Calibration of respiratory inductive plethysmograph during natural breathing

We describe a single-posture method for deriving the proportionality constant (K) between rib cage (RC) and abdominal (AB) amplifiers of the respiratory inductive plethysmograph (RIP). Qualitative diagnostic calibration (QDC) is based on equations of the isovolume maneuver calibration (ISOCAL) and is carried out during a 5-min period of natural breathing without using mouthpiece or mask. In this situation, K approximates the ratio of standard deviations (SD) of the uncalibrated changes of AB-to-RC volume deflections. Validity of calibration was evaluated by 1) analyzing RIP waveforms during an isovolume maneuver and 2) comparing changes of tidal volume (VT) amplitude and functional residual capacity (FRC) level measured by spirometry (SP) with RIP values. Comparisons of VT(RIP) to VT(SP) were also obtained in a variety of postures during natural (uninstructed) preferential RC and AB breathing and with voluntary changes of VT amplitude and FRC level. VT(RIP)-to-VT(SP) comparisons were equal to or closer than published reports for single posture, ISOCAL, multiple- and linear-regression procedures. QDC of RIP in supine posture with comparisons to SP in that posture and others showed better accuracy in horizontal than upright postures.     

Inductance plethysmography measurement of CPAP-induced changes in end-expiratory lung volume

The respiratory inductance plethysmograph (RIP) has recently gained popularity in both the research and clinical arenas for measuring tidal volume (VT) and changes in functional residual capacity (delta FRC). It is important however, to define the likelihood that individual RIP measurements of VT and delta FRC would be acceptably accurate (+/- 10%) for clinical and investigational purposes in spontaneously breathing individuals on continuous positive airway pressure (CPAP). Additionally, RIP accuracy has not been compared in these regards after calibration by two commonly employed techniques, the least squares (LSQ) and the quantitative diagnostic calibration (QDC) methods. We compared RIP with pneumotachographic (PTH) measurements of delta FRC and VT during spontaneous mouth breathing on 0-10 cmH2O CPAP. Comparisons were made after RIP calibration with both the LSQ (6 subjects) and QDC (7 subjects) methods. Measurements of delta FRC by RIPLSQ and RIPQDC were highly correlated with PTH measurements (r = 0.94 +/- 0.04 and r = 0.98 +/- 0.01 (SE), respectively). However, only an average of 30% of RIPQDC determinations per subject and 31.4% of RIPLSQ determinations per subject were accurate to +/- 10% of PTH values. An average of 55.2% (QDC) and 68.8% (LSQ) of VT determinations per subject were accurate to +/- 10% of PTH values. We conclude that in normal subjects, over a large number of determinations, RIP values for delta FRC and VT at elevated end-expiratory lung volume correlate well with PTH values. However, regardless of whether QDC or LSQ calibration is used, only about one-third of individual RIP determinations of delta FRC and one-half of two-thirds of VT measurements will be sufficiently accurate for clinical and investigational use.     

Calibration of respiratory inductive plethysmography in spontaneously breathing lambs and piglets

Respiratory inductive plethysmography is a method of assessing breathing pattern without an airway connection. We employ a single position graphic calibration technique for gain factor calculation. Nineteen studies were completed in piglets and 20 studies were completed in lambs. The single position graphic technique utilizes selection of two breaths from a 20 s run of breaths with different ribcage/pneumotachograph and abdomen/pneumotachograph ratios for gain calculation. Validation of gains was performed by comparing volumes obtained simultaneously by respiratory inductive plethysmography and pneumotachography. Total study time ranged between 15 and 30 min. Results suggest that the single position graphic calibration technique provides time-efficient and accurate calibration of respiratory inductive plethysmography in the spontaneously breathing, sedated lamb and piglet, allowing respiratory inductive plethysmography to become an additional tool for ventilatory parameter measurement.     

Respiratory adaptations to dead space loading during maximal incremental exercise

We examined the effects of dead space (VD) loading on breathing pattern during maximal incremental exercise in eight normal subjects. Addition of external VD was associated with a significant increase in tidal volume (VT) and decrease in respiratory frequency (f) at moderate and high levels of ventilation (VI); at a VI of 120 l/min, VT and f with added VD were 3.31 +/- 0.33 liters and 36.7 +/- 6.7 breaths/min, respectively, compared with 2.90 +/- 0.29 liters and 41.8 +/- 7.3 breaths/min without added VD. Because breathing pattern does not change with CO2 inhalation during heavy exercise (Gallagher et al. J. Appl. Physiol. 63: 238-244, 1987), the breathing pattern response to added VD is probably a consequence of alteration in the PCO2 time profile, possibly sensed by the carotid body and/or airway-pulmonary chemoreceptors. The increase in VT during heavy exercise with VD loading indicates that the tachypneic breathing pattern of heavy exercise is not due to mechanical limitation of maximum ventilatory capacity at high levels of VT.     

Enkephalin-induced changes in ventilation and ventilatory pattern in adult dogs

We studied the changes in ventilation induced by intracisternal administration of enkephalins in four unanesthetized adult dogs. Instantaneous minute ventilation (VT/TT) decreased markedly after D-Ala-Met-enkephalinamide (DAME). Mean VT/TT decreased maximally by 20-50 min after DAME and lasted an additional 15-60 min; by 2 h, VT/TT had returned to base line. Four doses (5, 25, 60, and 125 micrograms/kg) of DAME were used, and the ventilatory response depended on the dose. Mean inspiratory time decreased but mean expiratory time and mean TT showed a marked prolongation. Periodic breathing (2-3 breaths separated by long apneic pauses) occurred in every study and the frequency of sighs increased considerably. All these ventilatory changes were reversed by low doses of naloxone or naltrexone; in addition, VT/TT increased well above base line after the administration of these antagonists. However, naloxone did not increase VT/TT when injected without prior administration of DAME. We conclude that 1) the decrease in VT/TT is due to a decrease in respiratory duty cycle; 2) periodic breathing and increased frequency of sighs constitute part of the changes in the ventilatory pattern induced by DAME; 3) a ventilatory withdrawal reaction may occur after a receptor-agonist interaction of short duration; and 4) although enkephalins can modulate ventilation and the breathing pattern in a major way, these data provide no evidence suggesting that this modulation is tonic.     

Calibration of respiratory inductive plethysmography during quiet and active sleep in lambs

Respiratory inductive plethysmography provides a noninvasive method of measuring breathing patterns. Calibration of respiratory inductive plethysmography requires calculation of gain factors for ribcage and abdomen transducers utilizing 2 breathing patterns with different ribcage and abdomen contributions and tidal volume measured by either spirometry or integrated pneumotachography. The purpose of this study was to determine if respiratory inductive plethysmography can be calibrated to provide accurate measurements during quiet and active sleep in lambs. We used a least squares linear regression calibration technique with breaths selected from quiet sleep and active sleep to calculate gain factors in 6 tracheostomized lambs. Validation of gain factors was performed by comparing tidal volumes obtained simultaneously by respiratory inductive plethysmography and pneumotachography during quiet sleep and active sleep. Tidal volume differences between respiratory inductive plethysmography and pneumotachography on validation runs of 15 consecutive breaths each revealed 90% of validation breaths within +/- 20% during quiet sleep and 82% of validation breaths within +/- 20% during active sleep. These data provide evidence that respiratory inductive plethysmography can be calibrated to allow breathing pattern measurement during sleep.     

Ventilation and respiratory pattern and timing in resting awake cats

The purpose of this study was to characterize the variability and patterns of spontaneous respiratory behaviour in awake cats. Respiration was measured in six cats over 80 or 90 min by the plethysmographic technique. In three cats, arterial blood gases were measured. Breath frequency (f) and tidal volume (VT) varied considerably breath-to-breath, although on average, these measurements as well as average ventilation remained relatively constant. The incidence of breath ventilation (VT X 60/TTOT) and VT were distributed unimodally but the incidence of breath f had a bimodal distribution. In the low f range, average f was 22.5 breaths/min, and in the high f range, average f was 41.6 breaths/min. The latter range appeared to be associated with purring. Inspiratory duration (TI) was less than expiratory duration (TE) at low f but exceeded TE at high f. For a given breath ventilation there was a predictable f and VT. At shorter TI (higher f) mean inspiratory flow, an index of central respiratory drive, increased but VT decreased. This study indicates that "normal" control respiratory behaviour in awake cats is better described by the range and pattern of breathing than by average values.     

Mechanical aspects of chest wall distortion

During passive inflation of the respiratory system, the rib cage (RC) expands because the pressure applied to it [approximately equal to abdominal pressure (Pab)] increases. Similar Pab-tidal volume (VT) relationships between passive and spontaneous inspirations would occur only if 1) Pab acts on RC equally in the two situations (no distortion) or 2) the extradiaphragmatic inspiratory muscles expand RC, compensating for distortion. In anesthetized adult rats and in sleeping human infants the passive relationships between VT and Pab or abdomen motion (AB) were constructed by occluding the airways during expiration. For a given Pab (or AB) in active breathing VT averaged 55% (rats) and 49% (infants) of the passive volume change. With phrenic stimulation in rats VT was only slightly less than during spontaneous breathing, indicating that, in the latter case, the respiratory system was essentially driven only by the diaphragm. In both species occasional breaths with large RC expansion occurred, and VT was then equal to or larger than the passive volume at iso-Pab. We conclude that 1) RC distortion decreases VT to approximately half of the passive value and 2) being on the relaxation curve reflects "compensated" distortion and not absence of it.     

Short-term entrainment of ventilation to the walking cycle in humans

We describe a breath-by-breath method to test for entrainment of breathing and walking cycles. Thirty-eight normal subjects walked comfortably on a treadmill while breathing through a pneumotachograph. We analyzed the time intervals between heel strikes and the onset of inspiration (or expiration) for evidence of phase locking between steps and breaths, using Monte Carlo simulation to model the probability that n consecutive inspirations (or expirations) would begin at a constant time interval +/- 0.10 s from heel strikes by chance. We developed empirical criteria for rhythm synchronization during series of four or more breaths, while maintaining an estimated specificity of 95%. The majority of subjects showed some evidence of entrainment (29 +/- 23% of breaths on average), which occurred intermittently, usually lasting less than 10 breaths at a time. The precision of phase locking during spontaneous entrainment was similar to that in 10 subjects who attempted to maintain deliberate entrainment. The results suggest that the walking cadence provides a persuasive, but not dominant, input to the central breathing pattern generator. The present method can detect entrainment even when it occurs sporadically or with varying coupling pattern.     

Ventilatory and gas exchange responses under spontaneous and fixed breathing modes during arm exercise

To evaluate the difference of ventilatory and gas exchange response differences between arm and leg exercise, six healthy young men underwent ramp exercise testing at a rate of 15 W.min-1 on a cycle ergometer separately under either spontaneous (SPNT) or fixed (FIX) breathing modes, respectively. Controlled breathing was defined as a breathing frequency (fb; 30 breaths.min-1) which was neither equal to, nor a multiple of, cranking frequency (50 rev.min-1) to prevent coupling of locomotion and respiratory movement, i.e., so-called locomotor-respiratory coupling (LRC). Breath-by-breath oxygen uptake (VO2), ventilation (VE), CO2 output (VCO2), tidal volume (VT), fb and end-tidal PCO2 (PETCO2) were determined using a computerized metabolic cart. Arm exercise engendered a higher level of VO2 at each work rate than leg exercise under both FIX and SPNT conditions. However, FIX did not notably affect the VO2 response during either arm or leg exercise at each work rate compared to SPNT. During SPNT a significantly higher fb and lower PETCO2 during arm exercise was found compared with leg exercise up to a fb of 30 breaths.min-1 while VE and VT were nearly the same. During fixed breathing when fb was fixed at a higher rate than during SPNT, a significantly lower PETCO2 was observed during both exercise modes. These results suggest that: 1) FIX breathing does not affect the VO2 response during either arm or leg exercise even when non-synchronization between limb locomotion movement and breathing rate was adopted; 2) at a fb of 30 breaths.min-1 FIX breathing induced a hyperventilation resulting in a lower PETCO2 which was not associated with the metabolic rate during either arm or leg exercise, showing that VE during only leg exercise under the FIX condition was significantly higher than under the SPNT condition.     

Pulmonary afferent control of breathing as end-expiratory lung volume decreases

We studied reflex changes in breathing elicited by graded reductions in end-expiratory lung volume (EEVL) and the vagal nerves responsible. The chests of nine dogs anesthetized with alpha-chloralose were opened, and the lungs were ventilated by a phrenic nerve-driven servo-respirator. The immediate effects of a 50% reduction in end-expiratory transpulmonary pressure (EEPtp) from control (EEVL equivalent to functional residual capacity) were to significantly increase both tidal volume (VT) and breathing frequency (f) from 0.402 +/- 0.101 to 0.453 +/- 0.091 liter (mean +/- SD) and 11.8 +/- 5.4 to 15.7 +/- 6.4 breaths/min, respectively (P less than 0.05). Further reductions in EEPtp to 0 cmH2O did not change VT but augmented f to 19.6 +/- 6.6 breaths/min (P less than 0.05). The increase in f as EEVL decreased was due entirely to a reduction in expiratory time. Vagotomy abolished these reflexes. By 90 s after reduction in EEVL, arterial PCO2 fell significantly and VT returned to or below control values. We therefore repeated these experiments in five dogs whose blood gases were controlled by cardiopulmonary bypass. There were no secondary changes in VT and by 90 s breathing pattern could be characterized as rapid and deep. In another eight dogs submitted to the same collapse protocol, we recorded action potentials from all known categories of pulmonary vagal afferents. These studies demonstrated that the changes in breathing pattern induced by a 50% reduction in EEPtp were due to a withdrawal of slowly adapting stretch receptor activity; however, continued increases in f as EEVL was reduced further were due to increases in rapidly adapting stretch receptor activity.(ABSTRACT TRUNCATED AT 250 WORDS)     

Inter-Observer Agreement in Measuring Respiratory Rate

Background

Respiratory rate (RR) is an important vital sign which is strongly correlated with in-hospital mortality. At the same time, RR is the most likely vital sign to be omitted when assessing a patient. We believe that one reason for this could be the difficulty in measure the RR, since it is not read off a monitor, but counted manually. Also there is the possibility of assessment bias and the inter-observer reliability becomes important. We therefore set out to investigate how the nursing staff counting the actual number of respirations per minute would agree with the nursing staff using a predefined ordinal scale.

Methods

For this prospective study, we recorded five videos of a young healthy man breathing approximately 5, 10, 15, 30 and 60 times per minute. The videos were shown in a random order to a suitable sample of the nursing staff. The participants were randomized into two groups; one to count the exact number of breaths per minute, and one to use a predefined ordinal scale.

Results

Comparing the exact number of breaths per minute, the Intra Class Coefficient (ICC) was 0.99 (95% CI: 0.97–1.00). Comparing the RR using the predefined scale, the overall Kappa Fleiss Coefficient was 0.75.

Conclusions

The inter-observer agreement was high when comparing the use of the actual number of breaths per minute and substantial when comparing the use of the predefined scale. This is the largest inter-observer study on RR to date. However, further studies on the use of scaled comparisons of RR are needed.     

Effects of spontaneous swallows on breathing in awake goats.

The effects of spontaneous swallows on breathing before, during, and after solitary swallows were investigated in 13 awake goats. Inspiratory (TI) and expiratory (TE) time and respiratory output were determined from inspiratory airflow [tidal volume (VT)] and peak diaphragmatic activity (Dia(peak)). The onset time for 1,128 swallows was determined from pharyngeal muscle electrical activity. During inspiration, the later the swallowing onset, the greater increase in TI and VT, whereas there was no significant effect on TE and Dia(peak). Swallows in early expiration increased the preceding TI and reduced TE, whereas later in expiration swallows increased TE. After expiratory swallows, TI and VT were reduced whereas minimal changes in Dia(peak) were observed. Phase response analysis revealed a within-breath, phase-dependent effect of swallowing on breathing, resulting in a resetting of the respiratory oscillator. However, the shift in timing in the breaths after a swallow was not parallel, further demonstrating a respiratory phase-dependent effect on breathing. We conclude that, in the awake state, within- and multiple-breath effects on respiratory timing and output are induced and/or required in the coordination of breathing and swallowing.     

Control of breathing in the echidna (Tachyglossus aculeatus) during hibernation

Resting non-hibernating echidnas are characterised by low metabolic rates, but also have a very low respiratory frequency and a variable respiratory minute volume, often resulting in low levels of arterial O(2) and high CO(2). As the echidna lies at one physiological extreme among the hibernators, in terms of its large size and low metabolism and ventilatory requirement when not hibernating, a study of control of breathing during hibernation in echidnas should provide a useful test of the generality of various models. We used non-invasive techniques to study breathing patterns and the control of ventilation in 6 echidnas. Hibernating echidnas (T(b) range 7-10 degrees C) showed episodic breathing with bursts of breaths (average 36+/-16 breaths in 24+/-5 min) followed by a period of apnea (76+/-17 min) then a series (8+/-4) of slow breaths at 14+/-1 min intervals leading up to the next burst. Increasing CO(2) levels in the inspired air increased the number of breaths in a burst, eventually leading to continuous breathing. Inter burst breaths were controlled by O(2): hypoxia increased inter burst breaths, and decreased burst length, while hyperoxia abolished inter burst breaths and increased the apneic period. Overall, while CO(2) was a strong respiratory stimulus in hibernating echidnas, O(2) had little effect on total ventilation, but did have a strong effect on the breathing pattern.     

Compliance, blood gases, and acid-base balance in guinea pigs artificially ventilated with different IPPV patterns

In order to estimate optimum parameters for artificial ventilation of adult guinea pigs, the effect of four hours intermittent positive pressure ventilation (IPPV) was studied using different tidal volumes (VT), respiratory frequencies (f), and minute volumes (Ve). Total compliance was measured by placing the animal in a whole body plathysmograph, the arterial blood gases, pH and base excess by catheterizing the carotid artery. In Series I 9 guinea pigs were ventilated at parameters adapted to the spontaneous breathing pattern (VT = 2 ml, f = 70 breaths.min-1). This ventilatory pattern resulted in severe disorders in compliance, gas exchange, and acid-base balance. In Series II 3 different VT (2, 6, 10 ml) were studied by changing f so that Ve was kept constant. The results demonstrated a favourable effect of slow and deep ventilation upon lung mechanics and oxygenation. In Series III 3 different Ve (300, 250, 200 ml.min-1) were tested using a constant VT = 10 ml. Optimum parameters for artificial ventilation of adult guinea pigs were: VT = 10 ml and f = 20 breath--min-1 which resulted in stable compliance, good O2-saturation, normocapnia and normal acid-base balance.     

Variability of resting respiratory drive and timing in healthy subjects

Studies of breathing pattern have focused primarily on changes in the mean values of the breathing pattern components, whereas there has been minimal investigation of breath-to-breath variability, which should provide information on the constancy with which respiration is controlled. In this study we examined the variability of breathing pattern both on a breath-to-breath and day-to-day basis by calculating the coefficient of variation (i.e., the standard deviation expressed as a percentage of the mean). By examining breath-to-breath data, we found that the coefficients of variation of tidal volume (VT) and fractional inspiratory time (TI/TT, an index of timing) obtained with an inductive plethysmograph and spirometer were within 1% of each other. Examination of breath-to-breath variability in breathing pattern over a 15-min period in 65 subjects revealed large coefficients of variation, indicating the need to base calculations on a relatively large number of breaths. Less breath-to-breath variability was observed in respiratory frequency [f, 20.8 +/- 11.5% (SD)] and TI/TT (17.9 +/- 6.5%) than in VT (33 +/- 14.9%) and mean inspiratory flow (VT/TI, an index of drive; 31.6 +/- 12.6%; P less than 0.0001). Older subjects (60-81 yr) displayed greater breath-to-breath variability than young subjects (21-50 yr). Use of a mouthpiece did not affect the degree of variability.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of mechanical loading on expiratory and inspiratory muscle activity during NREM sleep

We investigated the effect of acute and sustained inspiratory resistive loading (IRL) on the activity of expiratory abdominal muscles (EMGab) and the diaphragm (EMGdi) and on ventilation during wakefulness and non-rapid-eye-movement (NREM) sleep in healthy subjects. EMGdi and EMGab were measured with esophageal and transcutaneous electrodes, respectively. During wakefulness, EMGdi increased in response to acute loading (18 cmH2O.l-1.s) (+23%); this was accompanied by preservation of tidal volume (VT) and minute ventilation (VE). During NREM sleep, no augmentation was noted in EMGdi or EMGab. Inspiratory time (TI) was prolonged (+5%), but this was not sufficient to prevent a decrease in both VT and VE (-21 and -20%, respectively). During sustained loading (12 cmH2O.l-1 s) in NREM sleep, control breaths (C) were compared with the steady-state loaded breaths (SS) defined by breaths 41-50. Steady-state IRL was associated with augmentation of EMGdi (12%) and EMGab (50%). VT returned to control levels, expiratory time shortened, and breathing frequency increased. The net result was the increase in VE above control levels (+5%, P less than 0.01). No change was noted in end-tidal CO2 or O2. We concluded that 1) wakefulness is a prerequisite for immediate load compensation (in its absence, TI prolongation is the only compensatory response) and 2) during sustained IRL, the augmentation of EMGdi and EMGab can lead to complete ventilatory recovery without measurable changes in chemical stimuli.     

Power spectral density of breathing pattern in patients with chronic obstructive lung disease

Newsom Davis and Stagg studying the interrelationship of the volume and time components of individual breaths in healthy resting man described a significant correlation between mean tidal volume (VT) and inspiratory time (TI) r = 0.704. The correlation between mean TI and expiratory time (TE) was lower, r = 0.381. Evaluation of these relationships and of the power spectral density of the breathing pattern was the aim of the present study. For breath by breath analysis we calculated power spectral density and cross correlations of VT, TI and TE. We found a significant correlation between VT and TI in 9 patients with global respiratory insufficiency (RI) (mean r = 0.52) and 7 patients with partial RI (mean r = 0.56). The correlation between TI, TE was lower, in 9 patients with global RI (mean r = 0.21) and 7 patients with partial RI (mean r = 0.35). The results of both groups did not differ from healthy subjects in power spectral density of the breathing pattern and in correlations of VT and TI as well as TI and TE.