Breathing through a diving snorkel; theory and experiment of air flow resistance and cost of breathing.

The snorkel allows a surface swimmer to observe the underwater world through the face mask without being disturbed by inhaling. The effect of a snorkel on breathing resistance and cost is widely held to be substantial. This study aims to model these parameters and to measure indirectly the actual increases. Further, resistances of differing designs and dimensions were assessed and recommendations were made concerning use and choice. Maximal voluntary ventilation in 12 seconds (MVV12) was measured in 19 volunteers seated on dry land with and without a classic J-type snorkel (inner diameter 20.5 mm). The extra and total resistances and costs were calculated using the MVV12 data and using estimated airways resistance extrapolated from subject's demography and spirometric literature data. MVV12 measurements with snorkel showed a minute volume of 152 ±38 L∙min-1, 6.0 ±3.7% lower than without snorkel (p = 7.0x10-6). The theoretical MVV12, calculated from snorkel and airways resistances, decreased by 3.2%. Experimental total breathing resistance (457±83 Pa∙s∙L-1) was 6.5 ± 3.2% higher than without snorkel (p = 2.6x10-7), but the total mechanical breathing cost was unaffected by the snorkel (13.58 Watts with; 13.64 Watts without). Divers' estimations of resistance increase were exaggerated (8.8% at rest, 23% swimming). Classical J-type snorkels with an inner diameter ≥19.5 mm add 3-16% resistance . There is no risk of hypercapnia. Scuba divers are recommended to use their snorkel to breathe more comfortably on the surface. It is recommended the snorkel be made a mandatory safety accessory. The best multipurpose snorkel (19-21 mm) has no top appendages and no water release valve.

Mental stress may cause high gas consumption and accelerated heart rate in fast-descending divers.

The descent is a critical part of a dive, both physically and mentally. Pulmonary ventilation, expressed as respiratory minute volume (RMV) and heart rate (HR) were recorded during fast and slow descents to 35 meters (m) in open water while breathing compressed air, and during swimming horizontally at moderate velocity at an 11-m depth. Values of both types of descents were compared with reference values recorded at 11 m, the "plateau" phase, halfway through the 35-m dives. It is hypothesized that the "slow-descent" and "plateau-phase" values will be less than 'fast-descent values. Depth, cylinder pressure, water temperature and HR were recorded with a dive computer yielding time-averaged means (mRMV and mHR) for the descent and for plateau. Of the 18 divers included, 16 performed the fast descents and 11 made the slow descents. The fast descents (23 m·min-1 vertically), performed with 0-8 fin kicks, yielded mRMVDescent=28 ambient L (aL)·min-1, which is 82% higher (P⟨0.001) than mRMVPlateau of 15 aL·min-1. Further, mHRDescent was121 beats·min-1 23% higher (P⟨0.001), than mHRPlateau of 100 bpm. Slow descents (2.4 m·min-1 vertically) yielded 17 aL·min-1 with mHR=101 beats·min-1, values only slightly higher than at Plateau. The 11-m dive (swimming horizontally) yielded 24 m·min-1 with 32 fin kicks·min-1, mRMV=35 aL·min-1 and mHR=115 beats·min-1. Fast descents cause a higher RMV and HR that cannot be explained by physiology alone. Presumably mental stress is a main contributor. For dives deeper than 20 m, a descent velocity of 10 m·min-1 is recommended to reduce cardiac stress, in particular for older divers.

Venous gas embolism after an open-water air dive and identical repetitive dive.

Decompression tables indicate that a repetitive dive to the same depth as a first dive should be shortened to obtain the same probability of occurrence of decompression sickness (pDCS). Repetition protocols are based on small numbers, a reason for re-examination. Since venous gas embolism (VGE) and pDCS are related, one would expect a higher bubble grade (BG) of VGE after the repetitive dive without reducing bottom time. BGs were determined in 28 divers after a first and an identical repetitive air dive of 40 minutes to 20 meters of sea water. Doppler BG scores were transformed to log number of bubbles/cm2 (logB) to allow numerical analysis. With a previously published model (Model2), pDCS was calculated for the first dive and for both dives together. From pDCS, theoretical logBs were estimated with a pDCS-to-logB model constructed from literature data. However, pDCS the second dive was provided using conditional probability. This was achieved in Model2 and indirectly via tissue saturations. The combination of both models shows a significant increase of logB after the second dive, whereas the measurements showed an unexpected lower logB. These differences between measurements and model expectations are significant (p-values < 0.01). A reason for this discrepancy is uncertain. The most likely speculation would be that the divers, who were relatively old, did not perform physical activity for some days before the first dive. Our data suggest that, wisely, the first dive after a period of no exercise should be performed conservatively, particularly for older divers.

Dynamic characteristics of retinal ganglion cell responses in goldfish.

A cross-correlation technique has been applied to quantify the dependence of the dynamic characteristics of retinal ganglion cell responses in goldfish on intensity, wavelength, spatial configuration, and spot size. Both theoretical and experimental evidence justify the use of the cross-correlation procedure which allows the completion of rather extensive measurements in a relatively short time. The findings indicate the following. (a) The shape of the amplitude characteristics depends on the energy per unit of time (power) falling within the center of a receptive field rather than on the intensity of the stimulus spot. For spot diameters of up to 1 mm, identical amplitude characteristics can be obtained by interchanging area and intensity. Therefore the receptor processes do not contribute to the change in the amplitude characteristics as a function of the power of the stimulus light. (b) For high frequencies the amplitude characteristics obtained as a function of power join together in a common envelope if plotted on an absolute sensitivity scale. For spontaneous ganglion cells this envelope holds over a range of three log units and the shape is identical for central and peripheral processes. (c) The amplitude characteristics of the central and peripheral processes converging to a ganglion cell are identical, irrespective of the sign (on or off) and the spectral coding of the response. Therefore we have no evidence for interneurons in the goldfish retina unique to the periphery of the receptive field.

A Golgi study of goldfish optic tectum.

A study of goldfish optic tectum was performed with rapid-Golgi, Golgi-Kopsch and a modified Golgi-Cox impregnation which proved quite suitable to impregnate cells in the middle tectal layers and to study more closely axonal properties. Fifteen cell types are distinguished, based upon the position of dendritic trees and axonal properties. Two cell types are found with dendrites in the marginal layer: type I with an axon terminating in the central gray layer and type II without an impregnated axon. Three cell types (III, IV and V) have dendrites in a single, specific tectal layer and an axon terminating within the tectum. Five cell types (VI-X) have dendrites in two horizontal planes. Two of them have myelinated axons leaving the tectum, whereas the axons of the remaining three types project to different tectal layers. While these first ten cell types have dendrites almost exclusively in the superficial half of the tectum, the remaining five types have dendrites in deeper layers too. This especially holds for the most conspicuous tectal cells (types XII and XIII), which have dendritic trees branching at three or more horizontal levels and a myelinated axon leaving the tectum, with sometimes a very peculiar course (XIII1). Also type XI has three or more dendritic trees, but its axon was not found. The numerous cells with cell bodies in the deepest tectal layer (type XIV) have dendrites and axonal terminations anywhere in the tectum, except in the most superficial and the deepest layer. However, most dendrites occur in the optic layers, whereas the axons, always originating from the dendritic shaft in the superficial tectal half, generally terminate in the middle tectal layers. Type XV cells have their soma in the deepest tectal layer as well, but their dendrites do not reach the optic layers. Per tectal lobe the following numbers are estimated: type I : 5,000-20,000 neurons; Type III : 2,500-10,000; types IV--XIII : each 500-2,000 and type XIV : 1,000,000-2,000,000. The total number of myelinated tectal efferents is estimated at 2,000-8,000. Comparison with other Golgi studies in teleosts leads to the conclusion that the tecta of these species of fish are basically similar.

O2 consumption and CO2 influence on the pH and electroretinogram of isolated retina of goldfish.

From micropolarographic O2 measurements, it can be concluded that the isolated goldfish retina is sufficiently supplied with oxygen when 100% O2 is blown over the retina. The retinal O2 consumption amounts to about 60 microliters of O2 per milligram dry weight per hour. Small amounts (0.25% to 2.0%) of CO2 added to the O2 gas diminish the retinal pH, which is 7.0 to 7.1 in situ and after isolation. Moreover, CO2 strongly reduces the ERG amplitude.

Acousticolateral and visual processing and their interaction in the torus semicircularis of the trout, Salmo gairdneri.

In the torus semicircularis of the trout, visual, auditory, visual-auditory, acousticolateral broadband, lateral line, visual-acousticolateral broadband and visual-lateral line units have been found, listed in decreasing frequency of occurrence. In contrast to the acousticolateral characteristics, the visual characteristics of toral units are poor; only sudden changes in the visual field seem to be of importance. When bimodal units are stimulated simultaneously with light and sound the responses of most units behave in a basically additive fashion. The activity of some units which responded to only one of the modalities could be modulated by the other modality.

Influence of GABA on the spectral and spatial coding of goldfish retinal ganglion cells.

The responses of center red ON and center red OFF ganglion cells of the isolated goldfish retina to a flickering stimulus light diminish by GABA. Without stimulation GABA blocks the maintained activity of the OFF cells. With a green flickering spot in the receptive field center, the green ON-response of the OFF cells becomes an OFF-response during GABA administration. It is concluded that, besides the common antagonistic green input, the receptive field center of these cells also has a synergistic green input, which is mediated by a pathway other than the center opponent red and green inputs.

Octavolateral projections to the torus semicircularis of the trout, Salmo gairdneri.

Microiontophoretic delivery of horseradish peroxidase in the torus semicircularis of the trout resulted in heavy labeling of somata in the rhombencephalic nucleus intermedius octavolateralis and nucleus octavus magnocellularis. In addition some labeled somata were found closely to the fasciculus longitudinalis lateralis and in the diencephalon. Efferents leave the torus to the diencephalon, the tegmentum and the tectum and, via the fasciculus longitudinalis lateralis, to the rhombencephalon and the spinal cord. It is concluded that in the trout the torus receives octaval and lateral line input mainly directly from the octavolateral area in the hindbrain, without involvement of a superior olive.

Mapping of sound direction in the trout lower midbrain.

In the trout lower midbrain 35% of the auditory neurons are directionally selective (DS). Most of these neurons have a higher directional selectivity than the sensory hair cells. DS units and non-DS units occur in vertical clusters, with the former more dorsally. The direction of preference is topographically mapped. Apparently, auditory space mapping is a common feature in the midbrain of vertebrates.

Temporal and spatial congruence of components of motion-onset evoked responses investigated by whole-head magneto-electroencephalography.

Motion-onset related components in averaged whole head co-recorded MEG and EEG responses of 24 adults to a low-contrast checkerboard pattern were studied. The aims were to identify these components, to characterize quantitatively their maps and to localize the underlying sources by equivalent-current-dipole (ECD) analyses with a spherical head model.After a weak P1, a large start-elicited negativity arises, comprising the novel N2a (occipital positive and parieto-central negative, peak-latency 141 ms) and the N2 like N2b (bilateral parieto-temporal, 175 ms) component. It is followed by a large positive stop-related component, P2 (156 ms after motion-offset). The corresponding MEG components N2am and N2bm showed bilateral dipole fields with considerable overlap. P1m has a single dipole field around the midline. N2a(m) and N2b(m) can be modelled with two bilateral ECDs with significant different locations. The study shows that accurate mapping and ECD analyses can distinguish two neighbouring areas of the visual cortex, 21+/-4 (SE) mm separated, which activities are reflected in both spatio-temporally closely related N2(m) components. N2a(m) and N2b(m) originate in the extrastriate cortex, possibly close to or in V3/V3A and MT/V5 respectively. Motion-evoked activity in (near) V3/V3A is novel on the basis of EEG data.

Computerized pattern recognition used for grain counting in high resolution autoradiographs with low grain densities.

Using a video-image system coupled to a minicomputer with commercial image handling software, autoradiographic grains displayed in dark-field are counted with a fast (ca. 3.5 min for 120,000 microns 2) and reliable (false scores less than 5%) grain-recognizing FORTRAN program executed in the users memory. The grain counts are printed in a raster of adjustable size overlying a bright-field image, so that the counts can be related directly to the underlying histological structures.

Transient and maintained changes of the spontaneous occipital EEG during acute systemic hypoxia.

BACKGROUND: It has been reported that during systemic hypoxia the spectral power of spontaneous EEG (closed eyes) increases, except for the power in the alpha band, which decreases. HYPOTHESIS: It is surmised that after a sudden decrease in inspired oxygen, the EEG parameters and SaO2 show similar temporal characteristics. METHODS: Normobaric acute hypoxia in 14 healthy subjects was evoked by breathing a hypoxic gas. Frequency spectra were calculated from occipital recordings and quantified together with the parameters of the spectral alpha peak. RESULTS: During the first 20 min of exposure, SaO2 decreases exponentially. With eyes open, the frequency bands show a transient increase in power (110-160%). The peak-time of the transient is shorter for the alpha and beta1 bands (4.5 min) than for the delta2 and theta bands (7 min). After the transient, the bands reach sustained power levels ca. 65% higher than the control value. The frequency of the alpha peak decreases with 0.61 +/- 0.16 Hz (mean +/- SE). In contrast to the eyes-closed condition, the peak amplitude increases (41 +/- 12%). CONCLUSIONS: During systemic hypoxia many characteristics of the spontaneous EEG with open eyes differ from those under normoxia, and alpha activity deviates strongly from that during hypoxia with closed eyes. During the first 20 min of exposure changes in power are most prominent and do not follow the change of SaO2. Monitoring total EEG power with open eyes, SaO2 and respiration gives a good impression of the hypoxia-related neurophysiological state of the subject.

Voluntary breath holding affects spontaneous brain activity measured by magnetoencephalography.

Spontaneous brain activity was measured by multichannel magnetoencephalography (MEG) during voluntary breath holds. Significant changes in the activity are limited to the alpha rhythm: 0.25 Hz frequency increase and narrowing of the peak. The area of alpha activity shifts slightly toward (fronto-) temporal. The topography of other rhythms is unaffected by breath holding. Electroencephalographic (EEG; 36 channels maximal) recordings generally made simultaneously with the MEG recordings show similar effects. However, EEG was inadequate to reveal the small topographic differences. Systemic hypercapnia caused by a long breath hold is unlikely to play an important role in producing the observed effects.

Effect of dysoxia and moderate air-hyperbarism on red-green color sensitivity.

The effect of acute air-hyperbarism (maximal 520 kPa), normobaric low and high FIO2 levels (minimal 0.1, maximal 1.0) and hyperbaric oxygen (HBO2) on the red-green sensitivity ratio (rgSR) and on color discrimination for foveal vision were studied. Effects were quantified by measuring the red-green flicker (16 Hz) fusion point for normals with the Oscar tester (Am J Optom Physiol Opt 1983; 60:892-901). Color discrimination was examined with the Lanthony's Desaturated 15 Hue test After 15-20 min of exposure rgSR is enhanced 4% (relatively increased red sensitivity) by normobaric acute hypoxia, and reduced 4% by normobaric hyperoxia (FIO2 = 1.0), but HBO2 gives a smaller reduction, and air-hyperbarism (FIO2 = 0.21) has no effect. Hypercapnia (increased FICO2), normobaric hypoxia (reduced FIO2), and HBO2 increase the duration of the Lanthony's test about 20-40%, but the number of errors were practically unchanged. The reduced effect during HBO2 upon rgSR is attributed to an opposing effect of hypercapnia. The absence of an effect during air-hyperbarism is probably due to a suppression by nitrogen of the effect of high PO2. In general, during moderate air-hyperbarism and HBO2 color vision seems to be normal, but evaluation of the colored scene is probably less stable and slightly slower.

Contrast sensitivity of air-breathing nonprofessional scuba divers at a depth of 40 meters.

Photopic contrast sensitivity of air-breathing scuba divers was measured with a translucent test pattern at depths up to 40 m. The pattern was composed of sine wave gratings with spatial frequency and contrast changing logarithmically. The spatial transfer characteristics were measured at various depths under controlled optical conditions in seawater and in fresh water. Analysis indicates that the visual contrast sensitivity, and therefore probably also acuity, of sport divers is not affected up to depths of 40 m. This holds under ideal as well as poor diving conditions.

Visual acuity at hyperbaric air pressure.

The visual acuity of five divers in a dry pressure chamber was measured with a Landolt C test. Only one of them showed a significant reduction at 7.0 atm. These results contrast sharply with open sea experiments, which show a considerable reduction at much lower pressures (Baddeley, 1968). It is suggested that besides stresses, luminance differences, also turbidity, as a function of depth may account for the discrepancy.