This lecture describes a series of clinical studies carried out by means of proton transfer reaction mass spectrometry (PTR-MS and PTR-ToF-MS).

Time and place of breath sampling can have a distinct impact onto exhaled substance concentrations and may easily override endogenous effects. Concentrations of breath biomarkers – but also those of typical contaminants – change rapidly and in a pronounced way within minutes in the clinical environment. Punctual breath sampling or simple subtraction of background air is not sufficient to account for this problem.

There are immediate effects of human physiological changes on VOC exhalations in breath. The extent of different physiological effects on VOCs depend on the origin and physico-chemical properties and distribution kinetics of the compounds. Changes in breathing patterns (e.g. breath holding or FEV maneuvers) and changes in body positions had profound effects on exhaled VOC concentrations. Physiology induced changes in exhaled VOC compositions returned to initial baseline within a certain timeframe. Breathing route induced changes depended on respiratory parameters, oral and nasal cavity exposers and physico-chemical characters of the compounds.

Volatile breath constituents such as acetone and ammonia have been linked to dextrose, fat, and protein metabolism. Non-invasive breath analysis, therefore, may be used for metabolic monitoring, identification of fuel sources actually used for energy production and determination of the anaerobic threshold (AT). Exhaled concentrations of acetone, ammonia, and isoprene were determined in 21 healthy volunteers under controlled ergometric exercise. In parallel, spiro-ergometric parameters (VO₂, VCO₂, respiratory rate and minute ventilation) and hemodynamic data such as heart rate were recorded. AT was determined from serum lactate, by means of respiratory exchange rate and by means of exhaled acetone concentrations. Exhaled acetone concentrations mirrored exercise induced changes of dextrose metabolism and lipolysis. Bland–Altman statistics showed good agreement between lactate threshold, respiratory compensation point (RCP), and determination of AT by means of exhaled acetone. Exhaled ammonia concentration seemed to be linked to protein metabolism and changes of pH under exercise. Isoprene concentrations showed a close correlation to cardiac output and minute ventilation. Breath biomarkers represent a promising alternative for metabolic monitoring under exercise as they can be determined noninvasively and continuously. In addition, these markers may add complementary information on biochemistry, energy production and fuel consumption.

Breath analysis could offer a non-invasive means of intravenous drug monitoring if robust correlations between drug concentrations in breath and blood could be established. Effects of changes in pulmonary blood flow resulting in a decreased cardiac output (CO) and effects of dobutamine administration resulting in an increased CO on propofol breath concentrations and on the correlation between propofol blood and breath concentrations were investigated in seven acutely instrumented pigs. Increasing cardiac output led to a deterioration of the relationship between breath and blood propofol concentrations. Decreasing pulmonary blood flow and cardiac output through banding of the pulmonary artery did not significantly affect the relationship between propofol breath and blood concentrations. Estimation of propofol blood concentrations from exhaled alveolar concentrations seems possible even when cardiac output is decreased. Increases in cardiac output preclude prediction of blood propofol concentration from exhaled concentrations.

In critically ill patients, VOC analysis may be used to gain complimentary information beyond global clinical parameters. This seems especially attractive in mechanically ventilated patients frequently suffering from impairment of gas exchange. Exhaled VOC concentrations varied with recruitment induced changes in minute ventilation and cardiac output. Ammonia exhalation depended on blood pH. The improvement in dorsal lung ventilation during recruitment ranged from 9%to 110%. Correlations between exhaled concentrations of acetone, isoprene, benzene sevoflurane and improvement in regional ventilation during recruitment were observed. Extent and quality of these correlations depended on physico-chemical properties of the VOCs. Combination of continuous real-time breath analysis and Electro Impedance Tomography (EIT) revealed correlations between exhaled VOC concentrations and distribution of ventilation. This setup enabled immediate recognition of physiological andtherapeutic effects in ICU patients.

Monitoring metabolic adaptation to chronic kidney disease (CKD) early in the time course of the disease is challenging. As a non-invasive technique, analysis of exhaled breath profiles is especially attractive in children. Ammonia accumulated already in CKD stage 1, whereas alterations of isoprene (linked to cholesterol metabolism), pentanal and heptanal (linked to oxidative stress) concentrations were detectable in the breath of patients with CKD stage 2 to 4. Only weak associations between serum creatinine and exhaled VOCs were noted. Non-invasive breath testing may help to understand basic mechanisms and metabolic adaptation accompanying progression of CKD. These results support the current notion that metabolic adaptation occurs early during the time course of CKD.

Strong points of Soft Chemical Ionization Mass Spectrometry are non-invasiveness, continuous analyses without any burden to the patient and unlimited repeatability. These features can be used to address important basic issues in clinical breath analysis such as influence of physiology onto results. In addition, this technique provides optimal conditions for longitudinal and cross sectional studies exploring short and even long term changes in breath biomarkers.