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Biomedical subjects

M K Sykes

Publications and source records attributed to M K Sykes.

At least 73 records · Page 4Linked to original sources
PubMed1983

Cardiorespiratory effects of an inspiratory hold and continuous positive pressure ventilation in goats.

The cardiorespiratory effects of three different patterns of mechanical ventilation were compared in sixteen anaesthetized goats. Intermittent positive pressure ventilation (IPPV), with an inspiratory: expiratory (I:E) time ratio of 1:3, was compared with an inspiratory hold pattern (IPPVH), with an I:E ratio of 3:1, and with continuous positive pressure ventilation (CPPV) adjusted to produce the same mean airway pressure. In eight animals with normal lungs, IPPVH reduced VD/VT and PaCO2, but produced no changes in oxygenation. CPPV did not significantly alter the efficiency of gas exchange. In a further eight animals, with oleic acid-induced lung damage, both IPPVH and CPPV produced a decrease in both VD/VT and PaCO2. Qs/Qt was significantly reduced by both CPPV and IPPVH, but the effect was more marked with CPPV, and the PaO2 was significantly increased only by CPPV. The increased effectiveness of CPPV in increasing PaO2 in this model may have been due to the greater increase in end-expiratory lung volume produced by this pattern of ventilation.

Adult↗
PubMed1983

Effect of changes in pleural pressure on fluid filtration rate in expanded and collapsed rabbit lungs.

Eight isolated rabbit lungs were suspended from a force-displacement transducer and perfused at constant flow. The pressure surrounding the lungs ("pleural" pressure) was then reduced from -0.5 kPa to -1.0 and -1.5 kPa, and the rate of weight gain recorded. The step reductions in "pleural" pressure produced greater increases in pulmonary vascular volume and fluid filtration rate when the lungs were collapsed than when they were expanded, thus suggesting that the change in "pleural" pressure was more effectively transmitted to the perimicrovascular space when the lung was collapsed. These observations may help to explain the phenomenon of re-expansion pulmonary oedema.

Animals↗
PubMed1983

Initial in vitro evaluation of a pediatric vortex-mixing membrane lung.

A new design for a pediatric membrane lung is described in this paper. The lung consists of eight blood compartments, each having six U-shaped blood channels, with microporous PTFE membranes supported on rigid plates in such a way that the membranes form furrowed blood channels. Two rolling diaphragm pumps are attached to the open ends of the U-shaped blood channels; these pumps are operated in antiphase. Mean flow is provided by a roller pump placed at the inlet end of the membrane lung. Pulsatile blood flow within the blood channels produces successive vortex formation and ejection, leading to good blood mixing and high efficiency in gas transport. The design of the rolling diaphragm piston pumps ensures that the blood prime volume is low (280 ml), and the grouping of the pumps at one end of the oxygenator allows the driving mechanism to be simple and compact. The relatively wide blood channels (minimum width 0.5 mm) and vortex mixing make priming the membrane lung particularly easy. The membrane area is 0.39 m2. Preliminary performance testing of the pediatric membrane lung was undertaken by pumping blood around a circuit containing a roller pump, the membrane lung, and a bubble oxygenator (to adjust the blood gases at the inlet to the membrane lung). In five such experiments it was shown that the membrane lung transferred 80 ml O2/min and 120 ml CO2/min at a blood flow rate of 1.5 L/min.

Adult↗
PubMed1983

The relationship between hypoxic pulmonary vasoconstriction and arterial oxygen tension in the intact dog.

The relationship between the magnitude of hypoxic pulmonary vasoconstriction (h.p.v.) and arterial oxygen tension (Pa,O2) has been studied in intact anaesthetized dogs. Radioactive 133Xe was infused continuously into the inferior vena cava. A tracheal divider made it possible to vary the inspired gas composition to each lung independently. With constant ventilation the 133Xe in the mixed expired gas from each lung was proportional to the blood flow to that lung. Unilateral ventilation of the left lung with 7% oxygen produced a diversion of blood flow away from the lung and a reduction in Pa,O2. Repeated hypoxic stimuli produced a progressively greater reduction in the blood flow to the hypoxic lung and a progressive increase in Pa,O2. Administration of a beta-adrenergic agonist, dobutamine hydrochloride, during ventilation of the left lung with 7% oxygen, resulted in an increased blood flow to the left lung and a further decrease in Pa,O2. Addition of CO2 to the inspired gas resulted in an increased diversion of blood flow away from the hypoxic lung but a decrease in Pa,O2. Respiratory alkalosis induced by over-ventilation decreased the hypoxic vasoconstriction and increased Pa,O2 slightly. However acid-base changes induced by infusion of 1 N-lactic acid or 8.4% NaHCO3 had no significant effects on the magnitude of the hypoxic vasoconstriction, or on Pa,O2, during hypoxic ventilation of the left lung. The magnitude of hypoxic vasoconstriction and Pa,O2 in the experiments described in 3, 4 and 5 above were positively correlated (r = 0.885), showing that the vasoconstriction may help to maintain Pa,O2. It is suggested that the effects of CO2 on h.p.v. and Pa,O2 may be explained largely by the changes in alveolar oxygen pressure (PA,O2) which are secondary to changes in PA,CO2.

Acid-Base Equilibrium↗
PubMed1982

The contribution of heart beat to gas mixing in the lungs of dogs.

The mixing efficiency for two gases of different gaseous diffusivity, helium (He) and sulphur hexafluoride (SF6) have been studied in anaesthetised dogs, closed and open chested, with and without the heart beating. Equilibration of He and SF6 was studied during rebreathing at frequency of 0.5 Hz and a tidal volume of either 0.3 or 0.5 L. Circulation and gas exchange were taken over by a complete heart and lung bypass circuit during the periods when the heart was stopped. The number of breaths required to reach 99% equilibration (n99) ranged from 4 to 14 for He and from 6 to 17 for SF6. There was no significant change in mixing efficiency in any situation. Stopping the heart increased the n99 for He by only 0.4 +/- 11% (1 SD) (n = 21). Opening the chest increased n99 by 1.4 +/- 13% with the heart beating and 2.5 +/- 19% with the heart stopped. The n99 for SF6 was 30 +/- 22% higher than that for He with the chest closed with or without the heart beating. This increased to 37% with the chest opened but was not altered by stopping the heart. The findings for the final phase equilibration rate constant were similar. We conclude that the beating action of the heart does not affect gas mixing in the lungs in the tidal breathing range.

Animals↗
PubMed1982

Changes in colloid osmotic pressure with plasma albumin concentration associated with extracorporeal circulation.

Colloid pressure (COP), plasma albumin concentration, haematocrit, and blood-gas tensions were measured in 16 patients undergoing open-heart surgery with cardiopulmonary bypass. The use of crystalloid priming and cardioplegia solutions resulted in a 60% decrease in COP, a 48% decrease in plasma albumin concentration and a 35% decrease in haematocrit. These measurements had returned to pre-perfusion values 6 h after the end of surgery. The alveolar-arterial PO2 difference increased significantly after by-pass and returned to pre-perfusion values within the same time scale. Right-to-left shunt increased from 7.9 to 10.3% 30 min after the end of by-pass. It is concluded that, in the absence of an increase in left atrial pressure, marked decreases in COP can be tolerated without the occurrence of pulmonary oedema.

Adult↗
PubMed1982

Cardiorespiratory responses to an i.v. infusion of dobutamine in the intact anaesthetized dog.

The cardiorespiratory responses to an i.v. infusion of dobutamine hydrochloride were assessed in eight anaesthetized, mechanically ventilated dogs. As the rate of infusion of dobutamine was increased from 2 to 30 microgram kg-1 min-1, there was a progressive decrease in arterial pressure, pulmonary wedge pressure and arterial pH. There was a significant decrease in arterial oxygen tension at the greater doses of dobutamine (15 and 30 microgram kg-1 min-1) from initial control values. Carbon dioxide output, arterial carbon dioxide tension, venous admixture and oxygen consumption increased during the infusion of dobutamine. However, oxygen supply increased further so that the oxygen consumption; supply ratio decreased. It is concluded that dobutamine may decrease arterial oxygen tension, but that the increased cardiac output and decreased arterial pH produced by dobutamine may increase oxygen supply to the tissues in spite of this.

Anesthesia, General↗
PubMed1981

Measurements of regional ventilation using nitrogen-13 and krypton-81m in mechanically ventilated dogs.

13N2 and 81Krm were used to measure changes in upper/lower (U/L) ratios of regional ventilation in supine anaesthetised dogs mechanically ventilated at different tidal volumes and the results compared. Analysis of the slopes of the 13N2 washout curves indicated that U/L specific ventilation was less than unity at low tidal volumes and decreased as tidal volume increased. U/L total ventilation, measured using the specific ventilation results and estimations of relative lung using 13N2, also decreased with increase in tidal volume. Preferential distribution of ventilation to dependent lung regions thus increased with tidal volume despite the opposing hydrostatic forces from the abdomen. Regional count density distributions in 81Krm images of the lungs were analysed. At high specific ventilation rates, correction of 81Krm count density is required in order to obtain correct regional ventilation information. The specific ventilation rates in this series were such that the uncorrected U/L count ratios of 81Krm increased as tidal volume increased in contrast to the decrease in U/L total ventilation recorded by the 13N2 method. When corrections using the 13N2 specific ventilation results were applied, the changes in U/L 81Krm count ratios with tidal volume showed a similar trend to the changes in total ventilation measured with 13N2. Sources of error in the two techniques are briefly discussed.

Animals↗
PubMed1981

Depression of hypoxic pulmonary vasoconstriction by sodium nitroprusside and nitroglycerine.

The pulmonary vasoconstrictor response to the unilateral administration of 7% oxygen was studied in 10 anaesthetized dogs in which the lungs were mechanically ventilated. The distribution of blood flow between the two lungs was measured continuously by recording the radioactivity in the mixed expired gas from each lung during the infusion i.v. of xenon-133. Infusions of nitroglycerine and sodium nitroprusside which produced the same decrease in mean aortic pressure produced similar decreases in hypoxic pulmonary vasoconstriction. The reduction in arterial PO2 during unilateral hypoxia was greater when the hypoxic vasoconstrictor response was depressed by drugs than it was during the control periods, although mixed venous PO2 was unchanged. It is concluded that both drugs may cause hypoxaemia by increasing the blood flow to alveoli with a low ventilation/perfusion ratio.

Animals↗
PubMed1980

Cardiorespiratory effects of high frequency intermittent positive pressure ventilation in the dog.

The cardiorespiratory effects of increasing respiratory frequency were investigated in two groups of dogs ventilated mechanically with a system which minimized apparatus and anatomical deadspace. In one group minute volume was constant whilst frequency was increased; in the second group minute volume was increased to maintain normal PaCO2. The reduction in tidal volume with increasing frequency was accompanied by a reduction in physiological deadspace. However, the reduction in deadspace was proportionally less than the decrease in tidal volume, so that the deadspace/tidal volume ratio increased. The peak airway pressures were minimal at a frequency of 45 b.p.m. At frequencies of 60 b.p.m. and more, the lung failed to empty so that peak airway pressures were increased and cardiac output decreased.

Animals↗
PubMed1980

Errors in oxygen concentration analysis: sensitivity of the IMI analyser to nitrous oxide.

The sensitivity to nitrous oxide of the IMI oxygen analyser (Model 3300) has been tested. Even if the manufacturer's checks are performed and the analyser appears to be functioning correctly, the reading given by the IMI oxygen analyser may be seriously affected by the presence of nitrous oxide. There is a wide variation in the sensitivity to N2O between different instruments, and this variation is partly a function of the electrode. The influence of nitrous oxide on a particular meter may be assessed by placing the probe in 100% nitrous oxide and noting the shift of the meter from zero.

Electrodes↗
PubMed1979

Preservation of the hypoxic pulmonary vasoconstrictor mechanism during methoxyflurane anaesthesia in the dog.

The effect of methoxyflurane on the hypoxic pulmonary vasoconstrictor response in the dog lung was assessed by measuring the redistribution of pulmonary blood flow in response to two levels of unilateral alveolar hypoxia. Methoxyflurane 0.5% had no significant effect on the redistribution of blood flow resulting from the unilateral administration of oxygen 7% or nitrogen and there were no significant differences in PaO2 between the measurements made during unilateral hypoxia with methoxyflurane and those made during unilateral hypoxia in the control periods.

Anesthesia, Inhalation↗
PubMed1979

Depression of hypoxic pulmonary vasoconstriction in the dog by dopamine and isoprenaline.

The effects of dopamine and isoprenaline on the pulmonary vasoconstrictor response to alveolar hypoxia were assessed by measuring the redistribution of blood flow between the lungs in response to unilateral hypoxia. Dose rates of dopamine 25 micrograms kg-1 min-1 and isoprenaline 0.25 micrograms kg-1 min-1 (which produced equal increments in thecontractile force of the heart in dogs) produced a similar degree of depression of they hypoxic vasoconstrictor response, whereas dopamine 2.5 micrograms kg-1 min-1 had little effect on the response PaO2 during unilateral hypoxia was inversely related to the blood flow through the hypoxic lung.

Animals↗
PubMed1979

Effects of heparin and protamine on left ventricular performance in the dog.

The effects of protamine on left ventricular (LV) function were measured under conditions of controlled heart rate and proximal aortic pressure in eight anaesthetized, heparinised dogs. Protamine 3 mg.kg-1 produced a 21% decrease in LV dP/dt max, a 43% decrease in cardiac output, a 47% decrease in stroke work and decreases in systolic and diastolic pressures (-16%, -19% respectively). Protamine 6 mg.kg-1 resulted in a 17% decrease in LV dP/dt max, a 26% decrease in cardiac output, a 50% decrease in LV stroke work and 25 and 30% decreases in systolic and diastolic pressures. These results show that an impairment of LV function plays an important part in the circulatory depression produced by protamine.

Animals↗