonary fibrosis, in whom vital capacity (VC) was reduced to 62% of predicted normal values. Maximum power output (POmax) was 53% predicted; there was a significant relationship between POmax and VC (r = 0.564). The maximum ventilation achieved during exercise was also related to VC (r = 0.614). Although arterial oxygen saturation (SaO2) fell by more than 5% in 13 of 31 patients, there was no relationship between either SaO2 at POmax or the exercise related fall in SaO2 and POmax. Heart rate responses were higher than normal predicted values in seven patients, all of whom showed a low POmax (36% predicted); this finding was due only in part to a fall in SaO2. The ventilatory response to exercise was within normal limits for the patients as a whole; those subjects with the lowest POmax showed relatively higher ventilatory responses to exercise but the difference was not significant. The pattern and timing of breathing was studied in 32 patients and compared with control subjects matched by sex, age, and size. Tidal volume (VT) was low in the patients; maximum VT was related to VC (r = 0.761), but at low values of VC VTmax was higher than in healthy subjects with comparable VC. The total breathing cycle time (Ttot) fell with progressive exercise in patients and controls; Ttot for a given ventilation was shorter in the patients. Inspiratory time (Ti) was shorter in patients than controls, as was Ti/Ttot. In most patients with diffuse pulmonary fibrosis exercise is limited by a reduced ventilatory capacity, despite the adoption of a short Ti and high inspiratory flow rate, both of which serve to optimise tidal volume and breathing frequency and presumably reduce both the force developed by inspiratory muscles and the sensation of breathlessness.

Breathing pattern in men during inspiratory elastic loads

Breathing pattern was studied in men during inspiratory elastic load applied throughout breathing cycle (CL) or inspiration only (DL). VT decreased similarly under both loads: 40% during 1st loaded breath (1st), 25% after 1 min (min); so did Ti (15% at 1st, 25% at min); Te decreased by 35% under CL, 25% under DL, at both times. Under DL expiratory flow started after a lag of about 0.4 sec, required to raise alveolar pressure above atmospheric. In all subjects the inspiratory muscles activity increased at 1st and in 6 out of 8 the ventilatory response at min was essentially neurogenic. At 1st all subjects recruited expiratory muscles, decreasing FRC by 100--300 ml under DL' in most this phenomenon was negligible at min. Rate of decrease of inspiratory muscle pressure during period of zero flow under DL was proportional to end-inspiratory muscle pressure (Pmuse.i.) and to Pmuse.i./Ti. At beginning of expiration dPmus/dt was greater under DL: under this period difference of Pmus between CL and DL seems mainly due to muscle intrinsic properties.

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.

Components of respiratory depression after narcotic premedication in adolescents

The effects of narcotics on ventilatory control were assessed in 13 adolescents and young adults. Both a narcotic and narcotic-phenothiazine significantly depressed the CO2 response curve. Using an occlusion pressure technique (Pm100) to evaluate those neuromuscular processes that generate forces acting on the ventilatory pump, it was found that narcotic agents reduced neuromuscular drive. In most subjects, narcotics had an additional action that contributed to the overall ventilatory depression. Using carbon dioxide to vary neuromuscular drive before and after drug administration at constant levels of neuromuscular drive the drugs reduced tidal-volume responsiveness of the pump. We conclude that narcotics impair ventilation through a combination of two effects; first, reduced neuromuscular drive, most probably due to central depression, and second, increased impedance of the ventilatory pump, most probably due to a decrease in chest-wall compliance.

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.

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.

Respiratory frequency control during external elastic loading and chest compression

Experiments were conducted to determine if extravagal (i.e. chest wall) respiratory mechanoreceptors are involved in the control of respiratory frequency during steady-state external elastic loading (EEL) and chest compression (CC) in the anesthetized (Dial), vagotomized cat and dog. Frequency versus PaCo2 curves obtained by breathing CO2 mixtures were compared with curves obtained during elastic loading and chest compression. There was no significant difference between the CO2 and EEL curves in cats or dogs indicating there is no extravagal mechanoreceptor information contributing to the control of respiratory frequency during EEL. Comparison of curves obtained with chest compression and CO2 breathing show that dogs respond to chest compression with an extravagal (i.e. involving chest wall mechanoreceptors) neural reflex increase in frequency.

The role of arterial chemoreceptors in the breath-by-breath augmentation of inspiratory effort in rabbits during airway occlusion or elastic loading

1. The breath-by-breath augmentation of inspiratory effort in the five breaths following airway occlusion or elastic loading was assessed in anaesthetized rabbits from changes of airway pressure, diaphragm e.m.g. and lung volume.2. When the airway was occluded in animals breathing air, arterial O(2) tension fell by 20 mmHg and CO(2) tension rose by 7 mmHg within the time of the first five loaded breaths.3. Inhalation of 100% O(2) or carotid denervation markedly reduced the breath-by-breath progression but had little or no effect on the responses at the first loaded breath.4. These results indicate that the breath-by-breath augmentation of inspiratory effort following addition of a load is mainly due to asphyxial stimulation of the carotid bodies, rather than to the gradual emergence of a powerful load-compensating reflex originating in the chest-wall, as postulated by some workers.5. The small residual progression seen in animals breathing 100% O(2) or following carotid denervation was not eliminated (a) by combining these procedures or (b) by addition of gas to the lungs to prevent the progressive lung deflation which occurred during airway occlusion.6. Bilateral vagotomy, when combined with carotid denervation, abolished the residual breath-by-breath progression of inspiratory effort.