In patients undergoing open thoracic and upper abdominal surgery, changes in lung function are seen as being inevitable
[1, 2]. Patients develop predictable pulmonary changes that include altered respiratory mechanics, reduced lung volumes, respiratory muscle dysfunction and alterations in oxygenation status. These changes may be transient and self-resolving or may predispose patients to the development of more substantial complications. Postoperative pulmonary complications (PPCs) have been defined as ‘…a pulmonary abnormality that produces identifiable disease or dysfunction that is clinically significant and adversely affects the clinical course’
. Consequences of PPCs include significant increases in length of hospital stay, patient discomfort, use of resources and overall hospital costs
. More importantly, PPCs are an important cause of postoperative morbidity and mortality following all types of major surgery
Over the past two decades, widespread developments in the identification and modification of risk factors, patient education, surgical and anesthetic techniques, pain management and postoperative rehabilitation have led to reductions in postoperative complications following major surgery and faster discharge from hospital
[6–9]. Despite this, PPCs continue to place a burden on health resources, particularly as surgery is now being offered to patient groups perceived to be at higher risk, such the elderly and those with comorbidities or more severe disease progression
More recently, the focus on strategies to reduce postoperative complications and improve postoperative health-related quality of life has shifted to include prehabilitation. Prehabilitation may be described as the process of improving the functional capacity of the individual prior to a planned intervention, commonly surgery, to enable the individual to withstand the anticipated cardiovascular, respiratory, neuromuscular or musculoskeletal stressors
. Whilst currently the concept of prehabilitation and the evidence for its effectiveness are in their infancy, the main aims of prehabilitation are to improve postoperative outcomes and reduce postoperative risk. Those studies that have investigated prehabilitation have used a variety of preoperative exercise interventions across a spectrum of clinical settings, including joint replacement surgery and cardiac, thoracic and abdominal surgery
. In addition, the concept of prehabilitation to improve functional capacity before and after anticipated admission to intensive care has also been considered
Some evidence suggests that adequate preoperative respiratory muscle strength and the ability to generate sufficient lung volumes may be protective against the development of PPCs
[10, 13]. Hence, preoperative training of the inspiratory muscles is one of several prehabilitation interventions currently coming under increasing investigation. Inspiratory muscle training (IMT) is a technique that targets the muscles of inspiration and aims to increase inspiratory muscle strength and endurance by applying an increased load to inspiration
IMT can be undertaken in several ways including isocapnic/normocapnic hyperpnoea training, inspiratory resistive flow training and inspiratory threshold pressure training
. With isocapnic/normocapnic hyperpnoea, the patient breathes at a high percentage of maximum voluntary ventilation while maintaining isocapnia via entrainment of CO2 into the circuit. Isocapnic/normocapnic hyperpnoea allows the generation of high flow load with minimal pressure load and is used as respiratory muscle endurance training. Because isocapnic/normocapnic hyperpnoea involves a complex breathing circuit, is relatively time-consuming and is physically demanding, it is seldom used clinically, however, the development of a portable device for normocapnic hyperpnoea may increase its use in clinical trials
. Inspiratory resistive flow training refers to the technique of providing load by inspiring through a small aperture. One limitation of inspiratory resistive flow training is that the resistance provided may alter if the patient varies the inspiratory flow rate. Thus careful monitoring and feedback is required to ensure consistency of training loads. This problem of variable flow rates is solved by inspiratory threshold pressure devices where a spring- or plunger-loaded valve within the device opens to allow airflow when a predetermined threshold negative pressure is achieved during inspiration. Thus reproducible pressures can be assured
. Consensus recommendations regarding load, repetition and duration of training programs have not been established and may differ depending on the population of interest but, nonetheless, inspiratory muscles are known to respond to training in the same way as other skeletal muscles
IMT has been shown to increase inspiratory muscle strength in healthy volunteers
[18, 19] and several patient populations, including patients weaning from mechanical ventilation
, patients presenting with chronic lung disease
 and patients with chronic heart failure
. As respiratory muscle dysfunction, particularly of the diaphragm, has been widely linked with the development of PPCs following major surgery
[23, 24], it is conceivable that strengthening the inspiratory muscles prior to surgery may impact upon postoperative recovery and reduce incidence of postoperative complications. This may shorten length of stay, reduce associated costs and improve patient outcomes.
Two recent systematic reviews are available that have estimated the effects of preoperative IMT (amongst a range of preoperative interventions) in patients undergoing major surgery
[12, 14]. However, the estimates of the effects of IMT may be affected by the methods used in these reviews. Olsén and Anzén
 considered a range of outcomes but did not attempt a meta-analysis. Conversely, Valkenet and colleagues
 performed a meta-analysis for the outcome PPCs, but did not consider important outcomes like oxygenation, mortality, quality of life, adverse events and costs. Also, both reviews excluded studies because of the language of publication and did not attempt to contact authors for unpublished data. Furthermore, since the searches were conducted for these reviews, additional data has been published (for example
[25, 26]), therefore a comprehensive review of this topic is appropriate.
The aims of this systematic review are to answer these questions:
Does preoperative IMT increase inspiratory muscle strength and endurance in patients undergoing open cardiothoracic or upper abdominal surgery?
Does preoperative IMT reduce the incidence of PPCs and length of hospital stay in these patients?
Does preoperative IMT affect the duration of mechanical ventilation, lung function, oxygenation, time to first sit out of bed and to first ambulation, exercise tolerance, adverse events, quality of life, mood, satisfaction, mortality and costs?