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

Sex-specific vagal and spinal modulation of breathing with chest compression

Lung volume is modulated by sensory afferent feedback via vagal and spinal pathways. The purpose of this study was to systematically alter afferent feedback with and without a mechanical challenge (chest compression). We hypothesized that manipulation of afferent feedback by nebulization of lidocaine, extra-thoracic vagotomy, or lidocaine administration to the pleural space would produce differential effects on the motor pattern of breathing during chest compression in sodium pentobarbital anesthetized rats (N = 43). Our results suggest that: 1) pulmonary stretch receptors are not the sole contributor to breathing feedback in adult male and female rats; 2) of our manipulations, chest compression had the largest effect on early expiratory diaphragm activity (“yield”); 3) reduction of spinally-mediated afferent feedback modulates breathing patterns most likely via inhibition; and 4) breathing parameters demonstrate large sex differences. Compared to males, female animals had lower respiratory rates (RR), which were further depressed by vagotomy, while chest compression increased RR in males, and decreased yield in females without changing RR. Collectively, our results suggest that balance between tonic vagal inhibition and spinal afferent feedback maintains breathing characteristics, and that it is important to specifically evaluate sex differences when studying control of breathing.

Ventilatory load compensation response to long-term chest compression in rat model

Short-term chest compression has been shown to decrease tidal volume and increase respiratory frequency. The present study was designed to assess and characterize the effect of long-term chest compression on breathing pattern and blood gases in awake rats. Chest compression was carried out by inflating a pneumatic cuff placed around the chest to a pressure of 25 mmHg and the pressure was maintained for 28 days. Respiratory frequency increased progressively until 14 days after chest compression whereas a decrease in tidal volume was stabilized within 3 days after chest compression. Although the changes in minute ventilation were small and no substantial change in Pa(CO2) was observed, an impairment of weight gain and a decrease in body temperature with a concomitant hypoxemia were evident during sustained chest compression. These observations suggest that the ventilatory response to chest compression may involve not only neural reflex mechanisms but also other non-reflex mechanisms. Sustained chest compression possibly impairs growth and metabolism.

Powerful respiratory stimulation by thin muscle afferents

The ventilatory responses to electrical stimulation of phrenic afferents were examined in spontaneously breathing dogs at different levels of sodium pentobarbital anesthesia. High intensity stimulation (activation of all the afferents, including thin fibers) increased ventilation (V(E)). The increase in V(E) was comparable to that of breathing 10% CO2 and was inversely related to anesthesia level. Under light anesthesia, V(E) increased to 282+/-36% of the control value when the phrenic nerve was stimulated at 130 times the twitch threshold (n = 15; P < 0.01). The increase in V(E) was due to increases in breathing rate (193+/-19%) and tidal volume (V(T)) (143+/-8%). On the other hand, inspiratory time (T(I)) decreased. Thus, average airflow rate (V(T)/T(I)) increased to 204+/-23%. After administration of 20 and 40% of the initial dose of pentobarbital, V(E) response was attenuated to 157+/-21 and 121+/-4%, respectively. We conclude that thin muscle afferents are capable of eliciting pronounced ventilatory stimulation. The small responses observed earlier were likely due to depth of anesthesia.

Control of breathing in a subset of patients with systemic lupus erythematosus

BACKGROUND

Inspiratory muscle weakness and abnormalities in breathing pattern and in respiratory drive have been reported in patients with multisystem disorders. In patients with systemic lupus erythematosus (SLE), data on respiratory muscle strength and control of breathing are scarce.

METHODS

We studied a subset of nine female patients with SLE with no major findings of cardiovascular, renal, or neurologic involvement, and with a normal routine chest radiograph. An age- and sex-matched normal group was also studied as a control. We evaluated lung volumes, diffusing lung properties (TLCO, TLCO/VA), maximal inspiratory (MIP) and expiratory (MEP) pressures, end-tidal carbon dioxide tension (PCO2), and breathing pattern: ventilation (VE), tidal volume (VT), inspiratory time (TI), and respiratory frequency (Rf). Neural respiratory drive, assessed in terms of mean inspiratory flow (VT/TI), mouth occlusion pressure (P0.1), and surface electromyographic activity of the diaphragm (Edi) and intercostal (Eps) muscles was also evaluated.

RESULTS

As a whole, patients exhibited mild decrease in MIP; vital capacity was slightly reduced in two patients and TLCO/VA was moderately reduced in three. During a hypercapnic rebreathing test, delta VT/delta PCO2 was lower, delta P0.1/delta PCO2 was normal, while delta Edi/delta PCO2 and delta Eps/delta PCO2 were higher in patients compared with normal control subjects. delta VT/delta PCO2 significantly related to MIP. At 60 mm Hg of PCO2 patients maintained the rapid and shallow pattern of breathing (RSB) exhibited during room-air breathing: lower VT, shorter TI, and greater Rf, with VE, VT/TI, and Edi being greater compared with the normal control subjects.

CONCLUSIONS

These data seem to indicate that in this SLE subset, mild decrease in respiratory muscle strength may accompany an increased respiratory drive, and contribute to a qualitatively abnormal ventilatory response (RSB) to carbon dioxide stimulation.

Respiratory responses to chest compression in human subjects

To test the hypothesis that respiratory compensation for chest compression in the conscious state is different from that in the unconscious state, a range of chest compression was applied to eight conscious and eight anesthetized subjects. Chest compression was carried out by inflating a pneumatic cuff inserted under a chest corset and placed over the anterior chest wall for 5 min. Cuff inflation pressures were 25, 50, and 100 mm Hg: light, moderate, and heavy chest compression, respectively. In both conscious and anesthetized subjects, chest compression immediately caused a decrease in tidal volume (VT) and a concomitant increase in respiratory frequency (f). These changes stabilized within 2 min and remained nearly steady for the rest of the sustained chest compression. The decrease in VT was more prominent the greater the loads, and the change in f was in reverse proportion to the change in VT. In conscious subjects the increase in f was due to shortening of both inspiratory time (T1) and expiratory time (TE), whereas in anesthetized subjects the increase in f was due solely to shortening of TE. In conscious subjects, the values of minute ventilation (VI) and end-tidal PCO2 (PETCO2) during light and moderate chest compression were not different from the control values, whereas VI increased and PETCO2 decreased during heavy chest compression. In anesthetized subjects, the values of VI and PETCO2 were not different from the control values at any different magnitudes of chest compression.(ABSTRACT TRUNCATED AT 250 WORDS)

Inspiratory muscle strength and respiratory drive in patients with rheumatoid arthritis

In 15 patients with rheumatoid arthritis (RA) and in 12 age- and sex-matched normal subjects, we evaluated inspiratory muscle strength and respiratory control system. Inspiratory muscle strength was assessed by measuring maximal inspiratory pressure (MIP). Respiratory drive was assessed by evaluating surface electromyographic activity of the diaphragm (EMGd) during both room-air breathing and hypercapnic rebreathing. Compared to the predicted value (mean +/- 1.65 SD), MIP was significantly reduced in nine patients (60%). All told, we noticed a significant inverse relationship in the patients between MIP and duration of steroid therapy (p less than 0.01). During room-air breathing, both EMGd and mouth occlusion pressure (P0.1), expressed both in actual values and as percentage of MIP, were significantly greater in patients than in the normal control group (p less than 0.001 for both). Both EMGd and P0.1 (%MIP) response slopes to CO2 were significantly greater in patients than in the normal control group (p less than 0.01 and p less than 0.001, respectively) and significantly related to the functional stage of disease. During quiet breathing and for a PETCO2 of 60 mm Hg, both EMGd (p less than 0.01 and p less than 0.05, respectively) and P0.1 (%MIP) (p less than 0.01 and p = 0.001, respectively) were inversely related to MIP. These results indicate that RA patients may exhibit inspiratory muscle weakness and increased respiratory drive. Steroid myopathy and rheumatoid myositis could explain the reduction in MIP, whereas neural afferents arising from respiratory muscle, lung, or joint receptors could be involved in the observed increase in neural drive.

Breathing pattern in sarcoidosis and idiopathic pulmonary fibrosis

We measured the volume and time components of the breathing cycle together with the P.1 in 16 sarcoidosis and 9 IPF patients. Patients in the IPF group were older, and their lung functions revealed a more restrictive pattern with small TLC, VC, and higher elastance values. We observed that in the two groups of patients there was a significant correlation between VT, f, and P.1 and lung elastance, and that the stiffer IPF patients had higher drive parameters. Hence, the main factor affecting the breathing pattern in both groups is the influence of the elastic load on the control of breathing. A significant relationship was also found between %VD/VT and elastance. We then compared the f values of the 16 sarcoidosis and 9 IPF patients with those of elastically loaded normal subjects. At equivalent levels of elastance, even though the patients and the normals showed qualitatively similar changes, the patients tended to have higher frequencies for the equivalent elastic load. In conclusion, ventilatory and drive parameters in interstitial lung diseases increased as a function of the elastic load, this load being greater in IPF than in sarcoidosis. Inflammation may stimulate the vagal receptors to increase the frequency of breathing, but this requires further study.

Comparison of the respiratory responses to external resistive loading and bronchoconstriction

The effects of resistive loads applied at the mouth were compared to the effects of bronchospasm on ventilation, respiratory muscle force (occlusion pressure), and respiratory sensations in 6 normal and 11 asthmatic subjects breathing 100% O2. External resistive loads ranging from 0.65 to 13.33 cm H2O/liter per s were applied during both inspiration and expiration. Bronchospasm was induced by inhalation of aerosolized methacholine. Bronchospasm increased ventilation, inspiratory airflow, respiratory rate, and lowered PACO2. External resistive loading, on the other hand, reduced respiratory rate and inspiratory flow, but left ventilation and PACO2 unaltered. FRC increased to a greater extent with bronchospasm than external flow resistive loads. With both bronchospasm and external loading, occlusion pressure increased in proportion to the rise in resistance to airflow. However, the change in occlusion pressure produced by a given change in resistance and the absolute level of occlusion pressure at comparable levels of airway resistance were greater during bronchospasm than during external loading. These differences in occlusion pressure responses to the two forms of obstruction were not explained by differences in chemical drive or respiratory muscle mechanical advantage. Although the subjects' perception of the effort involved in breathing was heightened during both forms of obstruction to airflow, at any given level of resistance the sense of effort was greater with bronchospasm than external loading. Inputs from mechanoreceptors in the lungs (e.g., irritant receptors) and/or greater stimulation of chest wall mechanoreceptors as a result of increases in lung elastance may explain the differing responses elicited by the two forms of resistive loading.

Involvement of thoracic nerve afferents in the respiratory response to chest compression

Chest compression elicits extravagal neural reflexes which can alter the respiratory pattern. Experiments were conducted to determine the source of the afferents responsible for the respiratory response to chest compression (CC). The effects of CC on VT, f, TI, TE, blood gases, end-tidal CO2, and blood pressure were studied in anesthetized, vagotomized dogs and cats. In dogs, thoracic wall afferents were eliminated by thoracic dorsal rhizotomies (TDR) and/or spinal blocks (SB). There were two different respiratory responses to CC. In one (I), Tt decreased and TE increased, resulting in a decreased f. The second (II) resulted in a decreased TI and TE. The I response was still present, but weaker, in animals after TDR (1--4), TDR (5--9), TDR (1--9, T5 or T10 SB and absent in those with T1SB. The II response was still present after TDR ()--4), TDR (5--9), TDR (1--9), or T10SB and absent after T5SB. The results indicate that: (1) afferents responsible for the I response to CC arise from the upper, middle and lower thoracic wall, (2) afferents responsible for the II response arise from the middle and lower thoracic wall, and (3) the responses are not due to changes in chemical drive, blood pressure or lung receptors.