RESUMO
Heat-induced hyperventilation may reduce PaCO2 and thereby cerebral perfusion and oxygenation and in turn exercise performance. To test this hypothesis, eight volunteers completed three incremental exercise tests to exhaustion: (a) 18 °C ambient temperature (CON); (b) 38 °C (HEAT); and (c) 38 °C with addition of CO2 to inspiration to prevent the hyperventilation-induced reduction in PaCO2 (HEAT + CO2 ). In HEAT and HEAT + CO2 , rectal temperature was elevated prior to the exercise tests by means of hot water submersion and was higher (P < 0.05) than in CON. Compared with CON, ventilation was elevated (P < 0.01), and hence, PaCO2 reduced in HEAT. This caused a reduction (P < 0.05) in mean cerebral artery velocity (MCAvmean ) from 68.6 ± 15.5 to 53.9 ± 10.0 cm/s, which was completely restored in HEAT + CO2 (68.8 ± 5.8 cm/s). Cerebral oxygenation followed a similar pattern. V Ë O 2 â m a x was 4.6 ± 0.1 L/min in CON and decreased (P < 0.05) to 4.1 ± 0.2 L/min in HEAT and remained reduced in HEAT + CO2 (4.1 ± 0.2 L/min). Despite normalization of MCAvmean and cerebral oxygenation in HEAT + CO2 , this did not improve exercise performance, and thus, the reduced MCAvmean in HEAT does not seem to limit exercise performance.
Assuntos
Dióxido de Carbono/uso terapêutico , Exercício Físico/fisiologia , Fadiga/prevenção & controle , Transtornos de Estresse por Calor/fisiopatologia , Temperatura Alta/efeitos adversos , Hiperventilação/terapia , Artéria Cerebral Média/fisiopatologia , Adulto , Desempenho Atlético/fisiologia , Velocidade do Fluxo Sanguíneo , Teste de Esforço , Fadiga/etiologia , Fadiga/fisiopatologia , Transtornos de Estresse por Calor/etiologia , Humanos , Hiperventilação/etiologia , Hiperventilação/fisiopatologia , Masculino , Consumo de Oxigênio , Método Simples-Cego , Resultado do TratamentoRESUMO
The penetration of ultrasonic waves through opaque media and the large difference in the acoustic properties between air bubbles and the fermentation broth were used to measure the energy attenuation of pulsed ultrasound by the bubbles as the waves passed through the broth. This leads to an on-line determination of the specific interfacial area provided information is available about the holdup or bubble mean diameter. This article gives the principle of the method and demonstrates how the measured interfacial area may be used in evaluating the mass transfer coefficient of a fermentation system in a bubble column.
