Session 2: Respiratory and cough failure in MND – an overview

Normal ventilation Ventilation is the movement of air between the environment and the lungs via inhalation and exhalation. Normal ventilation ensures effective control of oxygen and carbon dioxide. Normal tidal ventilation is around 400-500mls.

Normal Ventilation Normal Ventilation Requires an effective respiratory muscle pump. An appropriate load on those muscles. Open upper airways. A functioning control centre/normal drive.

Normal ventilation Respiratory centre , regulates breathing and determines how often and deeply we breathe Signal to breathe is carried to muscles via nerves The nerve signal is then conveyed to the muscles via neural junction

Respiratory muscles The respiratory muscles constitute the respiratory pump responsible for gas exchange Respiratory muscle weakness leads to respiratory pump failure Diseases that increase the load on pump or decrease its capacity or both, lead to breathlessness and eventually ventilatory failure Anything that drives an increase in ventilation (hypoxia, hypercapnia) will increase ventilatory load An increase in load: capacity ratio causes an increase in ventilatory drive (neural respiratory drive) Respiratory muscle weakness often advanced before clinical symptoms occur Cardinal symptom of respiratory muscle weakness is breathlessness

Muscular weakness Respiratory system Inspiratory muscles (diaphragm, external intercostal, scalene, sternocleidomastoid and trapezium) responsible for ventilation Expiratory muscles (internal intercostals and abdominal muscles) responsible for coughing Glosso-pharyngeal muscles for airway protection NMD involves all the respiratory muscles but mainly diaphragm and inspiratory muscles. Increase in respiratory rate along with small tidal volumes may cause lung atelectasis and reduction of compliance, therefore increasing mechanical load on already weakened respiratory muscles. This results in alveolar hypoventilation with increase in carbon dioxide

Hypoventilation Anything which interferes with the fine balance between load, pump and drive will cause hypoventilation

Neuromuscular disorders affecting respiratory function Neuropathic disease – MND, poliomyelitis, Spinal muscular atrophy Peripheral neuropathies – GBS, Critical illness polyneuropathy, Charcot –Marie-Tooth disease Disorders of neuromuscular junction Myasthenia Gravis, Botulism Acquired myopathies – Critical illness myopathy Inherited myopathy Progressive muscular dystrophies (Duchenne, Becker, Myotonic) Congenital muscular dystrophies (Ulrich congenital, Emery –Dreifuss) Metabolic myopathies (mitochondrial)

Respiratory muscle strength 1 minute in pairs. Nerve NMJ Muscle

Respiratory muscle strength UMN: MND, MS LMN: SCI, GBS, MND, Polio Nerve Myaesthenia, Botulism, drugs NMJ Congenital – muscle dystrophies, metabolic disorders Acquired – endocrine, inflammatory Muscle

Increased mechanical workload 1 minute in pairs Upper airway Chest wall Lung compliance

Increased mechanical workload Larynx Upper airway Pharynx Scoliosis Chest wall Obesity Atelectasis Fibrosis Lung compliance Inflammation Infection

Don’t forget the drive (1 minute in pairs) Structural Lesions Depressants

Factors affecting drive: Structural Lesions Trauma Vascular Depressants Drugs Sleep

Pathophysiology Respiratory Failure Oxygenation Failure Ventilation Failure

Remember in MND Muscle weakness Four factors may contribute to respiratory failure: Muscle weakness Increased load Airway (can also cause obstructive episodes with arousal) Obesity Lost compliance Drugs, patients are often on opiates MND is a disease of middle and later years, there may be other respiratory pathology

Physiological effects of hypoventilation Early effects Falling VC, Orthopnoea Sleep disordered breathing, sleep fragmentation Poor cough Later Effects *Hypercapnia PCO2>7.5 KPa Hypoxaemia *Tachypnoea Death *Poor prognostic signs if no treatment with NIV

Muscle tone at night If the diaphragm is damaged or weak or if there is increased load on the diaphragm then the patient will hypoventilate as no accessory muscles to support the diaphragm.

Sleep disordered breathing Umbrella term for several chronic conditions “partial or complete cessation of breathing occurs many times throughout the night, resulting in daytime sleepiness or fatigue that interferes with a person’s ability to function and reduces quality of life.” (BTS) Includes: Obstructive Sleep Apnoea Central Sleep Apnoea Nocturnal Hypoventilation

Symptomatic effect of hypoventilation Common symptoms fragmented sleep orthopnoea daytime somnolence groggy hangover nightmares

Symptomatic effect of hypoventilation Less common symptoms Ls common sympommpto morning headache poor concentration confusion poor appetite breathlessness

So what about the cough? The function of cough is to clear inhaled foreign materials captured in the mucociliary system, retained secretions or excessive secretions associated with respiratory infection or impaired mucociliary clearance, and aspirated materials.

So what about the cough? Three (four) phases to a cough: Phase 1: stimulation of cough receptors located primarily in the central airways or a voluntary initiation (trigger phase) Phase 2 (1) The inspiratory phase Phase 3 (2) The compression phase Phase 4 (3) The expiratory phase

The inspiratory phase A period of rapid inhalation, caused by contraction of the diaphragm and accessory inspiratory muscles. Normal pre-cough inspiration : 85-90% of total lung capacity

The compression phase The vocal cords close (0.2 secs) Expiratory muscles of the rib cage and abdomen contract in a coordinated effort, compressing the alveolar gas volume.

The expiratory phase The vocal cord open abruptly. The muscle of expiration continue to contract and air is exploded out of the airway. Initial PCEF can be as high as 720LPM in normal individuals.

Physiological effects of weak cough Weakened inspiratory muscles Reduced vital capacity Weakened bulbar muscles Weakened expiratory muscles Reduced explosive phase limited secretion clearance Increased risk of pulmonary congestion, acute respiratory failure and death

Physiological effects of weak cough Bulbar insufficiency, although important does not always cause a completely ineffective cough Remember that trache patient who shot phlegm across the room! Bulbar patients can be taught a huffing manoeuvre (forced exhalation through an open glottis) Insidious weak cough, particularly due to weak expiratory muscles often goes unnoticed until it is too late. !

Symptomatic effect of weak cough As well as increased significant complications and increased use of health care resources, weak cough causes: difficulty clearing phlegm choking, plugging and resultant anxiety distress decreased quality of life.

Summary Ventilatory Failure Cough Failure Weak inspiratory respiratory muscles Increased demand or load Upper airway issues Drugs Often in combination in NMD cause Hypoventilation Cough Failure Weak inspiratory respiratory muscles Weak expiratory muscles Bulbar weakness Often in combination cause weak cough