Current Techniques of Secretion Management

There are several modalities in current clinical practice, which are employed to manage airway secretions or to deal with foreign body aspiration. These include gravity, active suctioning with a catheter connected to a suction machine, manually assisted coughing whereby external force is applied to the abdominal wall and, use of a mechanical insufflation-exsufflation device. This latter device is used to apply a large positive pressure followed by a large negative pressure to the airway. This method has demonstrated efficacy and is frequently used in clinical practice ( ; ). While each of these techniques has demonstrated some benefit, they are also associated with significant limitations that restrict their overall effectiveness. For example, these methods are usually uncomfortable, typically labor intensive, require specialized equipment and provider-patient coordination, and lack uniform distribution of pressure within the intrathoracic cavity. Moreover, based upon the need for trained personnel alone, the annualized costs of application of these methods is also quite large. Perhaps, most importantly, despite use of these methods, the incidence of respiratory tract infections and the mortality from respiratory complications in the SCI population remains high ( ).

Experimental Methods to Restore Cough

Working under the premise that optimal generation of an effective cough is best achieved with the restoration of expiratory muscle function, several methods have been evaluated to activate the expiratory muscles. These include magnetic stimulation (MS) ( ), surface stimulation (SS) of the abdominal muscles ( ; ; ), and SCS ( ). Both MS and SS have the advantage of being noninvasive, whereas the SCS system requires surgical placement. The goal of each method is to activate the major portion of the expiratory muscles, which are typically engaged in the development of an effective cough. These include the abdominal muscle (obliques, rectus, and transversus abdominis muscles) and internal intercostal muscles of the lower rib cage.

MS involves the placement of a large coil over the back in the midline to activate the spinal roots innervating the expiratory muscles. While this technique has been shown to generate significant airway pressures in normal subjects, this could not be reproduced in tetraplegic subjects. It is likely that muscle atrophy contributed, at least in part, to low pressure development and that this would improve with muscle reconditioning ( ). However, it should also be noted that this method has significant limitations in that it is a large, bulky device requiring a 220 V power supply and therefore is not portable. Moreover, substantial heat can be generated at the stimulating coil creating the risk of thermal injury. Lastly, the presence of significant adipose tissue may interfere with successful stimulation due to the greater distance between the stimulating coil and neural activation points. It is possible with further refinement of this device, it may be a useful adjunct to secretion management in selected individuals.

SS of the abdominal muscles has been shown to produce large airway pressures with the application of electrodes with very large surface areas and large stimulation currents. McBain et al. demonstrated that SS, in patients with SCI, results in a plateau in expiratory cough flow in association with increasing expiratory pressures ( ). This finding is indicative of dynamic airway compression, characteristic of an effective cough. Despite very high stimulus intensities, which were as high as 480 mA, current density was low due to the large electrode surface area. While promising, this method has the disadvantages of tedious placement, and removal, of these large electrodes and risk of skin irritation and injury. As with MS, the presence of significant adipose tissue may interfere with its successful application in obese patients due the high electrical resistance of fat tissue. Moreover, long-term clinical efficacy of this method has yet to be demonstrated.

The SCS system results in very large airway pressure generation ( ) and peak airflow rates approximating that achieved by normal individuals. This is accomplished by applying electrical stimulation to disc electrodes, implanted on the dorsal epidural surface of the lower thoracic spinal cord. In contrast to MS and SS, this system is portable, does not require trained caregivers and is likely to be effective in virtually all SCI individuals, even those with significant adipose tissue. While this technique does require an invasive procedure, the surgical technique for placement of electrodes is a routine one that has been in clinical use for over 40 years for the management of chronic pain and spasticity. The incidence of operative complications such as infection and bleeding is also quite low. Consequently, the remainder of this chapter is devoted to this latter technique.

Spinal Cord Stimulation (SCS) to Restore Cough, Animal Studies

Prior to clinical application, extensive studies were performed in animals (dogs) to determine optimal electrode placement and evaluate the mechanism of SCS to activate the expiratory muscles ( ). These investigations demonstrated that SCS with disc electrodes placed over the T9–L1 spinal levels within the dorsal epidural surface resulted in significant activation of the expiratory muscles. Concerning the mechanism of expiratory muscle activation, SCS was shown to involve direct activation of motor roots near the electrode (∼2 segments cephalad and ∼2 segments caudal). Stimulation at the T9 level alone results in the development of large positive airway pressures due to activation of local motor roots but also more caudal motor roots via activation of spinal cord pathways ( ). However, stimulation at the T9 level does not result in complete expiratory muscle activation since stimulation with a second electrode at the L1 spinal level resulted in much greater changes in airway pressure. Use of more than two electrodes does not result in further increases in airway pressure generation.

Spinal Cord Stimulation (SCS) to Restore Cough, Human Studies

Electrical Stimulation System

The cough stimulation system can be surgically placed in a single procedure ( ).Partial hemi-laminotomies are required to place three, 4-mm single-lead, platinum-iridium disc electrodes at the T9, T11, and L1 spinal levels ( Fig. 113.2 ). In the initial clinical trial of this technique, three electrodes were placed due to the larger spinal cord in humans, compared to dogs, and the possibility that activation of the expiratory muscles would only involve direct motor root activation rather than spinal cord pathways. Electrodes are positioned in the midline in the epidural space overlying the thecal sac using fluoroscopic guidance. A single disc is positioned in the thoracolumbar fascia to serve as a ground electrode. A radio-frequency receiver (Finetech, Inc., London, England) is placed in a subcutaneous pocket over the anterior portion of the chest wall, in the area of the upper abdominal wall or preferably the lower rib cage. The electrode wires are tunneled subcutaneously and connected to the receiver. The function of the system should be assessed in the operating room by applying electrical stimulation and confirming contraction of the expiratory muscles. Postoperatively, stimulation can be applied by activating a small portable external control box (9.5 cm × 6 cm × 2.5 cm) connected to a rubberized transmitter (Finetech, Inc.), which can be secured to the skin with tape directly over the implanted receiver. The stimulus controller, powered by a rechargeable battery, delivers a radiofrequency signal to the implanted receiver, which is converted to an electrical signal that is transmitted to the electrodes. The stimulator provides a biphasic stimulus over a wide range of stimulus amplitudes (10–40 V), stimulus frequencies (2–105 Hz), and pulse widths (16–800 μs). Stimulus on-time can be adjusted between 0.2 and 50 s.

Cough system (see text for description).

Individuals with SCI can be trained to activate their expiratory muscles in phase with the other components of the cough reflex. In cases of subjects with respiratory failure and supported with mechanical ventilation or with DP, the ventilator or pacing system can be used to provide the inspiratory component of the cough reflex.

Implementation of the Cough Stimulator Following Surgical Implantation

Prior to application of the cough stimulator, 3–4 weeks of recovery time are necessary to allow for regression of edema and hemorrhage at the electrode and receiver sites and healing of all wounds. The expiratory muscles typically suffer from disuse atrophy and require a program of repeated muscle stimulation to restore strength and endurance. Subjects are instructed to apply electrical stimulation every 30 s for 5–10 min, 2–3 times/day while at home. Stimulus parameters were initially set at values resulting in near maximal positive airway pressure generation, as tolerated, because high intensity force generation for short periods results in the greatest increases in muscle strength. Subjects can also apply the device for expectoration of secretions, as needed.

Airway pressure and peak expiratory airflow are useful indices of the degree of expiratory muscle activation during electrical stimulation. Measurements need to be made with use of a tight-fitting face mask or through a tracheostomy tube, when present. While seated, airway pressure measurements should be made under conditions of airway occlusion at functional residual capacity (FRC) and total lung capacity (TLC). Peak expiratory airflow can be measured after release of airway occlusion after peak airway pressure is achieved during SCS.

Due to the concern of possible autonomic dysreflexia (AD) resulting from SCS, blood pressure and pulse rate should be closely monitored during the initial application of SCS. If absolute blood pressure exceeds 140 mmHg systolic or 100 mmHg diastolic, stimulation should be withheld until values return to baseline or below these values. Stimulation is then applied at less frequent intervals. Signs of AD typically abate and eventually disappear after the initial few weeks of SCS.

Results of Applied Electrical Stimulation

Using maximal stimulus parameters at TLC, the effects of lower thoracic SCS at different spinal levels is shown for one subject in Fig. 113.3 . SCS at each site (T9, T11, or L1) resulted in large airway pressures and peak expiratory airflows of similar magnitude ( ). Combined stimulation of two sites, however, resulted in significant increases in each of these parameters. There were no further increases observed with combined stimulation of three sites. From clinical trial results, mean effects of SCS at FRC and TLC are shown in Fig. 113.4 . As expected, values were significantly higher during SCS at TLC due to the greater mechanical advantage of the expiratory muscles at higher lung volumes. It is important to note that SCS was successful in achieving increases in airway pressure and peak expiratory airflow in each participant in this trial.

Representative trace of expiratory flow and airway pressure taken from one subject during T9, L1, and combined stimulation at total lung capacity (TLC). See text for further explanation.

Mean peak airflow rates (upper panel) and mean airway pressures (lower panel) during spinal cord stimulation (SCS) at the T9, T11, and L1 spinal levels alone and in combinations at total lung capacity (TLC) ( solid bars ) and at functional residual capacity (FRC) ( dotted bars ). Mean spontaneous airway pressure and peak expiratory flow rates are shown for comparison ( clear bars ). Large airway pressures and peak airflow rates of similar magnitude were generated during SCS during single site stimulation. Combined stimulation of two sites, however, resulted in significantly greater airway pressures and peak airflow rates ( P < .05, for each). There were no significant differences in either peak airflow rates or airway pressure generation between any two sites. Combined stimulation of three sites did not result in further increases in these parameters. See text for further explanation.

The relationships between stimulus frequency (10–50 Hz) and airway pressure generation (at maximal stimulus amplitude and pulse width of 200 μs) at TLC and FRC, expressed as a percentage of control values, are shown in Fig. 113.5 . With increases in stimulus frequency, there are significant increases in airway pressure generation during both single-site stimulation and combined stimulation at two sites. There is a plateau in pressure development between 40 and 50 Hz. The relationships between stimulus amplitude (10–40 V) and airway pressure generation with stimulus frequency and pulse width set at 50 Hz and 200 μs, respectively, are shown in Fig. 113.6 . During stimulation at each spinal cord level, there are progressive increases in airway pressure with increasing stimulus amplitude with no plateau in pressure development. With the 2-electrode combination, there are also progressive increases in pressure generation with increases in stimulus amplitude but with no differences between 30 and 40 V suggesting a plateau in pressure development. Increases in pulse width above 150 μs did not result in any increases in pressure development when utilizing supramaximal stimulus frequency and amplitude ( Fig. 113.7 ). Supramaximal parameters ranged between 30 and 40 V, 30 and 40 Hz, and pulse width of 150–200 μs for each subject. While mean maximum expiratory pressure generating capacity with SCS is less than that of healthy persons (∼200 cm H ₂ O for males and ∼150 cm H ₂ O for females), this occurs because of the reduction in inspiratory capacity in this study population. As mentioned, the expiratory muscles are positioned at their greatest length and achieve their greatest force-generating capacity at TLC. With decreases in lung volume, expiratory muscle force generation falls progressively.

Relationship between stimulus frequency and mean airway pressure generation (expressed as a percent maximum) during single site spinal cord stimulation (SCS) and combined stimulation of two sites at functional residual capacity (FRC) and at total lung capacity (TLC). There were progressive increases in airway pressure generation with increases in stimulus frequency. There was a plateau between 40 and 50 Hz, as there were only small changes in pressure generation between these stimulus frequencies. There were no significant differences between responses at TLC and FRC.

Relationship between stimulus amplitude (V) and mean airway pressure generation (expressed as a percent maximum) during single site spinal cord stimulation (SCS) and combined stimulation of two sites at functional residual capacity (FRC) and at total lung capacity (TLC). There were progressive increases in airway pressure generation with increasing stimulus amplitude. With two site stimulation, a plateau developed between 30 and 40 V, as there were no significant differences in pressure generation between these amplitude levels ( P > .05). There were no significant differences between responses at TLC and FRC.

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