The respiratory muscles

The respiratory muscles consist of two main groups: the primaries (consisting mostly of the diaphragm and intercostals) and the accessories (comprising mostly of the scaleni, abdominals and sternocleidomastoids). It is also important to remember the effects upon the oxygenation status and the balance of the blood pH with increasing metabolic demands should the recruitment of other additional muscles, for example the facial muscles with pursed-lip breathing, and the shoulder girdle and arm fixators, be required during times of respiratory distress/dysfunction. The primaries’ main role is that of ventilation. The accessories have other functions, but are recruited to facilitate ventilation when required. Normally, the respiratory muscles have both ventilatory and non-ventilatory motor functions, for example the diaphragm acts as the primary respiratory muscle, responsible for generating approximately 60–70% of the tidal volume while also being responsible for raising intra-abdominal pressure for postural stabilisation of the torso, parturition and micturition. Such considerations must be appreciated by the therapist: when the respiratory muscles are required for both motor and ventilatory functions, their ability to assist ventilation is reduced. This is of particular importance when ventilatory support has recently been reduced and motor activity is being encouraged during daytime hours, i.e. during the weaning and rehabilitative phases of recovery. Upper limb strengthening exercises may be a primary aim at this stage so increasing ventilatory support overnight may be appropriate (Table 7.1).

The respiratory muscles share the common features of other skeletal muscles and consist of a mixture of fibre types (Johnson et al. 1973). The proportions of fibre types and the metabolic constituents (e.g. capillaries; glycolytic and oxidative capacities; and time of recruitment in contraction) determine a muscle’s strength and endurance properties (Schauf et al. 1990). Type I fibres are important for endurance (slow twitch, high oxidative capacity are recruited first and are most resistant to fatigue). Type IIa fibres have a higher oxidative capacity, fast twitch and produce an intermediate level of force, and so are relatively resistant to fatigue, while Type IIb fibres have a low oxidative capacity, fast twitch, produce the greatest force on activation, are the last to be recruited for motor efforts and are easily fatigued when used repeatedly. Greater knowledge of muscle physiology is required if the aim is to train the respiratory muscles, rather than rest them (via ventilatory support). Muscle training may be an appropriate physiotherapy intervention to facilitate the weaning episode of the prolonged ventilatory supported individual, where muscle wasting and disuse atrophy are evident, though more research is required in this area.

Respiratory mechanics and airflow

Contraction of the respiratory muscles affects the overall motion of the chest wall, for example in the upright position, on inspiration, the diaphragm moves downwards on contraction while the abdomen moves out. Synchronicity is achieved when the rib cage and the abdomen move together, increasing in diameter during inspiration and decreasing in diameter during expiration. In the supine position, most movements are abdominal with little movement of the rib cage. Body position in both respiratory mechanics and ventilation to perfusion matching is of great importance, as respiratory muscle dysfunction alone, for example fatigue or weakness, can lead to dyspnoea, hypoventilation, hypercapnoea, reduced oxygenation of body tissues, respiratory failure, metabolic acidosis and, ultimately, death.

Normal resting ventilation is a tri-cyclical activity consisting of the inward flow of air (inspiration), the outward flow (exhalation) and the rest phase, which constitutes the zero-flow status. During the inspiratory and expiratory cycles, a volume of air moves in and out of the lungs. These changes occur as a result of pressure gradients between the airway opening (or mouth) and the alveoli. Prior to the beginning of inspiration, the pressures at the airway opening and in the alveoli are equivalent. As there is no pressure gradient, there is no air movement – this is known as the resting period of the respiratory cycle, where air neither enters nor leaves the lungs (see Figure 7.1).

As inspiration begins the respiratory muscles contract, causing an upward and outward movement of the chest wall (the bucket and pump handle effects) and the diaphragm descends (see Figure 7.2). This is known as the active phase of the breathing cycle and demands effort (termed the work of breathing). The changes in thoracic dimensions create a drop in the alveolar pressure; the pressure gradient between the airway opening and the alveoli results in an inward movement of air. The volume change that occurs is called the tidal volume and is, on average, 500 mL. (Joint guidelines by the British Thoracic Society and Association of Respiratory Technicians and Physiologists are available for a detailed underpinning of spirometric values and tests. Also downloadable from the internet are the American and European thoracic society recommendations for spirometry).

The opposing forces to ventilation

The work of breathing derives from the two resistive forces of the lungs and chest wall, i.e. the elastic (see Figure 7.3) and frictional forces. Forces within the respiratory system that oppose inflation of the lung and therefore ventilation can be grouped into two categories:

The elastic forces are encountered as a result of both the lungs and chest wall being ‘elastic’ structures, i.e. they resist changes in shape. When they have been inflated or deflated, they tend to return to the same resting/starting position of equilibrium once the driving force has been removed. The lungs naturally want to collapse and the chest wall naturally to expand. Thus, each exerts a pull on the other. In the absence of other forces (e.g. muscles) a position is reached in which the opposing forces are balanced.

Owing to these opposing forces (of lung and chest wall), the intrathoracic pressure is negative (sub-atmospheric). To inflate the lungs an extra force must be applied (by the muscles) and intrathoracic pressure falls lower.

Expiration is mostly passive as a result of the elastic forces returning the lungs and chest wall to a balanced position (Figure 7.4). It can, however, be active, for example forced breathing and coughing, where the expiratory muscles assist the elastic forces (resulting in a more rapid expiratory rate of flow and faster lung deflation).

The inspiratory muscles perform mechanical work through upsetting the balance of the elastic forces. Hence, the harder and faster the respiratory effort is, the more ‘elastic’ work is required. A certain amount of pressure is required to stretch the lungs to a certain volume. The normal value for elastance is around 10 cmH₂O/L. However, in disease states, the lungs become stiffer and the same pressure change may result in a smaller volume change, i.e. the elastance of the lung is higher. Pneumonia, acute respiratory distress syndrome (ARDS) and pulmonary oedema are common lung conditions affecting elastance. Others include fibrotic lung disease, pleural effusion, kyphoscoliosis and obesity.

Compliance

Compliance (demonstrated by the pressure volume curve) is a measure of the distensibility of the lungs or ease with which the lungs inflate (i.e. the reciprocal of elastance). It is measured as the change of volume in the lungs in response to a change in pressure. Normal lung and chest wall compliance is ~1L per kPa (100 mL per cmH₂O). It can be seen in Figure 7.5 that lung compliance is lowered at high and low lung volumes. Hence, it is favourable for the patient to sit on the steep slope portion of the curve (it is easier to inflate a partially opened lung than a collapsed lung).

The frictional forces

The second group of opposing forces encountered in ventilation is the frictional forces. Impedance to air movement through the airways is called airways resistance. A small, but measurable, amount of work must be done to maintain the flow. Frictional forces are therefore dependent upon the speed with which air moves through the airway. Resistance is defined as the ratio of the pressure change responsible for air movement and the rate of flow. A normal value for resistance is around 2.5 cmH₂O/L/sec. Factors affecting airways resistance include the size of the airway, its shape and calibre (Figure 7.5). Different diseases affect such properties, thus altering airway resistance, for example chronic obstructive pulmonary disease (COPD) is the most frequently encountered lung disease that increases airways resistance.

With the presence of lung disease, the opposing forces of ventilation are increased. To sufficiently ventilate the lungs, larger patient efforts may be necessary to generate the pressure change needed to overcome the increased elastance or resistance (see Figure 7.6). Sustaining large inspiratory efforts may lead to excessive workloads and eventually respiratory fatigue and failure.

When the work of breathing is excessive because of an increase in the elastic or resistive forces present, respiratory muscle weakness from fatigue may develop. The strength of the muscles will be inadequate to support normal levels of ventilation and muscle pump effectiveness is diminished resulting in inadequate ventilation and an increase in the arterial carbon dioxide level (PaCO₂) causing respiratory acidosis. Hypoventilation or respiratory muscle fatigue may also lead to severe hypoxia. Under these conditions, ventilatory support to unload the respiratory muscles and improve ventilation is indicated.

Body positioning, pharmacological management and oxygen therapy are of prime importance to reduce the opposing forces of respiration and maximise ventilation prior to, and during, the instigation of mechanical support.

Acute hypoxaemic (type I) respiratory failure

This is caused by intrinsic lung disease that interferes with oxygen transfer in the lung. Hypoxaemia results from increased right-to-left shunts (e.g. pneumonia, alveolar collapse, oedema and consolidation) or, more significantly, ventilation perfusion (V/Q) mismatch, for example pulmonary parenchymal disease, or a combination of the two. Where functional residual capacity (FRC) is reduced, airway closure is present throughout the respiratory cycle as tidal exchange occurs below closing volume. This results in an increased number of under-ventilated lung units (placing the patient lower down on the compliance curve).

The increased dead space (air in the airways which does not directly contribute to gas exchange) results initially in an increase in total ventilation to maintain a normal PaCO₂. This is because in areas of V/Q inequality, the raised arterial PCO₂ resulting from decreased CO₂ excretion in the under-ventilated alveoli stimulates the respiratory centre. Relative hyperventilation may ensue in response to severe hypoxaemia with a resultant drop in PaCO₂ to below normal range. (The degree of hypoxaemia is restricted by constriction of the blood vessels supplying under-ventilated alveoli, i.e. hypoxic pulmonary vasoconstriction.)

The mechanical disadvantage of a reduced FRC results from the consequential reduction of lung compliance and increased resistance, resulting in a greater work of breathing with a higher metabolic cost. The resultant clinical picture is that of rapid shallow breathing, which, in turn, further increases both oxygen consumption and carbon dioxide volume for excretion (Table 7.2).

Ventilatory (type II) respiratory failure

This is caused by failure of the respiratory pump (consisting of the respiratory muscles, chest wall, higher centres and nerves) where the amount of CO₂ excreted is less than that produced by metabolism. With respiratory pump failure, even with normal lung pathology, the arterial PaCO₂ is raised, with an inevitable fall in alveolar oxygen tension and hypoxaemia (alveolar hypoventilation). Hypoventilation results from a reduced respiratory effort, an inability to overcome resistive ventilatory forces or an inability to compensate for extra deadspace and/or CO₂ production. The respiratory muscle pump is as vital as the heart, failing in the same way and for the same reasons, but a large respiratory reserve is present and function may be markedly impaired without ventilatory failure accompaniment (Table 7.3).

It is worth noting at this point that a mixed picture of the two types of respiratory failure is frequently seen. While acute hypoxaemic failure may initially be present, some cases result in exhaustion. The patient is unable to compensate for the increased dead space, resulting in a raised PaCO₂ tension and mixed respiratory failure. Where individuals are unable to cough effectively or sigh, the risk of alveolar collapse, secretion retention and secondary infection is further increased. Others may be at risk of aspiration, for example unconscious or bulbar palsy, causing further damage to the lungs and further worsening ventilatory function.

Pathways to respiratory failure

The causes of respiratory failure are too numerous to detail but can be divided into two main categories:

• a reduction in neuromuscular competence resulting in the inability to generate adequate respiratory muscle force, for example reduced central drive, spinal injury, trauma, muscle disease (Figure 7.7);

• an increase in respiratory workload caused by raised ventilatory impedance, for example obstructive and restrictive lung defects.

Respiratory muscle dysfunction occurs as a result of various factors. This can be demonstrated by considering COPD, where respiratory muscle function is profoundly affected as a result of the increased work of breathing and the increased ventilatory load. The increased work of breathing arises from the pathological changes resulting in raised airway resistance and hyperinflation. At rest, the minute volume of these patients is higher than that of healthy subjects. As a result, the actual cost of breathing in terms of oxygen consumption is markedly increased and the accessory muscles of respiration are recruited.

Hyperinflation also has adverse effects upon the respiratory muscles. The increased lung volumes in COPD are thought to be compensated for by the lowering of the diaphragm or expansion of the rib cage. These changes result in the muscles functioning at a disadvantaged position on the length-tension curve, i.e. the diaphragm is at its optimal length for providing the maximum contractile force when it is in its resting domed position.

As lung volume increases, the inspiratory muscles shorten and their ability to generate a negative force on inspiration is reduced. Hence, the inspiratory effort required to obtain the same tidal volume is greater. A completely flattened diaphragm is incapable of generating any useful pressure and on contraction causes in-drawing of the lower rib cage (Hoover’s sign), effectively functioning as an expiratory muscle (see intrinsic positive end-expiratory pressure (PEEPi) below).

Respiratory muscle fatigue is an important precursor to respiratory failure. Factors affecting the endurance of the respiratory muscles include energy stores/nutrition, blood substrate concentration, arterial oxygen content, efficiency, mean inspiratory flow, minute ventilation, inspiratory duty cycle and maximum inspiratory pressure.

When fatigue of the respiratory muscles occurs, rest, not exercise, is indicated. However, a delicate balance between the two must be achieved as there is some evidence that if total rest is applied (through the application of full mechanical ventilation) disuse atrophy may occur causing weaning problems.

Increases in both elastic and resistive loads, as present with lung disease, will therefore increase the work of breathing and can lead to respiratory muscle fatigue and weakness, as described previously. When fatigue ensues, hypoventilation will result and weakened muscles will be unable to overcome the opposing forces of respiration to maintain adequate ventilation. Unless ventilatory support is provided to ‘unload’ the system, respiratory failure results.

The hypoxic drive concept

One of the causes of CO₂ retention in respiratory failure is the use of inappropriate levels of supplemental oxygen. This relates to only a small, but important, group of patients in whom the main ventilatory drive is hypoxaemia. Some patients with COPD develop severe hypoxaemia with some CO₂ retention. Owing to the degenerative pathological changes within the lungs, this is maintained over long periods of time and, while not referred to as respiratory failure, an increased work of breathing is usually encountered. Arterial pH is usually at the lower end of the normal range as renal compensation for the raised arterial CO₂ occurs with time and bicarbonate is retained (compensated respiratory acidosis). The cerebrospinal fluid also has a normal pH because of the raised bicarbonate levels and the main ventilatory drive now arises from the hypoxaemia (despite the raised arterial PCO₂).

The subsequent administration of high inspired oxygen fractions in such cases may pose a potentially lethal clinical scenario as the hypoxic drive will be abolished while the detrimental effects of the underlying lung condition and increased work of breathing continue. The result is gross respiratory depression with the arterial PaCO₂ climbing and arterial blood pH falling to extreme levels if left unnoticed or is misinterpreted in the clinical setting.