Estimating dead space ventilation: a computational modelling approach towards evaluation of clinical estimates of dead space fraction in critically ill patients

Naeem, Usra (2018) Estimating dead space ventilation: a computational modelling approach towards evaluation of clinical estimates of dead space fraction in critically ill patients. PhD thesis, University of Nottingham.

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Dead space is the part of tidal volume that does not participate in gas exchange and represents wasted ventilation. It is often increased in pulmonary diseases. Quantification of dead space by the original Bohr’s equation requires measuring mean alveolar pressure of CO2 (PACO2) and mixed expired partial pressure of CO2 (PĒCO2). Because of the difficulties and technical issues related with measuring PACO2 and PĒCO2, alternative methods have been proposed for the estimation of dead space. This thesis attempts to explore the performance of some methods proposed for the estimation of dead space to tidal volume ratio (VD/VT) in different pulmonary configurations and clinical scenarios.

In the first study, we compared the performance of 5 different methods for the estimation VD/VT with the gold standard method in multiple ventilation/perfusion (V/Q) relationships. Six pulmonary configurations all with same alveolar dead space fraction of 0.25, but with different specific pattern of V/Q distribution were created within the Nottingham Physiology Simulator (NPS). Next, variations in the methods of estimating VD/VT upon varying 4 physiological factors were analysed. We concluded that the estimation of alveolar dead space ratio by 5 methods of estimating VD/VT is influenced by pattern of V/Q distribution and alterations in the relevant physiological factors.

In the second study, we further analysed performance of 5 methods for estimating dead space ratio in patients with acute respiratory distress syndrome (ARDS). Nineteen ARDS subjects were created within the NPS and alveolar dead space fraction was measured by the gold standard method. Then, dead space fraction was determined by 5 different methods for estimating dead space fraction. We found that the estimates of dead space fraction measure different than the conventional equation in ARDS.

In the third study, we compared efficacy of three lung recruitment maneuvers (RMs) in patients with ARDS. Six virtual ARDS patients were created and changes in dead space fraction, (Pa-E’CO2)/PaCO2 and other parameters were observed following the RMs. The results of this study showed that changes in (Pa-E’CO2)/PaCO2 closely relate with changes in VD/VT. These findings suggest that in clinical settings where it is not possible to measure dead space fraction, a simple estimate of VD/VT may be used to monitor the efficacy of RMs and titration of positive end-expiratory pressure.

Simplified approaches for the estimation of dead space fraction may allow widespread use of this important physiological variable for diagnostic and prognostic purposes in critical care settings.

Item Type: Thesis (University of Nottingham only) (PhD)
Supervisors: Hardman, Jonathan G.
Harvey, Daniel J.R.
Keywords: Pulmonary gas exchange; Ventilation-perfusion ratio; Respiratory dead space
Subjects: W Medicine and related subjects (NLM Classification) > WF Respiratory system
Faculties/Schools: UK Campuses > Faculty of Medicine and Health Sciences > School of Medicine
Item ID: 51774
Depositing User: Naeem, Usra
Date Deposited: 10 Sep 2018 12:26
Last Modified: 12 Jul 2020 04:30

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