Many patients who are admitted to the intensive care unit (ICU) experience muscle weakness. This muscle weakness occurs whilst patients are unconscious, immobile in bed and on a breathing machine (ventilator). It can develop very quickly, as soon as they become unwell. The investigators know that this weakness can make it harder for patients to regain their normal level of functional ability, such as standing and walking independently. Physical rehabilitation, delivered by physiotherapists, is important for patients as they recover from their critical illness to help them regain strength and to practice the ability to perform physical activities. These activities include sitting on the edge of the bed, standing, stepping on the spot and walking.

However it is difficult to know how hard it is for patients who are recovering from critical illness to perform these types of activities - in other words, how much energy is required. Some patients may find certain activities harder or easier than others. By knowing the energy requirements of patients whilst they take part in different physical rehabilitation activities, physiotherapists may be able to be more accurate with prescribing exercises and designing rehabilitation sessions for patients to practice achieving those activities.

In this study, the investigators will measure the energy requirements of the patients when they take part in physiotherapy-led physical rehabilitation they will have during their admission. To do this, the investigators will use a different ventilator to the one normally used to help their breathing. This ventilator works in exactly the same way, but has an extra component built into it to measure energy requirements. After the rehabilitation session, the patient will return to using their normal ventilator.

Admission to the intensive care unit (ICU) with critical illness is typically associated with profound physical impairments including peripheral skeletal muscle wasting and dysfunction. These effects demonstrate a rapid onset from the point of ICU admission, affect those with higher illness acuity to greater levels, and contribute to the development of intensive care unit-acquired weakness (ICU-AW) defined as severe upper and lower limb muscle weakness. Clinically patients present with significant limitations in exercise capacity and performance of physical functional activities, with deficits persisting for many years following resolution of the original illness.

Physical rehabilitation is recommended to address the physical and functional deficits associated with ICU-AW, and delivery is advocated across the continuum of the patient recovery pathway, commencing whilst in the ICU, following transfer to the ward and beyond hospital discharge. Physical rehabilitation within the ICU typically incorporates early mobilisation (EM), characterised by a hierarchical progression of increasingly functional activities such as active-assisted bed exercises, sitting-on-the-edge-of-the-bed (SOEOB), standing, marching-on-the-spot and walking. These activities are summarised in the ICU Mobility Scale. In recent years, adjunctive technologies including electrical muscle stimulation, interactive video-games and passive cycle ergometry have also been utilised.

Delivery of early mobilisation is feasible, safe and effective in improving both short and long-term outcomes in critically ill patients even in the early stages of recovery whilst patients remain ventilated and receiving other forms of organ support. However, the optimum 'dose' of physical rehabilitation to deliver to patients whilst in the ICU remains undetermined. Early mobilisation reflects a clinically pragmatic approach to patient management, but there are few data to support the intensity, frequency and timing of interventions.

Furthermore, there is little known of the acute physiological response to undertaking this type of exercise in this patient population. Metabolic monitoring can provide information on the energy expenditure of patients. However, direct methods of data acquisition such as use of pulmonary artery catheters to measure whole-body oxygen consumption can be unreliable and their use in clinical practice has diminished in recent years. The non-invasive technique of indirect calorimetry (IC) has therefore emerged as a potential alternative that may be more practically applicable in the clinical setting. Indirect calorimetry measures total gas exchange, oxygen consumption and carbon dioxide production which can in turn determine energy expenditure. However the widespread clinical utility of the IC technique, can be offset by the number of clinical factors that may preclude its use across all patients e.g. those with cardiovascular or ventilator instability, those with open chest drainage or receiving renal haemofiltration.

Two studies to date have used IC to examine the response to physical rehabilitation activities performed by patients in the ICU. Collings et al conducted a randomised cross-over study in which 10 ICU patients completed either a passive chair transfer or an active sit-over-the-edge-of-the-bed (SOEOB) on one day, followed by the alternative activity on the consecutive day. The 'active' SOEOB activity elicited a significantly greater increase in oxygen consumption than the passive activity (90.69 ml/min (95% CI 44.04 to 137.34) vs 14.43 ml/min (95% CI -27.28 to 56.14), p = 0.007). Hickmann et al examined active or passive cycling in critically ill patients and healthy controls, demonstrating increased oxygen consumption during the active exercise compared to passive in patients with differing energy requirements compared to healthy individuals.

This limited existing evidence-base demonstrates the potential for use of IC to characterise physical rehabilitation in critically ill patients, but is restricted to particular types of activities and fails to capture sequential assessments of patients as their rehabilitation progresses through increasingly functional levels. Acknowledging the potential clinical limitations of applying the IC measurement technique to all critically ill patients, the aim of this study is to examine the feasibility of measuring energy expenditure of critically ill patients completing a range of physical rehabilitation activities (as characterised on the ICU Mobility Scale) performed sequentially during their ICU or ventilator weaning unit (Lane Fox Respiratory Unit, LFU) admission using indirect calorimetry.

Inclusion Criteria:

• Adult, ≥18years
• Receiving physical rehabilitation as part of routine physiotherapy management
• Invasively mechanically ventilated for ≥48hours
• Receiving invasive mechanical ventilation with stable settings

Exclusion Criteria:

• Endotracheal or tracheal leak >10%
• Burns
• Open chest drainage
• Inspired oxygen (FiO2) ≥0.6
• Extra-corporeal membrane oxygenation (ECMO)
• Pregnancy
• Cardiorespiratory instability requiring frequent adjustment of ventilator settings or FiO2, inotropic or sedative dosage, or positive end-expiratory pressure (PEEP) >10cmH2O
• Any neurological, orthopaedic or other factor contraindicating mobilisation
• Medical diagnosis mandating a specialist rehabilitation pathway e.g. amputee, acute neurological injury
• Palliative management

Adult critically ill patients undergoing physical rehabilitation in either the Intensive Care Units or the Lane Fox Unit at Guys and St Thomas' NHS Foundation Trust.