There is considerable interest in oxygen partial pressure (PO2) monitoring in physiology and biology, and in tracking PO2 changes dynamically in situations where PO2 varies rapidly. For example, arterial PO2 (PaO2) can vary within the respiratory cycle in cyclical atelectasis (CA), when PaO2 is thought to increase and decrease respectively during inspiration and expiration[1]. In CA, an intravascular sensor that detects these PaO2 variations could become a useful clinical diagnostic tool[2].Our group has developed an ultrafast fiber optic PO2 sensor[3, 4]. This sensor (200 µm diameter) is low-cost, has a very fast response time, and can measure PO2 continuously. We hypothesized that this sensor could show within-breath PaO2 variations in a model of lung injury.In order to determine the response time of the PO2 sensor, this was initially tested in the gas phase in vitro. A pressure step-change was generated in a small pressure chamber containing 5% O2 gas, and whose total pressure could be rapidly cycled between 100 and 300 kPa, giving an associated PO2 change from 5 to 15 kPa. Chamber pressure was recorded continuously with a piezoelectric transducer (response time: 1 ms), used as a means of comparison.The results from these in vitro tests are shown in Fig. 1A, which illustrates the step change detected by the piezoelectric transducer (dotted line), and the short response time of the fiber optic PO2 sensor (solid line). The sensor’s response time was shown to be less than 100 ms.An ovine saline lavage model of lung injury was used to determine whether the sensor was capable of detecting within-breath PaO2 variations. Experiments conformed to the National Institutes of Health Guidelines for the Use of Laboratory Animals, and the protocols were approved by the Home Office. Anaesthesia was induced with midazolam (0.4 mg/kg) and propofol to effect (approx. 2 mg/kg), and maintained with isoflurane (1% end-tidal) with the Anaconda system. The PO2 sensor was positioned in the carotid artery via a standard arterial catheter, the insertion of which was guided with ultrasound imaging. Ventilation was achieved with a Siemens Servo 900C mechanical ventilator in pressure-control mode, and physiological variables were continuously monitored with standard patient monitors (Datex Ohmeda Capnomac Ultima; multi-parameter patient monitor: Datex AS3).Figure 1B shows the first results recorded with our PaO2 sensor in the healthy lung (grey line, inspired fraction of oxygen (FIO2) = 30%), where PaO2 remained at about 12 kPa over a series of five breaths. Figure 1B also shows results from the injured lung (black line, FIO2 = 100%), where PaO2 oscillations with the same period as respiration were detected.We conclude that this novel sensor has the potential to become a diagnostic tool for CA, and to offer new insights into physiological scenarios where rapid PaO2 changes occur.