Bedside measurements of oxygen consumption and end-expiratory lung volume in mechanically ventilated patients

Thesis

Critical care medicine relies heavily on physiological monitoring, however, several fundamental variables remain difficult to measure at the bedside. This thesis focuses on two of these variables: oxygen consumption (V̇O₂) and end-expiratory lung volume (EELV). Both are central to understanding metabolic and respiratory function, but their reliable quantification in mechanically ventilated patients has historically been challenging. The overarching aim of this work was to determine whether V̇O₂ and EELV can be measured with sufficient accuracy to provide clinically meaningful information in the intensive care setting.

Chapters 1 and 2 provide the physiological background to V̇O₂ and EELV, outlining their importance and the challenges associated with their measurement in critical care.

Chapter 3 introduces Computed Cardiopulmonography (CCP), a technique composed of in-airway highly-precise and time-resolved measurements of respired gas composition and flow (Molecular Flow Sensor; MFS) along with robust cardiopulmonary modelling.

The first experimental strand (Chapters 4 and 5) investigated V̇O₂ measurement using the MFS. In healthy volunteers, the MFS showed strong agreement with the Douglas bag method, and in mechanically ventilated patients it provided precise V̇O₂ assessments over a prolonged measurement window. A novel quality control approach based on nitrogen balance was developed to construct confidence intervals around each individual V̇O₂ measurement. V̇O₂ trends were captured in tandem with fluctuations in body temperature, muscle activity, and haemodynamic status, supporting the feasibility of its real-time monitoring in critical care.

The second experimental strand (Chapters 6 and 7) examined the estimation of EELV using CCP. In healthy volunteers, CCP-derived volumes agreed with body plethysmography. EELV values obtained by CCP under partial and full multi-breath nitrogen washout stimulus were compared and proven to provide equivalent results. In mechanically ventilated patients, CCP delivered reproducible EELV estimates. An application of such estimates was conducted to assess the changes in absolute volume induced by lung recruitment manoeuvres: the total lung volume change was highly variable and did not correlate with existing predictors of lung recruitability.

Chapter 8 summarises the main conclusions of the thesis. Together, these studies demonstrate that V̇O₂ and EELV can be made measurable at the bedside with sufficient accuracy to inform clinical care and potentially aid in decision-making.

Authors: Luiz Segel, J

• DOI: 10.5287/ora-vje4rrmp5
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English