Learn — Lab values & diagnostic tests

2 notes · 15 questions on the topic

Thoracentesis

Thoracentesis is the aspiration of fluid or air from the pleural space through a needle or catheter passed between the ribs, done either to diagnose the cause of an effusion or to relieve the breathlessness it causes. The nurse's part matters because the position she achieves decides how safely the needle can be placed, and because the two complications that follow, a pneumothorax and re-expansion pulmonary oedema, are both detected by the nursing observations afterwards.

  • Before the procedure: check that consent has been taken, that the coagulation profile and platelet count are acceptable, that a recent chest X-ray or bedside ultrasound has marked the site and the side, and record baseline vital signs and oxygen saturation.
  • The position of choice is sitting upright leaning forward over a bedside table with the arms and head resting on a pillow, which widens the intercostal spaces and lets the fluid settle at the base; if the patient cannot sit up he is positioned lying on the unaffected side with the head of the bed raised 30 to 45 degrees.
  • The needle is inserted at the UPPER border of the rib, because the intercostal nerve, artery and vein run in the groove along the lower border of the rib above; the usual site is an interspace below the level of the fluid in the posterior axillary line, and not below the eighth or ninth rib, to avoid the diaphragm, liver and spleen.
  • The patient is told before the skin is cleaned that he must not cough, talk, take a deep breath or move while the needle is in, and that he should raise a hand and report any sharp chest pain, an urge to cough or breathlessness.
  • No more than about 1 to 1.5 litres is removed at one sitting and it is drained slowly, because a larger or faster drainage causes re-expansion pulmonary oedema and vasovagal hypotension; the procedure is stopped if the patient develops persistent coughing, chest tightness or breathlessness.
  • Fluid is sent for cell count and differential, protein and lactate dehydrogenase for Light's criteria, glucose, adenosine deaminase where tuberculosis is suspected, Gram stain, acid-fast staining, culture and cytology, and the volume, colour and character are recorded.
  • After the procedure a small sterile occlusive dressing is applied with pressure and the patient is turned onto the unpunctured side for about an hour so the puncture site seals; a chest X-ray is arranged to exclude a pneumothorax.
  • Observe closely for the first few hours: respiratory rate, oxygen saturation, breath sounds on the punctured side, pulse and blood pressure, the dressing for leakage of fluid, and the patient for cough, chest pain or breathlessness.
  • The complications to know are pneumothorax (the commonest), haemothorax, re-expansion pulmonary oedema, infection or empyema, puncture of the liver or spleen, and a vasovagal faint during the procedure.

Lumbar puncture — procedure and position

A lumbar puncture is the introduction of a needle between two lumbar vertebrae into the subarachnoid space to measure the cerebrospinal fluid pressure and to collect fluid for examination, or to give a drug or an anaesthetic. It matters because the level chosen keeps the needle below the end of the spinal cord, and because the one contraindication that can kill, raised intracranial pressure, has to be excluded by the assessment the nurse helps to make before the needle is ever opened.

  • The spinal cord ends at about the level of L1 to L2 in an adult and lower, at about L3, in a newborn, so the needle is placed at L3-L4 or L4-L5 in an adult and at L4-L5 or L5-S1 in an infant; the line joining the highest points of the iliac crests crosses the body of L4 and is the surface landmark used.
  • The needle passes through skin, subcutaneous tissue, the supraspinous and interspinous ligaments, the ligamentum flavum and the epidural space before piercing the dura and arachnoid.
  • Position the patient on his side at the edge of the bed with his back at the very edge and perpendicular to the mattress, the knees drawn up to the abdomen and the chin tucked to the chest, which flexes the spine and opens the interspinous spaces; the sitting position leaning forward over a pillow is the alternative, but an opening pressure can only be measured in the lateral position.
  • Contraindications are raised intracranial pressure with focal signs or papilloedema, an infection of the skin over the puncture site, a coagulopathy, a platelet count below about 50,000 per microlitre or full anticoagulation; a CT scan is done first when there are focal neurological signs, papilloedema, new seizures, immunosuppression or a markedly depressed consciousness.
  • Normal cerebrospinal fluid is clear and colourless like water, with an opening pressure of about 10 to 20 cm of water, protein 15 to 45 mg/dL, glucose 45 to 80 mg/dL which is about two-thirds of the blood glucose, and 0 to 5 lymphocytes per microlitre with no organisms.
  • The classical patterns are examined: bacterial meningitis gives turbid fluid with a high neutrophil count, high protein and low glucose; viral meningitis gives clear fluid with lymphocytes, a mildly raised protein and normal glucose; tuberculous meningitis gives fluid that forms a cobweb clot, with lymphocytes, a very high protein and low glucose; and subarachnoid haemorrhage gives fluid that is uniformly blood-stained in every tube and xanthochromic after centrifugation, unlike a traumatic tap, which clears from the first tube to the last.
  • Tubes are numbered in the order they are collected and sent for cell count, biochemistry and microbiology with a spare; the microbiology sample goes to the laboratory at once and is not refrigerated when meningococcus is suspected.
  • Before the procedure the nurse takes consent to the ward, asks the patient to empty his bladder and bowel, records baseline vital signs, pupil size and neurological observations, explains that he must stay absolutely still and must report any shooting pain down a leg, and gathers the sterile pack, local anaesthetic, manometer and labelled tubes.
  • Afterwards the patient lies flat for the period the unit specifies, commonly 4 to 6 hours, though the evidence that flat bed rest prevents headache is weak; he is encouraged to drink 2 to 3 litres, the puncture site is watched for leakage, and neurological observations and limb power are recorded hourly at first.
  • Complications are post-dural-puncture headache (the commonest, worse on sitting up and relieved by lying flat), backache, transient radicular pain, local bleeding or infection, and cerebral herniation if the intracranial pressure was raised.

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