The Larynx

The larynx is a functional sphincter at the beginning of the respiratory tree. It protects against foreign bodies and is used for phonation. It is lined with ciliated columnar epithelium.

larynx

Innervation

larynxvagus

Sensation
Internal brand of superior laryngeal nerve: Above the cords.
Recurrent laryngeal nerve: Below the cords.

Power
Recurrent laryngeal nerve: All muscles except the cricothyroid muscle which is innervated by the external brand of the superior laryngeal nerve.

Of note is the different sensory innervations affected during tracheal intubation and the haemodynamic effects these can have. The valeculla has sensory supply from the glossopharyngeal nerve, whereas beneath the epiglottis has sensory innervation from the vagus nerve. Using a standard Macintosh blade seated in the valeculla produces a sympathetic response both due to nociception and due to the glossopharyngeal nerve’s connection to the nucleus tractus solitarius and subsequent effects on heart rate and blood pressure (see Control of circulation). However when a Miller blade is used this stimulates the sensory afferents of the vagus nerve which can in turn produce vagal stimulation and bradycardia. This is particularly evident in children who do not have a high sympathetic resting tone.

Blood supply

Arterial supply from the laryngeal branches of superior and inferior thyroid arteries. Venous drainage from laryngeal brances of superior and inferior thyroid veins.

Laryngeal muscles

laryngealmuscles

There are three extrinsic and six intrinsic muscles.

Extrinsic

  1. Sternothyroid – Arises from manubrium, inserts into thyroid cartilage lamina. Functions as a depressor of the larynx.
  2. Thyrohyoid – Connects thyroid lamina to greater horn of hyoid. Functions as an elevator of the larynx.
  3. Inferior constrictor – Constricts laryngeal inlet. Propofol relaxes these muscles very effectively and so aids placement of a laryngeal mask airway.

Intrinsic

These are all paired muscles, except transverse arytenoid which is a midline structure.

  1. Cricothyroid – Anterior horn of cricoid to inferior horn of thyroid cartilage. Contraction tilts cricoid upwards, moving arytenoids posteriorly and therefore tensing the vocal cords.
  2. Posterior cricoarytenoid – Posterior cricoid to muscular surface of arytenoid. Contraction externally rotates arytenoids causing abduction of the cords.
  3. Lateral cricoarytenoid – Outer cricoid to muscular surface of arytenoid. Contraction adducts vocal cords.
  4. Transverse arytenoid – Posterior surface of both arytenoids. Contraction narrows distance between the two arytenoids, constricting glottis.
  5. Aryepiglottic – Causes a minor constriction of laryngeal inlet.
  6. Thyroarytenoid – Thyroid lamina to anterior arytenoid. Contraction pulls arytenoid anteriorly relaxing the cords.

Summary

Abductors Adductors Tenses cords Relaxes cords
Posterior cricoarytenoids Lateral cricoarytenoids Cricothyroids Thyroarytenoids
Transverse arytenoid
Thyroarytenoids

Recurrent laryngeal nerve injury

This is a problem because all intrinsic muscles except the cricothyroid muscles are supplied by these nerves. Therefore the only muscle with any tone after a RLN injury is a muscle that moves the arytenoids posteriorly and tenses the cords. A bilateral RLN injury can therefore cause upper airway obstruction.

Transverse section through the neck at C6

Landmarks in relation to the cervical vertebrae

  • At C1, base of the nose and the hard palate
  • At C2, the teeth of a closed mouth
  • At C3, the mandible and hyoid bone
  • At C4, the common carotid artery bifurcates
  • From C4-5, the thyroid cartilage
  • From C6-7, the cricoid cartilage

Transverse section through C6

transversesectionC6

The thyroid covers the 2nd to 4th tracheal rings. When performing a surgical tracheostomy the isthmus of the thyroid is general displaced downwards by blunt dissection, through if this is not possible it may need to be divided. When performing a percutaneous dilatational tracheostomy the space between the 2nd and 3rd tracheal ring is usually chosen. Tracheostomy higher than this increases the risk of tracheal stenosis. The use of ultrasound to select puncture site to avoid aberrant midline vessels can make the procedure safer, but the use of bronchoscopic guidance is essential for a percutaneous approach.

Coronary anatomy

Sinoatrial node: At the junction of the SVC and right atrium on the posterolateral surface.

Atrioventricular node: Lies in atrial septum above coronary sinus.

Left coronary artery arises from the posterior aortic sinus, and the right coronary artery arises from the anterior aortic sinus. The sinuses of valsalva (also known as the aortic sinuses) are shaped to encourage equal bilateral flow.

Coronary arteries

  • Right Coronary Artery (RCA):  Supplies the  RA, RV, and interatrial septum.  It usually supplies both the SA and AV nodes.
  • Posterior Descending Artery (PD):  Supplies the inferior portion of the LV and the posterior septum.  The PD arises from the RCA in 70% of cases and the CFX in the remaining 20%.
  • Left Main Coronary Artery (LCA):  Gives rise to the LAD and CFX.
  • Left Anterior Descending artery (LAD):   Supplies the LV, RV, and interventricular septum.  Arises from LCA.  May also be called the anterior interventricular artery.
  • Circumflex artery (CFX):  Supplies the LA and LV.  Arises from the LCA and anastamoses with the RCA.

Schematic of coronary arteries

coronaryarteries

The right coronary artery supplies to SA node in 60% of people, and it supplies the AV node in 90% of people.

Dominance refers to which side supplies the posterior interventricular artery (also called the posterior descending artery). 70% of people are right side dominant, 20% co-dominant and 10% left side dominant.

coronaryvesselsschematic

Coronary perfusion pressure

CPP = aortic pressure – intraventricular pressure

Left ventricle:

Systole: [SBP-LVESP] = 120-120 = 0mmHg

Diastole: [DBP-LVEDP] = 70-10 = 60mmHg

In the right ventricle flow occurs throughout the cardiac cycle.

Drug distribution in pregnancy

Volume of distribution of drugs is increased by 5 litres at term. This affects predominantly water soluble (polar) molecules.

A fall in albumin concentration affects the binding of acidic drugs. Basic drugs predominantly bind to alpha-1-glycoprotein which falls to lesser extent and are therefore less affected. Most anaesthetic drugs are basic so are not affected, however fentanyl can bind to albumin so may have an exaggerated effect at term.

Plasma cholinesterase levels fall by 25% during pregnancy, though an increased volume of distribution balances this so the duration of action of drugs like suxamethonium is not really affected in vivo.

Drug distribution to the foetus

The placental membrane seperates foetal and maternal blood. This is phospholipid in nature, fused to form a single membrane. It is much less selective than the blood brain barrier, even molecules with only moderate lipid solubility can cross relatively easily.

Placental blood flow and free drug concentration in foetus affect the rate of transfer as governed by the Fick principle:

The rate of transfer of a gas through a sheet of tissue is proportional to the tissue area and the difference in gas partial pressures between the two sides, and inversely proportional to tissue thickness.

Vgas  alpha  frac{A}{T}  x  D  x  (P1-P2)

Where D is the diffusion constant (Graham’s law), stating that diffusion is proportional to solubility but inversely proportional to the square root of the molecular weight (or density).

D  alpha  frac{Solubility}{sqrt{MW}}

Ion trapping

The foetus has a lower pH than the mother due to increased pCO2 and immature kidneys not able to excrete organic acids as well as mature kidneys. These acids instead diffuse out via the placenta which is slower.

Basic drugs: [BH+] ⇐⇒[B] + [H+]

Foetal pH is lower, therefore more negative than the drugs pKa. Therefore more drug is ionised in the foetus and unable to diffuse back across the placenta.

e.g. Local anaesthetic toxicity, pethidine (metabolised to norpethidine by foetus which can be trapped).

Drugs at the time of birth

A newborn will commonly have anaesthetic drugs in it’s circulation.

Thiopentone crosses the placenta rapidly, peak umbilical artery levels occur within three minutes of maternal injection.

Non-depolarising muscle relaxants are large polar molecules that do not cross the placenta. However if the mother has suxamethonium apnoea, maternal levels will remain high and some transfer will occur – this is particularly a problem if the foetus has inherited the same enzyme deficiency.

Propofol is not licensed for use in late pregnancy, which is historically why thiopentone is used.

Foetal circulation

The foetal circulation is unique in the sense that less than 10% of cardiac output passes through the lungs. This is suited to life in utero because the foetus does not breathe, instead gets all it’s gas exchange via the placenta, however after birth this situation has to rapidly change to ensure survivial outside the womb.

Schematic

foetal circulation

Blood leaves the placenta via the umbilical vein with an oxygen saturation of around 80%. The ductus venosus shunts half of the blood across the liver directly into the inferior vena cava. The mixed venous blood oxygen saturation is around 65%. Two thirds of blood is shunted directly into the left atrium via the foramen ovale.

There is intense hypoxic pulmonary vasoconstriction, therefore the majority of the blood in the pulmonary artery flows through the ductus arteriosus into the aorta. Less than 10% of the cardiac output passes through the pulmonary circulation.

The umbilical arteries arise from the internal iliac arteries and pass to the placenta.

Birth

With the first breath a negative intrathoracic pressure of around -50cmH20 is generated, expanding the FRC and encouraging blood flow through the lungs. Ventilation of the alveoli reduces hypoxic pulmonary vasoconstriction and therefore reduces pulmonary vascular resistance. This reduces the amount of blood flowing across the ductus arteriosus and when the umbilical cord is clamped this raises the systemic vascular resistance and can reverse the flow of blood through the ductus.

Exposure to oxygenated blood and a drop in prostaglandin E2 causes closure of the ductus arteriosus in less than 24 hours.

Oxygen saturation at different points

Umbilical vein: 80%
IVC pre-ductus venosus: 25%
Mixed IVC: 65%
Aorta pre-ductus arteriosus: 60%
Descending aorta: 50%

Foetal haemoglobin

Contain two alpha and two gamma chains, the gamma chains do not bind to 2,3-diphosphoglycerate shifting the oxyhaemoglobin dissociation curve to the left. The P50 of foetal haemoglobin is 2.5kPa, compared to adult haemoglobin which is 3.5kPa. The Hb concentration is around 160g/l at birth.

A baby starts to synthesis haemoglobin A (adult haemoglobin: two alpha, two beta chains) a few weeks before birth and by the age of 2 years haemoglobin F is no longer present.