Weaning Algorithm for Mechanical VEntilation

Patients admitted to the intensive care unit typically receive invasive mechanical ventilatory support when they are critically ill. Whilst mechanical ventilation is a life-saving intervention, it can also lead to deleterious consequences and cause lung damage (known as ventilator-associated lung injury) if not implemented carefully. Hence, reducing the duration of mechanical ventilation should reduce complications such as ventilator-associated lung injury, ventilator-acquired pneumonia, respiratory and skeletal muscle wasting, and patient discomfort, leading to decreasing mortality and economic costs etc. Importantly, prolonged weaning perpetuates these complications which further increase the duration of mechanical ventilation thereby creating a viscous cycle leading to greater morbidity and mortality.

The availability and education of intensive care unit (ICU) staff are important considerations in minimizing the duration of mechanical ventilation through weaning protocols. It is common practice that the attending physician decides upon patient's therapy and ventilatory management according to recommendations. This usually occurs as part of medical rounds. In addition, nurses often manage the weaning of patients from mechanical ventilation either by following attending physicians' instructions or local guidelines protocols. Such protocol-directed, nurse-driven weaning has been shown to reduce the duration of mechanical ventilation. However, it has been shown that the quantity and quality of nursing are important factors if duration is to be reduced.

Several decision support systems have been developed to help select optimal mechanical ventilator strategies. Those finding their way into routine clinical practice have typically been based on clinical guidelines or rules rather than detailed physiological description of the individual patient. A recent Cochrane review of weaning trials with these systems concluded that use of these systems may reduce duration of weaning, but pointed out that many of these trials are based on patients that are 'simple to wean'. Such patients are usually less complex, without lung pathology, and ventilated for less than 48 hours. However, there is a need to develop and validate protocolised systems that utilize a more detailed physiological description of individual patients to aid in the management of complex patients ventilated for longer durations.

The Beacon Caresystem is a model-based decision support system using mathematical models tuned to the individual patient's physiology to advise on appropriate ventilator settings. Personalised approaches using individual patient description may be particularly advantageous in complex patients, including those who are difficult to mechanically ventilate and wean; precisely those where previous systems have not been sufficiently evaluated. The Beacon Caresystem is a commercial version of the system previously known as INVENT, which has been retrospectively evaluated in post-operative cardiac patients and patients with severe lung disease, and prospectively evaluated in advising on the correct level of inspiratory oxygen. Furthermore, studies are near completion showing that the system provides safe and appropriate advice on inspired oxygen, respiratory frequency, tidal volume, pressure support/control and positive end expiratory pressure (PEEP) in a wide variety of patients ranging from patients with severe respiratory failure to patients close to extubation (unpublished data). However, previous and ongoing studies with the Beacon Caresystem have focused on safety and efficacy of advice under limited time periods, and have not focused on weaning from mechanical ventilation.

The core of the Beacon Caresystem is a set of physiological models including pulmonary gas exchange, acid-base chemistry, lung mechanics, and respiratory drive. The Beacon Caresystem tunes these models to the individual patient such that they describe accurately current measurements. Once tuned, the models are used by the system to simulate the effects of changing ventilator settings. The results of these simulations are then used to calculate the clinical benefit of changing ventilator settings by balancing the competing goals of mechanical ventilation. For example, an increased inspiratory volume will reduce an acidosis of the blood while detrimentally increasing lung pressure. Appropriate ventilator settings therefore imply a balance between the preferred value of pH weighted against the preferred value of lung pressure. A number of these balances exist, and the system weighs these, calculating a total score for the patient for any possible ventilation strategy. The system then calculates advice as to changes in ventilator settings so to as improve this score. The Beacon Caresystem functions as an "open loop" system. This means that the advice provided by the system is presented to the clinician. The ventilator settings are then changed by the clinician, and the patient's physiological response to these changes is automatically used by the system to re-tune the models and repeat the process of generating new advice.

In calculating appropriate advice, selecting the correct level of positive end expiratory pressure (PEEP) is particularly challenging. The nature of the challenge is however, very different depending upon the presence or type of lung abnormality, and the function of the heart. Patients with severe lung abnormalities such as acute respiratory distress syndrome (ARDS), which often result in small, stiff lungs, are often in control ventilation mode with little or no spontaneous breathing. For these patients, PEEP is often increased to try to recruit units of the lung which are collapsed. This can be difficult, as increasing PEEP may result in elevated lung pressure and hence an increased the risk of lung injury, incomplete expiration and air trapping, and haemodynamic compromise, especially in those with heart failure.

Patients in support ventilation modes have some degree of spontaneous breathing, and the correct selection of PEEP therefore includes different criteria. It is important that these patients be weaned as quickly as possible, and PEEP is reduced as part of that process. If the setting of PEEP is too low, there is a risk of increased resistance to airflow with added respiratory work and consequent risk of respiratory muscle fatigue. If the patient has intrinsic PEEP due to dynamic hyperinflation, reducing PEEP below the level of intrinsic PEEP would also cause increased inspiratory threshold load on the respiratory muscles, and potential muscle fatigue. If PEEP is too high the respiratory muscle fibres may be shortened reducing their pressure generating capacity and endurance thus increasing the risk of respiratory muscle fatigue. Changing pressure support may help to work against an additional workload, as in cases of increased resistance or autoPEEP, whilst correct PEEP may counter the additional load.

The above factors are taken into account by the physiological models of the Beacon Caresystem, and patient specific advice is also provided on PEEP. In addition to providing advice on changing individual ventilator settings, the system also advises on when measurement of arterial blood gas is necessary, when it is important to change ventilator mode, and when a spontaneous breathing test is passed, and as such extubation should be considered.

However, previous and ongoing studies with the Beacon Caresystem have focused on safety and efficacy of advice under limited time periods, and have not focused on weaning from mechanical ventilation. A current study is underway in a French hospital (Clinical Trials number: NCT02842944), and at a UK hospital (IRAS 226610), to assess the benefit of the Beacon Caresystem in general medical intensive care patients. However, as mechanical ventilation therapy can vary with different patient populations it is important that investigation of the effects of use of the Beacon system be studied in numerous different clinical situations. In contrast to other studies, this study will investigate the effects of the Beacon Caresystem in ICU patients with primary cardio-thoracic disease, with these patients representing a substantial sub group of all ICU patients worldwide.

The purpose of this study is to compare mechanical ventilation following advice from the Beacon Caresystem to that of routine care in cardio-thoracic ICU patients from the start of requiring invasive mechanical ventilation until ICU discharge or death. The Beacon Caresystem will be compared to routine care to investigate whether use of the system results in similar care or reduced time for weaning from mechanical ventilation.