Chest wall and diaphragm

What it normally does

• Breathing in is an active pump. The diaphragm contracts, flattens and drops; the ribs swing up and out. The chest gets bigger, pressure inside it falls below atmospheric, and air is pulled in (negative pressure ventilation). Breathing out at rest is passive elastic recoil and costs essentially no muscle work.

  Anything that breaks the mechanical continuity of the wall, or the muscle that drives it, causes ventilatory failure with normal lung tissue — flail chest and neuromuscular failure.

• The lung and the chest wall are coupled by a thin film of pleural fluid. Over the normal breathing range the lung recoils inwards while the wall springs outwards (the wall only recoils inwards at high lung volumes), and that tug-of-war leaves the pleural pressure negative. Where the two recoils balance at rest is the functional residual capacity.

  A stiff, heavy or splinted chest wall pulls that resting point down and lung volumes shrink — the restrictive defect.

• The motor supply of the diaphragm is the phrenic nerve, from C3, C4 and C5 — nerve roots that come down from the neck, not from the chest. The phrenic also carries sensation from the central part of the diaphragm.

  Explains why a high cervical injury stops breathing altogether, why surgery or a mediastinal tumour can paralyse one hemidiaphragm, and why blood under the diaphragm is felt at the shoulder tip.

• Lung volumes and the work of breathing are set by how easily the whole system stretches (compliance). Stiffen either the lung or the wall and every breath costs more muscle work, so the muscles fatigue.

  Explains the spirometry pattern in chest wall disease, and why these patients retain CO2 at night before they do by day.

What goes wrong

• Flail chest← from “Breathing in is an active pump. The diaphragm …”

  Blunt trauma fractures adjacent ribs, each in two or more places — commonly defined as two or more contiguous ribs, though many texts require three or more. That segment is now an island, disconnected from the rest of the wall, so it no longer follows the muscles — it follows pressure. When the chest expands and pleural pressure drops, the segment is sucked inwards; it bulges out on expiration (paradoxical movement). But the paradox is not what harms the patient. The force that broke the ribs also bruised the lung beneath (pulmonary contusion), and the pain stops the patient breathing deeply or coughing, so the lower lobes collapse.

  Flail chest kills through pulmonary contusion and splinting, not through the paradox. Treat the pain properly, give oxygen, be careful with fluids, and be ready to support ventilation. In Australia the injury is mostly road trauma and falls, and injury hospitalisation rates are substantially higher for Aboriginal and Torres Strait Islander people.

  You would find: High-energy trauma, severe localised chest pain, a segment of chest wall that visibly moves in as the rest moves out, and hypoxia that worsens over the first 24 to 48 hours as the contusion evolves.

• Diaphragmatic paralysis (phrenic nerve injury)← from “The motor supply of the diaphragm is the phren…”

  Cut, stretch, cool or compress the phrenic nerve — cardiac surgery, a mediastinal tumour, neck trauma, a cervical cord lesion — and that hemidiaphragm loses its motor supply. It goes flaccid. When the rest of the chest expands and pleural pressure falls, the paralysed dome is dragged upwards instead of pushing down, so it wastes part of the breath.

  Count the level. Above C3, the diaphragm is effectively denervated and the patient cannot breathe unaided. C3 to C5, partial and variable. Below C5, the diaphragm works but the intercostals and abdominals are lost, so the patient breathes but cannot cough — and then drowns in their own secretions.

  You would find: Breathlessness lying flat or in the water (the abdominal contents push the flaccid dome up), a raised hemidiaphragm on chest x-ray, and paradoxical upward movement of that dome when the patient sniffs, seen on ultrasound or fluoroscopy. Vital capacity falls further on lying down (roughly 15-25 percent with unilateral paralysis, and by half or more when both sides are affected).

• Restrictive defect from the chest wall (obesity hypoventilation, kyphoscoliosis, ankylosing spondylitis, circumferential burn eschar)← from “The lung and the chest wall are coupled by a t…”

  The wall is stiffened or loaded, so it will not expand for the usual muscle effort. Compliance falls, every breath costs more work, and the resting volume of the lung is squeezed down. Patients compensate with small fast breaths, which minimises elastic work but wastes a larger fraction of each breath on dead space. At night, when muscle tone and respiratory drive fall, ventilation drops below what is needed and CO2 rises.

  Gas transfer corrected for alveolar volume (KCO) separates wall from lung: preserved or high when the chest wall is the problem, low in fibrosis. Obesity hypoventilation is an increasingly common cause in Australia, and Aboriginal and Torres Strait Islander adults carry a higher burden of obesity and chronic lung disease and present younger.

  You would find: Spirometry shows FVC and FEV1 both reduced with the FEV1/FVC ratio preserved or increased (judged against the lower limit of normal for the patient, not a single fixed cut-off), and total lung capacity reduced on formal lung volumes. Morning headache, daytime sleepiness and a raised bicarbonate on the blood gas — the kidney's evidence that CO2 has been high overnight for weeks.

• Neuromuscular ventilatory failure (Guillain-Barre syndrome, myasthenia gravis, motor neurone disease)← from “Breathing in is an active pump. The diaphragm …”

  The pump muscles weaken while the lungs stay clear. The patient keeps minute ventilation up by breathing faster and shallower until the muscles fatigue. Then tidal volume collapses, CO2 climbs quickly, and the chest x-ray still looks normal.

  Do not wait for hypoxia or a rising CO2 — by then you are intubating an exhausted patient. Serial vital capacity is the measurement that decides; a falling trend matters more than any single number.

  You would find: A falling forced vital capacity measured at the bedside, orthopnoea, a weak cough, and speech broken into short bursts because the patient runs out of air mid-sentence. Oxygen saturation usually stays normal until very late.

• Referred pain from an irritated diaphragm (Kehr sign)← from “The motor supply of the diaphragm is the phren…”

  The phrenic nerve carries sensation from the central diaphragm back to C3, C4 and C5. The brain has no cortical map for the diaphragm, so it reads those segments as the skin over the shoulder tip. Blood, pus or gas sitting under the dome therefore hurts in the shoulder. The peripheral rim of the diaphragm is supplied by the lower intercostal nerves instead, so irritation there is felt as local chest wall or upper abdominal pain.

  Shoulder tip pain in a trauma patient is intra-abdominal bleeding until proven otherwise. Lying the patient flat or head-down spreads the blood under the dome and classically makes the pain worse.

  You would find: Left shoulder tip pain after blunt abdominal trauma with a deceptively soft abdomen — a ruptured spleen bleeding under the left dome. The same pain appears after laparoscopy from residual carbon dioxide, and with a subphrenic abscess.

What we give, and how it works

Open a drug to see what it binds, what that does to the cell, and what you then see in the patient.