Effect of lung volume on breath holding

The mechanism by which large lung volume lessens the discomfort of breath holding and prolongs breath-hold time was studied by analyzing the pressure waves made by diaphragm contractions during breath holds at various lung volumes. Subjects rebreathed a mixture of 8% CO2-92% O2 and commenced breath holding after reaching an alveolar plateau. At all volumes, regular rhythmic contractions of inspiratory muscles, followed by means of gastric and pleural pressures, increased in amplitude and frequency until the breakpoint. Expiratory muscle activity was more prominent in some subjects than others, and increased through each breath hold. Increasing lung volume caused a delay in onset and a decrease in frequency of contractions with no consistent change in duty cycle and a decline in magnitude of esophageal pressure swings that could be accounted for by force-length and geometric properties. The effect of lung volume on the timing of contractions most resembled that of a chest wall reflex and is consistent with the hypothesis that the contractions are a major source of dyspnea in breath holding.     

Shape and size of the human diaphragm in vivo

Serial computerized tomograph (CT) sections at 5-mm intervals of a human diaphragm in relaxed and contracted states were obtained in one subject while he held his breath and lay supine in a CT scanner. All sections for one state were scanned at the same chest wall configuration as monitored by rib cage and abdominal dimensions, using magnetometers. Sections were scanned at relaxed functional residual capacity and after inspiring approximately 1 liter in such a way that rib cage dimensions increased only slightly. Models of the diaphragm dome in the two states were constructed from the sets of serial sections. Diaphragm length and volume displaced were measured, the zone of apposition of diaphragm to rib cage was mapped, and the line of the diaphragm silhouette in anteroposterior and lateral X-rays identified. Coronal and sagittal sections were constructed. In the inspiration studied, the diaphragm movement displaced 680 ml. Meridian lines in sagittal, coronal, and transverse directions over the right hemidiaphragm dome shortened by 6.7-7.2 cm, but over the left dome by only 4.0-4.3 cm. Lines of X-ray silhouettes were close to meridian lines, and estimates of shortening were similar to those made previously from X-rays. The peculiar saddle shape of the muscle may help the hemidiaphragms to operate independently, the fibers of the saddle acting as an anchor for midline directed fibers of the hemidiaphragm domes. The shape of the diaphragm also has implications for the distribution of transdiaphragmatic pressure and for the kind of distortion of the lower rib cage margin that is seen during inspirations at high lung volume.     

2-Aminoethylphosphonic acid concentrations in some rumen ciliate protozoa.

The concentration of 2-aminoethylphosphonic acid has been measured in seven genera of rumen ciliate protozoa. Expressed as milligrams per gram of total nitrogen, 2-aminoethylphosphonic acid concentrations ranged from 17.2 in Ophryoscolex spp. to 72.4 in Eremoplastron spp.     

OBSERVATIONS ON HUMAN MONOCYTE KINETICS AFTER PULSE LABELING

Monocyte kinetics were studied in seven hematologically normal individuals using in vivo pulse labeling with tritiated thymidine. Although occasional labeled cells appear in the peripheral blood within 4 or 5 hr of the administration of label, a significant outflow from the marrow begins 13–26 hr later. This interval is occupied by the G₂ and M phases of the mitotic cycle since mitotic cells are not observed in the peripheral blood. The duration of the DNA synthesis phase of monocytes is measured at 34 hr ≈ 1.8 hr. Cells do not enter this phase while circulating since exposure of circulating cells to tritiated thymidine does not result in any uptake. If monocytes are not 'end'cells which have completed their mitotic activity before leaving the marrow they must at least be inhibited from further proliferative activity until they are permanently sequestered in other tissues.
The generation time is probably not less than 40 hr and data derived from the mean grain counts of labeled cells suggest that it is often more than 70 hr. The total daily output of monocytes in man is 9.4 × 10⁸ cells per 24 hr ≈ 3.3 × 10⁸.
Cells leave the bloodstream with a half-time of about 71 hr thereby proving themselves to be considerably more durable than neutrophils which have a half-life in the neighborhood of 6 hr.     

Patterns of intercostal muscle activity in humans

Coordination of activity of inspiratory intercostal muscles in conscious human subjects was studied by means of an array of electromyograph (EMG) electrodes. Bipolar fine wire electrodes were placed in the second and fourth parasternal intercostal muscles and in two or three external intercostal muscles in the midaxillary line from the fourth to eighth intercostal spaces. Subjects breathed quietly or rebreathed from a bag containing 8% CO2 in O2 in both supine and upright postures. Respiration was monitored by means of flow, volume, and separate rib cage and abdominal volumes. Onset of EMG activity in each breath was found near the beginning of inspiration in the uppermost intercostal spaces but progressively later in inspiration in lower spaces, indicating that activity spreads downward across the rib cage through inspiration. At higher ventilation stimulated by CO2, activity spread further and faster downward. In voluntary deep breathing, external intercostal muscles tended to be recruited earlier in inspiration than in CO2-stimulated breathing. The change from supine to sitting resulted in small and inconsistent changes. There was no lung volume or rib cage volume threshold for appearance of EMG activity in any of the spaces.     

Relationship between inspiratory effort and breathlessness in pregnancy.

Using open-magnitude scaling, we compared the relationships between breathlessness, inspiratory esophageal pressure swing (delta Pes), and ventilation in pregnancy and postpartum. Thirteen healthy women performed progressive cycle exercise tests at 33 +/- 2 wk gestation and 12 +/- 3 wk postpartum. Pulmonary function and maximal transdiaphragmatic pressure did not change. Minute ventilation (VE) was greater in the third trimester. This increase was entirely due to the increase in tidal volume (VT; 0.74 +/- 0.18 vs. 0.54 +/- 0.18 liters at rest, P less than 0.01; 1.56 +/- 0.3 vs. 1.24 +/- 0.24 liters at 48 W, P less than 0.001). delta Pes (15.3 +/- 3.0 vs. 11.9 +/- 3.5 cmH2O at 48 W, P less than 0.01) and breathlessness (1.8 +/- 1.4 vs. 1.0 +/- 0.9 at 48 W, P less than 0.05) were greater in the third trimester. However, the relationships between VT and delta Pes and between delta Pes and breathlessness were identical in the two conditions. The VT-tidal abdominal volume (Vab) and Vab-tidal gastric pressure swing (delta Pga) relationships were similar in the two conditions. In conclusion, the relationship between delta Pes and breathlessness is the same in the third trimester and postpartum. The increased VE is responsible for the breathlessness in the third trimester. Despite progressive abdominal distension by the gravid uterus, the VT-Vab and Vab-delta Pga relationships were the same in the two conditions.