This is a feasibility study of the use of a novel technique called 'Inspiwave' to measure heart and lung function in adult patients in critical care.

The preliminary work has already been undertaken in animal models and in healthy volunteers. The ultimate aim of this study is to develop a clinical tool for measuring (and therefore being able to make treatment changes based on) indices of heart and lung function in critical care patients. This study is the first assessment of the technique in this population, and whilst we know it works in patients undergoing general anaesthesia, we now need to assess whether Inspiwave can be used at all in critical care patients who may have much more physiological derangement. The purpose of this phase of the research is to determine whether it is feasible to use Inspiwave in critical care.

Inspiwave generates a sinusoidally modulated tracer gas signal in the inspired air. It also measures the resulting signal in the expired air. The unique handling of this signal by the patient can used be to derive key variables related to cardiopulmonary function such as lung volume, pulmonary blood flow, the deadspace (wasted ventilation) and the degree to which ventilation and blood flow are non-uniform. These are 'vital signs' in sick patients, yet are currently technically difficult to measure, particularly non-invasively.

Inclusion Criteria:

• patients requiring mechanical ventilation in a critical care area

Exclusion Criteria:

• decision of participant not to be included
• consultee declaration not to be included
• Severely impaired gas exchange as declared by the managing clinician
• oxygen requirement exceeding 90% FIO2

-mechanically ventilated patients in a critical care area