Pediatric research
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Elastic unloading [otherwise known as negative ventilator compliance (Cv) or proportional assist ventilation] is a new mode of assisted mechanical ventilation. The ventilator continuously measures the volume of spontaneous breathing (V) and adjusts the pressure at the airway opening in proportion to V. The quotient of pressure above the baseline end-expiratory level per unit of V (the gain of the assist) is constant at any point in time and can be preset. ⋯ An elevation of Ctot decreased the expiratory airflow. Tidal volume increased slightly in healthy lungs and arterial PCO2 decreased. We conclude that the effects of Cv on the total compliance of the combined lung-respirator system can accurately be predicted.
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Biography Historical Article
Acceptance of the Howland Award: childhood nutrition--50 years later.
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This study was carried out to determine the influence and site of action of N omega-nitro-L-arginine methylester, an L-arginine analogue, on basal pulmonary vascular tone and hypoxic vasoconstriction in neonatal pig lungs. We studied isolated lungs from pigs, age 14.5 +/- 0.5 (SD) d and weight 3.6 +/- 0.7 kg, perfused with autologous blood at a constant flow rate. The arterial-venous occlusion method was used to determine sites of action upstream and downstream of the double occlusion pressure (Pd) during baseline, infusion of acetylcholine, and ventilation of the lung with a hypoxic gas mixture. ⋯ Infusion of acetylcholine resulted in downstream dilation, and hypoxia resulted in an increase in both upstream and downstream resistance. After adding N omega-nitro-L-arginine methylester to the blood, there was an increase in both upstream and downstream resistances; acetylcholine infusion resulted in an increase in total vascular resistance, which was entirely due to upstream constriction; and the hypoxia response was much larger both upstream and downstream of Pd. These results suggest that nitric oxide synthase helped maintain a low level of basal pulmonary vascular tone both upstream and downstream of Pd in these neonatal pig lungs; that the vascular effect of acetylcholine was changed from downstream dilation to upstream constriction by N omega-nitro-L-arginine methylester; and that nitric oxide synthase activity modulated both the upstream and downstream vasomotor response to hypoxia.
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In extremely preterm infants, the protective capacity for cerebral blood flow (CBF) autoregulation may be impaired or absent, which increases the risk for developing cerebral lesions. The purpose of this study was to quantify the simultaneous influence of several vital parameters, such as mean arterial blood pressure (MABP), PCO2, and PO2, on cerebral blood flow velocity (CBFv), which is used as a measure for CBF. In 16 mechanically ventilated infants of < 33 wk gestation, the CBFv in the internal carotid artery was measured every minute for 1 h by a computer-controlled pulsed Doppler device. ⋯ A multiple linear regression analysis was performed in each patient to determine the individual MABP, PCO2, and PO2 reactivities as a measure for CBF autoregulation. The medians (and ranges) of the whole group were an MABP reactivity of 7.5% (-12.5 to 20.1%) rise in CBFv/1 kPa rise in MABP, a PCO2 reactivity of 32.7% (-8.1 to 79.5%) rise in CBFv/1 kPa rise in PCO2, and a PO2 reactivity of -3.1% (-14.2 to 7.9%) fall in CBFv/1 kPa rise in PO2. In preterm infants, the individual's capacity for MABP-, PCO2-, and PO2-dependent CBF autoregulation can be estimated by means of the present method, even if the vital parameters change simultaneously.
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The pathogenesis of bradycardias in preterm infants is poorly understood. Because their pathogenesis may involve both apnea and hypoxemia, we set out to analyze the proportion of bradycardias that were associated with an apneic pause and/or a fall in arterial oxygen saturation (SaO2), and the temporal sequence of the three phenomena, in overnight tape recordings of SaO2 (Nellcor N100 in beat-to-beat mode), breathing movements, nasal airflow, and ECG in 80 preterm infants at the time of discharge from hospital. A bradycardia was defined as a fall in heart rate of > or = 33% from baseline for > or = 4 s, an apneic pause as a cessation of breathing movements and/or airflow for > or = 4 s, and a desaturation as a fall in SaO2 to < or = 80%. ⋯ Where all three phenomena occurred in combination, the time from the onset of apnea to the onset of the fall in SaO2 was shorter (median interval, 0.8 s; range -4.9-+ 11.5 s) than that from the onset of apnea to the onset of bradycardia (median, 4.8 s; range -4.0-+ 14.0 s). Hence, most bradycardias (86%) commenced after the onset of the fall in SaO2. We conclude that bradycardia, apnea, and hypoxemia are closely linked phenomena in preterm infants.