Experimental paperTidal volume measurements via transthoracic impedance waveform characteristics: The effect of age, body mass index and gender. A single centre interventional study
Introduction
Capturing reliable ventilation parameters during manual positive pressure ventilation (PPV) with bag-valve systems is challenging, especially in the prehospital setting and during cardiopulmonary resuscitation (CPR). The transthoracic impedance (TTI) signal is measured in Ohms (Ω) by applying a high frequency and low amplitude current between two electrodes placed on the chest, such as the defibrillation pads.1., 2. Changes in TTI are caused by ventilation and circulation induced variations in tissue density and fluid content in the thoracic region. TTI can be used to estimate ventilation parameters such as ventilation frequency2., 3. and insufflation time,3., 4. and to detect oesophageal intubation.5 Although the amplitude of the TTI waveform (TTI-amplitude) has been shown useable to precisely reflect tidal volumes (TV) for a given patient, the poor interpatient reliability has hindered its clinical applicability.6
A recent cardiac arrest study by Chang et al. found that patients with TTI-amplitude above a 0.5 Ω threshold during CPR had better outcomes.7 These results raise the question of whether this threshold is associated to the formerly suggested adequate TV of 400 ml,8., 9. and if TTI holds more information on TV of relevance for prehospital emergency medicine care such as resuscitation. Anatomical morphometric studies have revealed that easily available data like body mass index (BMI), gender and age, affect chest dimensions and the level of chest wall displacement produced by ventilations.10., 11., 12. By adjusting for such individual factors a better fit of the TTI amplitude to the true physiology may be established. If the TTI ventilation waveform is automatically delineated, as described in Jaureguibeitia et al.,3 TTI data could then enable detailed analyses on the ventilation-outcome relations in retrospective resuscitation data analyses, and possibly allow detailed real-time feedback.
In a recently published study of TTI in consenting patients with PPV during anaesthesia, we found a consistent relationship between the timing of TTI changes and airway pressures during the ventilation cycle. 4 For the present study, we hypothesized that TTI-amplitude would correlate with TV and that this correlation would improve when adjusted for BMI, gender and age in this dataset. We also wanted to analyse what TTI amplitude thresholds corresponded to ventilations with adequate TV suggested to be of at least 400 ml.8
Section snippets
Study design and experimental protocol
Data was recorded in a controlled ventilation setting from anaesthetised patients, and the ventilator settings were adjusted to control for TV, frequency and insufflation time. Advanced signal processing techniques were applied to obtain the TTI ventilation waveforms, and their amplitudes and durations. These variables were then associated to the TV, frequency and insufflation times of the ventilations and adjusted for age, gender and BMI.
The study was approved (REK2015/1287) by the ethical
Results
The study cohort comprised 21 patients (9 female), from which 9 (3 female) were overweight and 12 (6 female) were normal weight. The median BMI (interquartile range) for females and males were 23.6 (19.0–25.9) and 24.9 (23.2–28.2), with no significant differences in BMI by gender (, Wilcoxon rank-sum test). There was a significant difference in age between male 69 (53–78) and female 48 (40–54) patients (). None of the patients exceeded the 25 cmH2O airway pressure limit and none
Overall findings
We have investigated the relationship between TV created by PPV and different characteristics of the changes induced by ventilations in the TTI signal. Our main finding is a strong positive linear covariation between the TTI-amplitude and TV, although the responding TTI-amplitude to a specific TV varied between patients. However, when the models were adjusted for the three accessible patient variables (age, BMI, gender), this variation could be partially explained. The TTI-amplitude threshold
Conclusions
TTI-amplitude measurements, obtained through defibrillation pads can, using specific filtering and signal processing algorithms that could be incorporated into defibrillators, convey important information on ventilation quality. TV to TTI-amplitude relations can be established, proper cut-offs (around 0.5 Ω) for adequate TV ventilations can be defined, and patient morphometric variables can be used to explain the variability in the measurements. These results could be used in the future to
Conflict of interest
LW is the NAKOS representative in the Medical Advisory Board of Stryker and holds patents licensed to Stryker and Zoll. The other authors declare that they have no conflicts of interest related to the research.
CRediT authorship contribution statement
P.O. Berve: Conceptualization, Methodology, Investigation, Data curation, Formal Analysis, Writing – original draft. U. Irusta: Methodology, Software, Data curation, Visualization, Formal Analysis, Writing – original draft. J. Kramer-Johansen: Writing – original draft. T. Skålhegg: Investigation. E. Aramendi: Validation, Writing – original draft. L. Wik: Conceptualization, Methodology, Investigation, Writing – original draft, Supervision.
Acknowledgements
This work was supported by the Spanish Ministerio de Ciencia, Innovacion y Universidades through grant RTI2018-101475-BI00, jointly with the Fondo Europeo de Desarrollo Regional (FEDER), by the Basque Government through grant IT1229-19, and by the university of the Basque Country (UPV/EHU) under grant COLAB20/01. Author POB is employed by Oslo University hospital (OUH), but his position is sponsored as a donation to the OUH from the Norwegian national union for heart and lung patients (LHL).
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