When injecting lidocaine as a local anaesthetic, it’s really rather important that you don’t give too much, what with the whole making-the-nerves-stop-working aspects of its pharmacological profile.

When given locally it’s a terrific anaesthetic, however if too much is allowed into the general circulation, everything gets a bit tingly, the brain goes a tad bezerk and heart decides to stop cooperating. In essence, don’t give too much.

So how much to give?

Well on it’s own, Lidocaine doses shouldn’t exceed 3mg/kg however when given with adrenaline, to induce vasoconstriction and thus limit systemic spread, the dose can be increased to 7mg/kg.

How to remember this?


Li-do-caine = 3 syllables

Li-do-caine-ad-ren-a-line = 7 syllables


Induce or Inhibit?

Which common drugs inhibit CYP450 enzymes and which induce them?

Here’s a couple of stories to help remember them – it’s by no means a comprehensive list, but should highlight the common drugs to remember.


Inducers – Mr Rifampicin

Mr Rifampicin tries St John’s Wort for the first time, which causes him to have two seizures.

He then rolls over and gets caught in barbed wire, where he proceeds to stay, and passes the time by having a long drinking session.

A wandering grizzly bear then mauls him to death.


Enzyme inducers:

  • Rifampicin
  • St John’s Wort
  • Phenytoin, Carbamazepine
  • Barbiturates
  • Chronic alcohol use
  • Griseofulvin
  • (also smoking, but via CYP1A2)


Inhibitors – Mrs Isoniazid

Mrs Isoniazid rolls in the New Year with a whopper of a booze binge.

This sets off her gout and causes her to vomit twice.

This then makes her feel depressed, despite the two antidepressants that she’s taking, and gives her palpitations.

She attempts to remedy the situation by having unprotected sex, which gives her HIV and a nasty infection.

She’s prescribed four separate antibiotics and two antifungals to clear everything up.


Enzyme Inhibitors:

  • Isoniazid
  • Acute alcohol use
  • Allopurinol
  • Cimetidine, Omeprazole
  • Fluoxetine, Sertraline
  • Amiodarone
  • Ritonavir
  • Antibiotics
    • Erythromycin
    • Metronidazole
    • Ciprofloxacin
    • Quinupristin
  • Antifungals
    • Ketoconazole
    • Fluconazole



Just remember than in questions they’ll often ask something like, “which of the following drugs would increase a warfarin patient’s INR” – an increased INR implies the enzymes have been inhibited so you need to think about the inhibitors…


Don’t get fooled!

Why you should care about Magnesium

I never learned much about Magnesium in the first few years of med school, which was a mistake, because it’s awesome.

Here I’ve tried to outline why it’s important and when to think about it in a clinical setting.

1. There’s loads of it

Magnesium is the ninth most abundant element in the universe, and the eleventh (by mass if you’re being pedantic) in your body. It’s required by every single cell as it is crucial to the basic nucleic acid chemistry that makes life possible, over 300 enzymes need it as a co-factor, and even ATP likes to hang out as a chelate of magnesium ions.

2. It’s useful as medicine

Magnesium has been used for many medicinal uses. It is a common laxative, antacid and it can stabilise muscle spasm in eclampsia. It increases production of prostaglandins and reduces that of thromboxane and angiotensin II. It also, in low doses, makes mineral water taste tart apparently (that was just for interest).

3. You can get it from loads of places

Clearly the ninth most abundant element in the universe isn’t going to be difficult to find, and you can find buckets of it in foods such as:

  • Spices
  • Nuts
  • Cocoa
  • Vegetables (given it’s crucial to chlorophyll, green stuff’s your best bet)

4. It does a lot of stuff in the body

Magnesium’s pretty busy in the world of metabolism. Most of it (60%) just chills in the bones, and the rest is mostly intracellular (20% in skeletal muscle). About 1% sits in the extracellular space, so clearly measuring your blood levels of magnesium isn’t going to tell you much about how much there is in your body.

It interacts with three major ions and their conduction channels:

  • Sodium
  • Potassium
  • Calcium


Magnesium is required for the NaKATPase to function correctly in myocytes. Thus low magnesium means the sodium isn’t being pumped out and potassium isn’t being brought back in. This leads to a tachycardia and a predisposition to arrhythmias.


To begin with, magnesium inhibits potassium leaving the cell through its channels. This means that too little magnesium, and you’re going to start losing potassium from the kidneys, and develop hypokalaemia. So – if you have a patient who’s hypokalaemic, and they’re not responding to potassium supplementation, start thinking about magnesium! In addition, patients in DKA (diabetic ketoacidosis) should have their magnesium monitored, so that the hypokalaemia caused by insulin driving the potassium into the cells isn’t compounded by renal losses.


Magnesium suppresses release of calcium from the sarcoplasmic reticulum. This has several effects

  • reduced muscle contraction
  • reduced neuronal excitation through two mechanisms:
    • blockage of NMDA glutamate receptors
    • inhibition of acetylcholine release
  • reduced production of parathyroid hormone
    • interestingly mild reduction of magnesium stimulates PTH release, however
    • severe drops in magnesium reduce PTH release
  • hypocalcaemia
  • reduces sensitivity of skeletal muscle to parathyroid hormone 

Clinically this is important, as magnesium sulphate can be used as a bronchodilator in severe asthma.

5. Too little is bad 

As explained in point 4, you can have a normal blood magnesium while still being significantly deficient, and there isn’t really a reliable biomarker to measure. The best way to test whether someone is deficient is to load them with magnesium (a magnesium loading test… weirdly) and see how much of it they retain. If they hold onto more than 20%, they were definitely deficient in the first place. You’ve also just fixed the problem, so win win!

Symptoms of magnesium deficiency include:

  • weakness
  • muscle cramps
  • abnormal heart rhythm
  • tremors and athetosis (fidgeting)
  • extensor plantar reflex
  • confusion
  • hallucinations
  • depression
  • seizures
  • tetany

It’s a scary list but it makes sense – the muscle cramps largely due to the increased calcium that results from low magnesium, and the potassium leakage from cells that occurs.

Reasons for low magnesium in the blood are put into three categories:

  1. not enough uptake
  2. redistribution into the cells
  3. too much loss

Not enough uptake

Magnesium is absorbed in the duodenum and secreted by the colon, so malabsorption or excessive throughput will prevent it being absorbed into the blood. Conditions such as Crohn’s or Ulcerative Colitis will cause hypomagnesaemia by causing diarrhoea.

Redistribution into the cells

Adrenaline and other catecholamines tend to shove magnesium ions into the cells. Thus after a heart attack, 80% of patients will have low serum magnesium. Drugs that cause intracellular shift of calcium will drag magnesium with it, so things like Digitalis are also a risk factor.

Too much loss

A huge cause of hypomagnesaemia is alcohol. 30% of alcoholics have low magnesium, as do 85% of those with delerium tremens. In terms of medications, anything that inhibits reabsorption in the Loop of Henle is going to result in shedding of magnesium into the urine. The most common are:

  • loop diuretics
  • antibiotics
  • proton pump inhibitors

6. Too much is equally bad

Equally, too much is rarely good for you, and magnesium toxicity can happen even if the serum levels are normal, so knowing what to look out for in a clinical setting can be life-saving for the patient. Usually this is rare because your kidneys are seriously good at shifting it from the blood, so you don’t tend to get it from dietary overdose. Typically it’s your renally-impaired patient who’s on magnesium supplementation who sees their blood levels gradually climbing, and as expected, there is usually concurrent hyperkalaemia and hypocalcaemia, which generate most of the symptomatology:

Symptoms of hypermagnesaemia:

  • arrhythmias
  • hypotension
  • respiratory depression
  • hypo or areflexia
  • dizziness and somnolence

Treatment of hypermagnesaemia is done by antagonising it with calcium. Calcium gluconate IV help to reduce the cardiac manifestations of hypermagnesaemia, while diuretics and dialysis can reduce the magnesium levels in the body to a more tolerable level.

So there you go, a brief intro into why you should care about magnesium. It’s a puppet-master, controlling potassium and calcium, and for the most part it’s your friend, as long as you don’t have too much of it!

Steve can’t have NSAIDs

Steve can’t have NSAIDs. Poor Steve. Steve needs to take something else. Anything but NSAIDS. And here’s why:

Steve has a hurty, and was told to take a painkiller to make his hurty go away.

diagnosis ‘hurty’

Unfortunately for Steve, the painkiller he decided to take was an evil non-steroidal anti-inflammatory drug, which didn’t agree with him.

The NSAID made Steve’s stomach give up all hope and bleed catastrophically, resulting in quite a mess.

It totally ruined the Nativity

All of this blood loss made his kidneys and liver give up hope too, and he watched in vain as they left to search for a sunnier place to live.

This made Steve really angry, and despite his frankly incredible blood loss and dehydration, his blood pressure still managed to go so high that his heart gave up and fell out as well.

At this point, Steve thought an aspirin might help. In fact, it just sparked off his asthma and his lungs fell off too.

Ok this is just stupid now

He then had a banana. Bananas have potassium. Steve also likes bananas.

Yeah, that’ll definitely make up for the lack of vital internal organs

Absolute contraindications to NSAIDS:

  • History of sensitivity to NSAIDS
  • History of GI bleeding or ulceration
  • Severe liver dysfunction
  • Renal impairment
  • Aspirin-induced asthma
  • Uncontrolled hypertension
  • Cardiac failure
  • Dehydration
  • Hypovolaemia
  • Hyperkalaemia

Mike’s morphine toilet adventure

Mike had an absolute blinder of an evening as one beer led to another and then took the logical step toward heavily sedative opioid narcotics, as all good nights out in London do.

Yes mate

He’s now sat astride the porcelain throne, fast asleep. He tried to poo but the straining made him tired, and he’d long since given up trying to urinate. A warm aromatic pool of vomit laps gently around his toes and when he does finally wake up, he has absolutely no idea where he was or why.

Feeling an itch crawling its way up his back, he stands up to scratch it, but everything goes black and he slumps onto the floor.


Side effects of opioid medications:

Nausea and Vomiting
Urinary retention

Sedation and respiratory depression

Someone Else’s Trousers

I always put my money down someone else’s trousers, so people don’t talk about me.

Want to remember what to include on an anaesthetic pre-operative checklist? Well look no further – now you’ll be even more confused!

  • I
    • Identity
      • name
      • date of birth
      • wristband
  • Always
    • Allergies
      • allergies
      • reactions to medications
  • Put
    • Proposed Procedure
      • what do you understand about the procedure you’re having today?
  • My
    • Medical History
      • do you have any medical problems?
      • have you ever been to hospital before?
      • what’s the background to you having this procedure?
  • Money
    • Metalwork
      • do you have any metalwork in your body?
  • Down
    • Drug History
      • what medications are you on?
      • why are you on drug X?
      • when did you last take your medication?
  • Someone
    • Systems Review
      • CVS
        • pacemaker
        • hypertension
        • stroke
        • chest pain
      • Neuro
        • epilepsy
      • Gastro
        • acid reflux
      • Resp
        • asthma
        • lung disease
      • Endo
        • diabetes
        • thyroid
      • Renal
      • Liver
      • Joints
  • Else’s
    • Exercise Tolerance
      • how far could you walk before you get short of breath?
      • how many stairs could you climb?
  • Trousers
    • Today (current health)
      • how is your health today?
      • coughs or colds?
      • recent fevers?
  • So
    • Social History
      • do you smoke?
        • how much?
      • do you drink?
        • how much?
      • other drugs?
  • People
    • Previous General Anaesthetics
      • have you had a general anaesthetic before?
      • were there any problems?
      • do you suffer from motion sickness?
      • has anyone in your family reacted badly to anaesthetic drugs?
  • Don’t
    • Drink/Eat (last meal)
      • when was the last time you ate and drank?
      • what did you have?
  • Talk
    • Teeth
      • are those your own teeth?
      • do you have any loose teeth, caps or crowns?
  • About
    • Airway Assessment
      • jaw protrusion
        • can you put your bottom teeth in front of your top teeth for me?
      • neck extension
        • could you put your chin on your chest and then look up
        • touch chin to each shoulder
  • Me
    • Mallampati
      • please could you open your mouth as wide as you can?
      • now stick out your tongue?