Abdominal Functional Electrical Stimulation Training and Its Effect on Mechanical Insufflation-Exsufflation

Respiratory infections are a leading cause of morbidity and mortality for the tetraplegic population. They are often caused by the build up of secretions in the lungs as a result of being unable to generate an effective cough. This inability to cough leads to many tetraplegic people requiring some type of intervention to aid the removal of secretions. Manually assisted coughing or routine suctioning through the patient's tracheostomy tube are commonly used. Both of these techniques are associated with problems: (i) Tracheostomy suctioning is uncomfortable and often misses the left bronchus, one of the contributing factors to 80% of pneumonia occurring in the left lung of the spinal cord injured population. (ii) A manually assisted cough is a safe and effective procedure but needs to be provided by a trained care giver with associated resource implications for the local health care provider. An alternative method to help clear secretions and improve ventilation is Mechanical Insufflation-Exsufflation (MI-E) where a combination of positive and negative pressure is applied to the user's airway in order to induce a cough. A number of studies have shown that MI-E is more effective at removing secretions and reducing respiratory infections than conventional suctioning and manually assisted cough techniques, with the advantage that secretions are removed from both bronchi. MI-E has also been shown to significantly reduce the length of ICU stay and reduce the rates of reintubation.

Individuals with tetraplegia have reduced exhaled tidal volume (VT) and reduced Cough Peak Flow (CPF) (maximum air flow rate during a cough. The CPF value can be used to assess the risk of respiratory infection, with a CPF < 160 L/min associated with no functional cough and a high risk of infection and a CPF of <270 L/min suggesting that a person will not be able to adequately clear secretions from their airway. The use of MI-E is therefore suggested for anyone with a CPF of <270 L/min.

Functional Electrical Stimulation (FES) is a technique which can be used to make paralysed muscles contract. The most effective contraction is observed when train of electrical pulses are applied close to the motor point of a paralysed muscle. When FES is applied to the abdominal muscles it is called Abdominal Functional Electrical Stimulation (AFES). AFES has previously been used to increase the VT and CPF of tetraplegic patients in a number of studies.

MI-E is routinely combined with manually assisted coughing in order to loosen lung secretions and to increase the user's CPF. In the same way as a manually assisted cough, AFES leads to increased abdominal movement, resulting in larger CPF. During recent studies the investigators have observed that the use of AFES with acute tetraplegic patients appears to be associated with an increased need for secretion clearance, indicating that AFES aids the loosening of lung secretions. Although the abdominal movement achieved with AFES is typically significantly smaller than that achieved with a manually assisted cough, it may have beneficial effects in combination with MI-E. In addition, AFES has the advantage that it can be applied consistently and requires less involvement of the caregiver.

The investigators hypothesise that (i) maximal respiratory improvements due to an AFES training programme will be achieved after more than 3 weeks of training, and (ii) that the combined use of AFES with MI-E may improve the respiratory function and ability to clear secretions of tetraplegic patients, resulting in a reduction in the number of respiratory infections in this group.

Methods

Participants Ten tetraplegic participants will be recruited for this study. All participants will be inpatients at the Queen Elizabeth National Spinal Injuries Unit, Southern General Hospital, Glasgow.

Experimental Procedures Each participant will undergo a pre study assessment. The study will take the form of a random crossover design involving a four week control period and an eight week training period which will be followed by a two week follow up period. The order of the control and training periods will be randomised for each participant. If the training period precedes the control period the follow up period will serve as a two week wash out period to avoid any carryover of acute effects from the training intervention. At the pre study assessment, and at the end of every second week, the participant will take part in an assessment session.

Pre study assessment Before the study commences each participant will undergo a pre study assessment which will also serve as a familiarisation session. Stimulation will be applied to the participant's abdominal muscles to test whether they respond to electrical stimulation. The optimum stimulation parameters (electrode position/frequency/current and pulsewidth) required to induce a strong contraction of the abdominal muscles will then be established. These stimulation parameters will be used as a starting point for the following training and assessment sessions. An assessment session, as outlined below, will then be conducted at the end of the pre study assessment.

Training sessions Participants will take part in AFES training sessions five times per week (Monday-Friday) for a total of 8 weeks. During these sessions participants will receive AFES for 40 minutes.

Assessment sessions Each participant will take part in bi-weekly assessment sessions throughout the study. At each assessment session a series of Forced Vital Capacity (FVC) tests will be performed with and without abdominal stimulation, which provide a measure of the participant's Vital Capacity (VC), Forced Exhaled Volume in one second (FEV1) and Peak Expiratory Flow (PEF). Respiratory flow and volume will be measured using a spirometer. To perform the FVC manoeuver the participant will be asked to inhale to Total Lung Capacity (TLC) and then exhale as quickly and as fully as they can. This will be repeated for each condition up to a maximum of 5 times, or until 3 attempts within 0.15L of each other are recorded. Many of the participants on this study will have a FVC of <1L. The ATS/ERS standards for spirometry suggest that for an FVC of <1L the range of acceptable values should be plus or minus 0.1L. All participants will perform the assessment sitting in an upright position.

The participant's cough peak flow (CPF) will also be measured under three conditions: (i) unassisted (ii) unassisted mechanical insufflation-exsufflation (MI-E) and (iii) MI-E assisted with Abdominal Functional Electrical Stimulation (AFES). To measure the CPF, five cycles of MI-E will be applied for both MI-E conditions. The maximum CPF value recorded from five attempts will be taken as the participant's CPF in each condition.

During the pre study assessment and at the end of the training and the control period an extended assessment session will be performed. The additional measurements will be (i) MI-E with manually assisted cough, and (ii) ultrasound measurement of diaphragm movement with and without AFES.

The CPF achieved using MI-E with a manually assisted cough will be recorded while the manually assisted cough is applied by the patient's regular caregiver, with the same caregiver applying the manually assisted cough in all three sessions.

The movement of the diaphragm will be measured during a FVC manoeuvre using an ultrasound probe, with and without AFES assistance. This will allow an evaluation of the effect of AFES on diaphragm movement.

Follow up/ Wash out period After the training period the participant will undergo a two week follow up period. If the participant has still to complete the control period this follow up will also act as a wash out. During this wash out period the participant will undergo no training sessions. The aim of this wash out period is to minimise the effect of any acute response from the training period to the control period.