Seizures Caused by Exposure to Hyperbaric Oxygen in Rats Can Be Predicted by Early Changes in Electrodermal Activity

Hyperbaric oxygen (HBO2) is breathed during undersea operations and in hyperbaric medicine. However, breathing HBO2 by divers and patients increases the risk of central nervous system oxygen toxicity (CNS-OT), which ultimately manifests as sympathetic stimulation producing tachycardia and hypertension, hyperventilation, and ultimately generalized seizures and cardiogenic pulmonary edema. In this study, we have tested the hypothesis that changes in electrodermal activity (EDA), a measure of sympathetic nervous system activation, precedes seizures in rats breathing 5 atmospheres absolute (ATA) HBO2. Radio telemetry and a rodent tether apparatus were adapted for use inside a sealed hyperbaric chamber. The tethered rat was free to move inside a ventilated animal chamber that was flushed with air or 100% O2. The animal chamber and hyperbaric chamber (air) were pressurized in parallel at ~1 atmosphere/min. EDA activity was recorded simultaneously with cortical electroencephalogram (EEG) activity, core body temperature, and ambient pressure. We have captured the dynamics of EDA using time-varying spectral analysis of raw EDA (TVSymp), previously developed as a tool for sympathetic tone assessment in humans, adjusted to detect the dynamic changes of EDA in rats that occur prior to onset of CNS-OT seizures. The results show that a significant increase in the amplitude of TVSymp values derived from EDA recordings occurs on average (±SD) 1.9 ± 1.6 min before HBO2-induced seizures. These results, if corroborated in humans, support the use of changes in TVSymp activity as an early “physio-marker” of impending and potentially fatal seizures in divers and patients.

Metabolic Cage for Urine Collection from Small Animals with High Level of Performance and Low Cost

The results of designing the original metabolic cage for urine collection from small laboratory animals consisting of a case, a cylindrical animal chamber with the floor, a funnel, a urine collection vessel and two graded drinking bottles that can be placed at a different height depending on animal age are presented. The case was made of laminated particle board; a cylindrical animal chamber was made of polyethylene terephthalate; a circular floor of the animal chamber was made of stainless steel wire cloth mesh. As a funnel for urine collection, a ribbed glass funnel SIMAX (Czech Republic) was used. To prevent rat feces from entering the urine collection vessel, there were installed two stainless steel wire mesh filter discs, namely a larger disc located on the internal ribbed surface of the funnel and a smaller disc located close to the hole of the funnel tube. To support the urine collection vessel, a metal vessel stand with a deepening was made. Between the vessel and the funnel, there was placed a fine stainless steel metal cylinder preventing urine evaporation. In addition to low cost, the proposed design of the metabolic cage provides high levels of performance as confirmed by its high ability to allow urine to flow freely, as well as to collect urine, significantly smaller volume of urine evaporated, improved housing conditions for animals and allows us to collect the amount of urine more fully reflecting animal diuresis.

Flow-through Respirometry: Excurrent Flow Measurement

This chapter describes the setup, plumbing, and equations for implementing a respirometry system wherein the flow rate of the air leaving the animal chamber is known. Such systems are usually referred to as pull systems, because the air is usually pulled from a chamber or mask at a known rate, and the concentrations of incurrent and excurrent gases are alternately measured. Such systems are often the only practical way of measuring the metabolic rates of large animals. Setups and equations for oxygen-only, carbon dioxide-only, and combined oxygen and carbon dioxide systems are described. Methods for creating multiple-animal pull mode respirometry systems, for compensating flow rate, and for the automatic baselining (that is to say, measuring incurrent gas concentrations) of respirometry systems are discussed.

Flow-through Respirometry: Incurrent Flow Measurement

This chapter describes the setup, plumbing, and equations required for applying a respirometry system wherein the flow rate of the air entering the animal chamber is known. Such systems are usually referred to as push systems, because the air is usually pushed into a sealed respirometer chamber at a known rate, and the concentrations of incurrent and excurrent gases are alternately measured. Setups and equations for oxygen-only, carbon dioxide-only, and combined oxygen and carbon dioxide systems are described. Methods for creating multiple-animal push mode respirometry systems and for the automatic baselining (that is to say, measuring incurrent gas concentrations) of respirometry systems are also discussed.

A potential early physiological marker for CNS oxygen toxicity: hyperoxic hyperpnea precedes seizure in unanesthetized rats breathing hyperbaric oxygen

Hyperbaric oxygen (HBO2) stimulates presumptive central CO2-chemoreceptor neurons, increases minute ventilation (V̇min), decreases heart rate (HR) and, if breathed sufficiently long, produces central nervous system oxygen toxicity (CNS-OT; i.e., seizures). The risk of seizures when breathing HBO2 is variable between individuals and its onset is difficult to predict. We have tested the hypothesis that a predictable pattern of cardiorespiration precedes an impending seizure when breathing HBO2. To test this hypothesis, 27 adult male Sprague-Dawley rats were implanted with radiotelemetry transmitters to assess diaphragmatic/abdominal electromyogram, electrocardiogram, and electroencephalogram. Seven days after surgery, each rat was placed in a sealed, continuously ventilated animal chamber inside a hyperbaric chamber. Both chambers were pressurized in parallel using poikilocapnic 100% O2 (animal chamber) and air (hyperbaric chamber) to 4, 5, or 6 atmospheres absolute (ATA). Breathing 1 ATA O2 initially decreased V̇min and HR ( Phase 1 of the compound hyperoxic ventilatory response). With continued exposure to normobaric hyperoxia, however, V̇min began increasing toward the end of exposure in one-third of the animals tested. Breathing HBO2 induced an early transient increase in V̇min ( Phase 2) and HR during the chamber pressurization, followed by a second significant increase of V̇min ≤8 min prior to seizure ( Phase 3). HR, which subsequently decreased during sustained hyperoxia, showed no additional changes prior to seizure. We conclude that hyperoxic hyperpnea ( Phase 3 of the compound hyperoxic ventilatory response) is a predictor of an impending seizure while breathing poikilocapnic HBO2 at rest in unanesthetized rats.

On the barometric method for measurements of ventilation, and its use in small animals

The barometric method is a common technique for measurements of pulmonary ventilation in unrestrained animals. It basically consists of recording the changes in chamber pressure generated during breathing. In fact, as the air inspired is warmed and humidified from the ambient to the pulmonary values, the total pressure in the animal chamber increases; the opposite occurs in expiration. The present commentary is an introduction to this method, briefly reviewing its historical development, the conceptual pitfalls, and potential sources of errors during practical applications.Key words: barometric technique, plethysmography, pulmonary ventilation, respiratory techniques, tidal volume.