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ABSTRACT. Electrical impedance plethysmography was evaluated in lambs as a method of measuring tidal volume (VT). Over tidal volumes ranging from 15 to 414% of estimated dead space, and frequencies of 300 to 1000 breathslmin, correlation between VT measured by the impedance technique and VT measured by whole-body plethysmography was 0.98. Above 600 breathdmin, the correlation between the two methods was 0.94. Independent calibration of the impedance technique using a pneumotachograph at conventional rates of ventilation yielded absolute values of VT which closely corresponded to values obtained with the whole-body plethysmograph (slope = 1 &, baseline transthoracic impedanceDirect measurement of VT during HFV is difficult if not impossible in most clinical circumstances due to constraints resulting from limited frequency response and high gas velocity. These difficulties are compounded in newborn and premature infants because of their small size, critical dependence on a closely regulated thermal environment, and inability to cooperate.Variations in transthoracic electrical impedance have been shown to correlate closely with VT during conventional ventilation (1). This investigation was undertaken to determine if impedance plethysmography could also be used during HFV to measure VT. Inasmuch as the technique is noninvasive and unobtrusive, it would be well suited to meet the requirements of safe application in newborn infants. METHODSFour lambs weighing 4.0 to 10.2 kg were studied on 6 days between 1 and 7 wk after birth. During each study session, the lamb was sedated with chloralose, but spontaneous breathing was not abolished. After placement of a cuffed endotracheal tube through the mouth, the lamb was placed in a pressure-compensated, integrated flow, whole-body plethysmograph having a volume of 165 liters. The endotracheal tube extended through a sealed port in the plexiglass wall of the plethysmograph. VT was measured by pressure compensating the integrated pressure signal from the plethysmograph. The plethysmograph had been tuned to eliminate phasic distortion up to 20 Hz.Transthoracic electrical impedance was measured by a DCcoupled SAAB Respimeter operating at a frequency of 50 kHz anda current of 0.2 mA peak to peak (2). A tetrapolar electrode system was used, with one pair of electrodes attached to each side of the lamb's chest wall after shaving. Each electrode had a circular silver-silver chloride surface 0.8 cm in diameter and each electrode pair was housed in a soft rubber casing that maintained a constant interelectrode distance of 2.25 cm center to center. Within each electrode pair, one electrode was used for current injection and the other was used to detect voltage drop. This type of impedance bridge minimizes the effect of resistance variations at the skin-electrode interface. The frequency response of the impedance bridge was found to be flat below 20 Hz. HFV was provided by a Bird VDR. The Bird VDR is a flowintermvter tvve of high frecluency ventilator that has a pneumatic inspira~ory/exp...

Caffeine is an efficient respiratory stimulant during theneonatal period, but its action on respiration beyond this period remains unclear. This study was performed to evaluate the effect of caffeine on ventilation in relation to post-natal maturation. Nineteen lambs divided in 3 groups of different ages were studied;I: 7 lambs with a mean age of 1 week;II: 6 lambs, mean age of 2 112 months and 111: 6 sheep, 6 months old. After a baseline period, saline (as a control infusion) then, 10 minutes later, caffeine 10 mg/kg I.V. were injected to the animals. Ventilation was measured using a mask adapted to the animal's facial contour and connected to a pneumotachograph; on-line values of t~ (minute ventilation), Vt/Ti, (mean inspiratory flow) and Ti/Ttot (timing of respiration) were derived from the flow signal using a computerized system.No response was seen after sa1ine;but in all three groups, VE (ml/min/kg) increased significantly after caffeine; this increase was 36% in groue 1; 46% in group 11; and was 48% in group 111. This increase in VE was due mainly to an increase in Vt/Ti (ml/sec/kg), which increased in groups I,II, 111 by 36,41 and 26%. Ti/Ttot did not change significantly at any time. The plasma concentrations were 9.7.8.8.10.0 mg/l in groups I,II,III. We conclude that: the magnitude of the ventilatory response and the strategy of breathing in response to the therapeutic blood concentration of caffeine persists withoutmajor difference between I week to 6 months of age in lambs. Caffeine is frequently used as a respiratory stimulant in the therapy of infants with apnen.The site of action of caffeine remains unclear and this present study was undertaken to evaluate the role of the chemoreceptors in the increase of ventilation following caffeine therapy. To achieve this goal we decided to study the response to caffeine after carotid body denervation (CBD). Twelve animals were studied, 6 had CBD and 6 had a sham denervation (intact) at a mean age of 13 days. The present study was carried out at a mean age of 80 days in an awake and not sedated state. The caffeine dose used was that recommended for human infants:lO mg/kg I.V. and the plasma concentration achieved was in the therapeutic range;CBO:9.0, intact:8.8 mg/l. The intact lambs responded significantly to caffeine by an increase in tE of 46% from 274+10(+SEM) to 400+21 ml/kg/min at I min. This response gradually faxed,-but at 2,-hours CE was still greater than baseline being 314+21. This increase in ventilation was mainly caused by a change in-vt/~i;from baseline 9.9 ml/kg/min it reached 14.0 at I min and was still increased at 120 min:11.2. In the CBD lambs no increase in VE (from baseline 263225) and VtjTi wasseen. No change in Ti/Ttot was seen in both groups. We concluded that: for the usual therapeutic dose of caffeine, CBD abolishes the increase in ventilation seen in intact lambs. This suggests that the carotid body receptors play an important role in the mediation of the ventilatory response to caffeine. This study was designed to assess the mecha...