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Biomedical subjects

B A Hills

Publications and source records attributed to B A Hills.

13 recordsLinked to original sources
PubMed1979

The contact angle induced by DPL at pulmonary epithelial surfaces.

A contact angle has been measured at the surface of dog tracheal epithelium by means of two surface balances simultaneously monitoring the apparent surface tension of the same film in a Langmuir trough - one method dependent upon contact angle and the other independent of contact angle. This was confirmed by direct observation. The contact angle was absent for Ringer's solution alone but was induced by DPL deposited on the fluid surface in physiological concentrations. The contact angle varied from 9 degrees to 67 degrees for compression of the DPL film from 100% to 27.5% of its original area and displayed hysteresis with respect to area. These findings show that DPL has unusual surface properties in reducing surface tension yet decreasing wettability, i.e. acting as an anti-wetting agent. The physiological advantages of this unusual combination of properties are discussed in relation to maintaining pulmonary homeostasis and the forward propulsion of mucus which could be facilitated by the differential wettability of cilia induced by DPL. One possible disadvantage of DPL is indicated where fluid plugs might seal an airway - such as in the newborn.

Animals↗
PubMed1979

A critical assessment of the Wilhelmy method in studying lung surfactants.

1. The surface properties of over 250 films of diplamitoyl lecithin (DPL) and Tween 20 on distilled water have been investigated using two different surface balances simultaneously, the Wilhelmy balance, popular in physiological studies, and the Du Nuoy ring method whose readings are independent of contact angle. 2. Using concentrations of DPL ranging from 0.08 to 1.90 microgram cm-2 on a Langmuir trough where the pool area was cycled from 100 to 27.5% of maximum, the Wilhelmy balance registered virtually the same force per wetted perimeter as the ring method for both pure water and Tween 20, but appreciably lower values for DPL over the whole cycle. 3. The above differences can be explained on the basis of a significant (45-70 degrees) contact angle, a surface property also demonstrated photographically and by direct measurement. 4. Contact angle was shown to vary with pool area, a relationship exhibiting hysteresis. 5. This study indicates that the Wilhelmy balance has been an unfortunate choice of instrument for studying DPL films whose surface tensions are appreciably higher than previously estimated.

Methods↗
PubMed1979

Surface tension induced by dipalmitoyl lecithin in vitro under physiological conditions.

1. The surface tension of 161 films of DPL have been measured on a Langmuir trough using a Wilhelmy balance under conditions controlled to simulate the state of the alveolar lining in vivo.2. The parameters controlled were temperature (maintained at 37 degrees C), humidity (100% at 37 degrees C), surfactant concentrations (encompassing the best available estimates), area changes (consistent with normal respiration), frequency adaptation to continuous cycling and composition and pH of the aqueous hypophase.3. Simultaneously maintaining all of these parameters within the best estimates of physiological limits, the relationships between surface tension and surface area showed appreciable differences from previous studies, our results showing higher minimum values of surface tension, appreciably less change in surface tension with compression and far less hysteresis between surface tension and surface area.4. The higher minimum values are consistent with original estimates of alveolar surface tension made by von Neergaard, namely 35-41 dyne cm(-1).5. Although appreciably smaller than hitherto reported, the change in surface tension with change in area is still adequate to impart alveolar stability.6. The reversibility between surface tension and surface area under physiological conditions is discussed in connection with compliance hysteresis which is considered to be more dependent upon geometric irreversibility of the alveolar surface than upon any intrinsic property of the surfactant.

In Vitro Techniques↗
PubMed1979

Intermittent flow in tendon capillary bundles.

The capillary bed has been observed in the Achilles tendon of 40 bullfrogs and 10 guinea pigs for periods of up to 2 h. The opening and closing of adjacent capillaries in a perfused area follows the pattern originally described by Krogh for skeletal muscle but the frequency is slower, as anticipated from the lower metabolic rate of tendon. However, superimposed on this "flickering is a much slower process whereby whole bundles of 20-147 capillaries open and close with little overlap in the tissue areas perfused by each. Periods of no flow averaged 39 min in 70 bundles followed in bullfrogs and 43 min in 34 bundles followed in guinea pigs, although a few failed to open in 100 min. This bundle phenomenon is discussed in relation to the serious implications in mathematically modeling the exchange of gases and nutrients between blood and tissue and the possible errors in assuming time averaging when determining blood perfusion rates. Also mentioned are the likely effects of decompression on closed bundles and the notion is introduced that the bundle phenomenon may be a factor determining which tissues can be injured by decompression.

Achilles Tendon↗
PubMed1979

The lung as a filter for microbubbles.

A new ultrasonic Doppler device has been used noninvasively over the femoral artery of anesthetized dogs to prove that it can detect carefully calibrated microbubbles of 14--189 micrometers diam when these are infused directly into the aorta. The same evaluated technique has then been employed to detect any bubbles escaping into the arterial system when gas was infused into the venous system either as microbubbles or as a bolus. Results from 18 dogs showed that, under normal conditions, the lungs are a superb filter for bubbles and that any cutoff diameter is less than 22 micrometers. However, bubbles escaped entrapment when the lungs were severely overloaded with gas (20 ml) or were pretreated with a pulmonary vasodilator (aminophylline). The dog preparation and arterial Doppler device appear to be ideal for future studies to determine what other factors might compromise the capability of the lungs to filter microbubbles. Physiological parameters showed dramatic changes when bubbles were detected as escaping into the arterial system by comparison with their effect when retained within the lungs. Changes in respiration profile indicated that they may offer a useful index of the degree of venous embolization and, hence, a warning of impending overload leading to arterial embolization.

Air↗
PubMed1979

Method for estimating absolute lung volumes at constant inflation pressure.

A method has been devised for measuring functional residual capacity in the intact killed animal or absolute lung volumes in any excised lung preparation without changing the inflation pressure. This is achieved by titrating the absolute pressure of a chamber in which the preparation is compressed until a known volume of air has entered the lungs. This technique was used to estimate the volumes of five intact rabbit lungs and five rigid containers of known dimensions by means of Boyle's law. Results were found to agree to within +/- 1% with values determined by alternative methods. In the discussion the advantage of determining absolute lung volumes at almost any stage in a study of lung mechanics without the determination itself changing inflation pressure and, hence, lung volume is emphasized.

Animals↗
PubMed1979

Mechanical vs. ischemic mechanisms for decompression sickness.

We used 20 kangaroo rats to investigate the effect of exposure to low oxygen levels (0.11 Atm 02 inspired partial pressure) prior to decompression from a steady-state condition. This hypoxia was found to afford significant protection against limb bends as simulated in those animals by tail biting. Yet, it potentiated neurologic symptoms compared with a control exposure on air with the same level of nitrogen supersaturation. However the incidence of simulated limb bends in the same animals was the same with hypoxia as with another control exposure at a pressure estimated to give extravascular bubbles of the same size upon decompression. The results are, therefore, consistent with a simple mechanical basis for limb bends, but are difficult to explain by any ischemic mechanism since a general hypoxia exacerbates any pain produced by oxygen deficiency in the tissues. However, the reverse may be true for some forms of neurologic decompression sickness and the two such cases reported here are consistent with that view, although not statistically significant.

Animals↗
PubMed1978

Effect of decompression per se on nitrogen elimination.

The elimination of nitrogen from each of 10 unanesthetized guinea pigs has been monitored after switching to a nitrogen-free breathing mix (normoxic He:O2), either without decompression or on decompression to 2.21 or 1 ATA, following an exposure of 2 h at 4 ATA on normoxic N2;O2. Normoxic conditions were maintained throughout to avoid vasomotor effects of oxygen that could have complicated interpretations derived from previous studies. Results confirmed that inert gas washout rates decrease with decompression per se. This can be explained simply on the basis of the decrease in driving force for nitrogen elimination caused by depositing gas into bubbles where they form in tissue in a somewhat random manner. A very rough estimate shows that about 83% of all body tissue retained its gas in supersaturated solution on decompression to 2.21 ATA but only 79% did so at 1 ATA.

Animals↗
PubMed1978

The kangaroo rat as a model for type I decompression sickness.

This study involved 720 exposures of 70 kangaroo rats trapped in West Texas and showed that decompression-induced tail biting in this animal provides a good animal model for marginal limb bends in man. That this phenomenon can be reversed by recompression and pathological examination of the tail both indicated that a similar mechanism is probably involved in kangaroo rats and humans. Quantitatively, the most susceptible 20% of kangaroo rats can reproduce the no-stop decompression limits for man for exposure times ranging from 5 min to 8 h, for both air and helium-oxygen. Even the average minimum no-tail-biting depth of 46.2 fsw (2.40 ATA) for this species is much closer to the minimum bends depth of man than to the equivalent depth for other animals of its size, and is as good as the goats'. Its size and habits make the kangaroo rat much more convenient than other animals to use as a model for marginal decompression sickness, and particularly attractive economically for testing long helium-oxygen schedules and other means of decompression sickness prevention.

Animals↗
PubMed1977

Inert gas narcosis.

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Anesthesia, General↗
PubMed1977

Supersaturation by counterperfusion and diffusion of gases.

Gaseous supersaturation can be induced under steady-state conditions when two inert gases are transmitted in opposite directions across any system comprising a diffusion barrier adjacent to a zone of limited convective capacity. This has many implications for bubble formation in vivo and can explain the occurence of symptoms of decompression sickness without decompression.

Decompression Sickness↗
PubMed1975

Evaluation of ultrasonic bubble detectors in vitro using calibrated microbubbles at selected velocities.

Two standard Doppler ultrasonic devices, currently used for detecting bubbles in vivo, have been evaluated and compared in vitro using carefully calibrated uniform micro-bubbles rising at terminal velocity through a static aqueous medium. Two unexpected findings were observed: (a) the focal length of the transducer apparently decreases for smaller bubble sizes, and (b) a significant horizontal convection current was produced by one of the instruments. When the medium was in motion, it was found that the sensitivity varied markedly with bubble velocity, varying from a minimum detectable diameter of 40 mum at 55 cm/sec to 170 mum at 20 cm/sec. These findings are discussed with regard to the limitations of the Doppler technique for monitoring gas emboli in vivo and as an early warning for decompression sickness in divers.

Decompression Sickness↗