Continuous non-invasive monitoring of tidal volumes by measurement of tidal impedance in neonatal piglets

Abstract

Background: Electrical Impedance measurements can be used to estimate the content of intra-thoracic air and thereby give information on pulmonary ventilation. Conventional Impedance measurements mainly indicate relative changes, but no information concerning air-volume is given. The study was performed to test whether a 3-point-calibration with known tidal volumes (VT) during conventional mechanical ventilation (CMV) allows subsequent calculation of VT from total Tidal-Impedance (tTI) measurements using Quadrant Impedance Measurement (QIM). In addition the distribution of TI in different regions of the thorax was examined.

Methodology and principal findings: QIM was performed in five neonatal piglets during volume-controlled CMV. tTI values at three different VT (4, 6, 8 ml/kg) were used to establish individual calibration curves. Subsequently, each animal was ventilated with different patterns of varying VT (2-10 ml/kg) at different PEEP levels (0, 3, 6, 9, 12 cmH(2)O). VT variation was repeated after surfactant depletion by bronchoalveolar lavage. VT was calculated from tTI values (VT(calc)) and compared to the VT delivered by the ventilator (VT(PNT)). Bland-Altman analysis revealed good agreement between VT(calc) and VT(PNT) before (bias -0.08 ml; limits of agreement -1.18 to 1.02 ml at PEEP = 3 cmH(2)O) and after surfactant depletion (bias -0.17 ml; limits of agreement -1.57 to 1.22 ml at PEEP = 3 cmH(2)O). At higher PEEP levels VT(calc) was lower than VT(PNT), when only one fixed calibration curve (at PEEP 3 cmH(2)O) was used. With a new calibration curve at each PEEP level the method showed similar accuracy at each PEEP level. TI showed a homogeneous distribution over the four assessed quadrants with a shift toward caudal regions of the thorax with increasing VT.

Conclusion: Tidal Impedance values could be used for precise and accurate calculation of VT during CMV in this animal study, when calibrated at each PEEP level.

Figure 1. Placement of electrodes.

a) Schematic map of the piglet's body and position of electrodes; b) detailed illustration of the sternum and distances of the electrodes; the length of the sternum (s) is quartered. Cranial Right (CrR) and Cranial Left (CrL) ventral are set at the end of the first quarter from the cranial end of the sternum right and left with the distance ¼ s from the axis. The central electrode for current injection (CE) is set in the middle (m) of the remaining ¾ of the sternum. Caudal Right (CaR) and Caudal Left (CaL) are placed right and left from the caudal end of the sternum with the distance between m and the caudal end of the sternum. Dorsal electrodes are set mirror-inverted to the ventral electrodes.

Figure 2. Blood Gases and Heart Rate.

Shown are mean (SD) values of blood gases and heart rate for n = 5 animals. Blood gas analyses were performed after measurements at one PEEP level before changing to the next PEEP level.

Figure 3. Tracing signal of impedance measurement during 5 breathing cycles.

Shown are tracing signals of QIM measurements from 4 quadrants during 5 respiratory cycles. In addition flow and pressure curves from the ventilator are shown below in order to illustrate the time course of impedance changes with respect to ventilatory cycles from the ventilator.

Figure 4. Bland-Altman Plot for comparison between VT_calc and VT_PNT during pre-lavage measurements.

Shown are Line of Identity (grey line), Bias (blue line), and 95%-Limits of Agreement (dashed line). Calibration was performed at each PEEP level in all animals. Data points indicate the means of measurements in individual piglets at different PEEP levels.

Figure 5. Bland-Altman Plot for comparison between VT_calc and VT_PNT during post-lavage measurements.

Shown are Line of Identity (grey line), Bias (blue line), and 95%-Limits of Agreement (dashed line). Calibration was performed at each PEEP level in all animals. Data points indicate the means of measurements in individual piglets at different PEEP levels.

Figure 6. Distribution of fTI in different quadrants at different VT, pre-lavage.

Shown are mean (grey line) fTI values for different VT, with 95%-Confidence Interval of the mean (green diamonds).

Figure 7. Distribution of fTI in different quadrants at different VT, post-lavage.

Shown are mean (grey line) fTI values for different VT, with 95%-Confidence Interval of the mean (green diamonds).

Figure 8. Distribution of fTI at different VT stratified according to PEEP, pre-lavage.

Shown are mean fTI values (grey line) for different VT at different PEEP with 95%-Confidence Interval of the mean (green diamonds).