Mg/Phos, and Warfarin questions

[LouisianaDoctor]

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Hi all,

I have two questions:

1. Magnesium. When and why do we measure this? I recent followed a pt with CHF (EF<20), alcoholism, and PE/DVT. The cardio that was consulted measured his Mag, and it was slightly decreased (1.2, I think the normal lower limit was 1.5). He did not replace it. Are there any clinical pearls to replacing Mag (ie, for K, generally if you give 10, it raises serum K by 0.1)? And what is Mgs association with Phos?

2. Warfarin. Ok, so patient has DVT/PE, we cover with Lovenox 1 mg/kg twice daily, and we give warfarin, bridge them, and once the INR is therapeutic for 24 hours, we d/c the Lovenox and send them home. ... But throughout the hospital course, the warfarin dose stayed the same and the INR crept up until therapeutic. Why wouldn't the INR just keep increasing? Do we adjust the dose of warfarin during that 48 hour overlap? Is there a "rule of thumb?" for this? I know when they are discharged they should follow up with PCP and have weekly office visits to check INR, is this where additional adjustments would be made? And what would you do for a patient that is likely to have poor follow up?

Thank you.

 

[armybound]

Mag levels aren't always ordered, though it becomes very important to measure in pregnant women and those prone to seizures. I'm not sure about any role it plays in the cardiovascular system, but I know very little about Mg in general. I don't know anything about a Mg/Phos relationship other than that they come together when you order a Chem10 (BMP + Mg/Phos)

INR can keep rising depending on what your patient's cytochrome profile is. You have to remember the pharmacokinetic principle of a steady state. Due to the long half life of warfarin, it takes a while to build a steady state. Adjusting the warfarin dose too quickly muddies the waters.. you may think the INR isn't rising quickly enough and increase the dose before you really see what your patient's INR is on the current dose. This often gets people in trouble. Warfarin is notoriously difficult to manage. If you think your patient may not be followed in coumadin clinic due to compliance you can consider an anticoagulant with less monitoring like Pradaxa

 

[link2swim06]

Mag levels aren't always ordered, though it becomes very important to measure in pregnant women and those prone to seizures. I'm not sure about any role it plays in the cardiovascular system, but I know very little about Mg in general. I don't know anything about a Mg/Phos relationship other than that they come together when you order a Chem10 (BMP + Mg/Phos)

INR can keep rising depending on what your patient's cytochrome profile is. You have to remember the pharmacokinetic principle of a steady state. Due to the long half life of warfarin, it takes a while to build a steady state. Adjusting the warfarin dose too quickly muddies the waters.. you may think the INR isn't rising quickly enough and increase the dose before you really see what your patient's INR is on the current dose. This often gets people in trouble. Warfarin is notoriously difficult to manage. If you think your patient may not be followed in coumadin clinic due to compliance you can consider an anticoagulant with less monitoring like Pradaxa

To add to your post....Mg is significant for cardio pts too. A low Mg increases the risk of arrhytmias 2-3 times in cardiac pts. Also it can increase the chance of torasodes de points. In my brief couple weeks on medicine it seems pretty common with ICU pts.

 

[armybound]

It would make sense that Mg could stabilize the membrane in cardiocytes as well.

 

[DrSnips]

The most frequent patients requiring mag repletion outside of OB seem to be the alcoholics. Pretty much any person with suspected EtOH abuse you should get a Mag level.

 

[LouisianaDoctor]

The most frequent patients requiring mag repletion outside of OB seem to be the alcoholics. Pretty much any person with suspected EtOH abuse you should get a Mag level.

So in the patient up above, who already has a pretty crappy heart, it would be very important to check his mag because he is an alcoholic with a likely alcoholic cardiomyopathy. Interesting!

Any hints on replacing Mag? Which Magnesium do you use?

 

[greatnt249]

So in the patient up above, who already has a pretty crappy heart, it would be very important to check his mag because he is an alcoholic with a likely alcoholic cardiomyopathy. Interesting!

Any hints on replacing Mag? Which Magnesium do you use?

It's also worth noting that Mg levels are important to know if you have a patient with refractory hypokalemia despite adequate K+ supplementation.

 

[maldini99]

Mag levels aren't always ordered, though it becomes very important to measure in pregnant women and those prone to seizures. I'm not sure about any role it plays in the cardiovascular system, but I know very little about Mg in general. I don't know anything about a Mg/Phos relationship other than that they come together when you order a Chem10 (BMP + Mg/Phos)

INR can keep rising depending on what your patient's cytochrome profile is. You have to remember the pharmacokinetic principle of a steady state. Due to the long half life of warfarin, it takes a while to build a steady state. Adjusting the warfarin dose too quickly muddies the waters.. you may think the INR isn't rising quickly enough and increase the dose before you really see what your patient's INR is on the current dose. This often gets people in trouble. Warfarin is notoriously difficult to manage. If you think your patient may not be followed in coumadin clinic due to compliance you can consider an anticoagulant with less monitoring like Pradaxa

Warfarin takes a while to work not due to its own long half, life but due to very long half life of factors it inhibits. so once they are depleted, then the inr is at steady state.

 

[jdh71]

Any hints on replacing Mag? Which Magnesium do you use?

Mag sulfate 2grams IV Q8 for 9 doses will tank up anyone

Thing to remember about mag is that it is an INTRAcellular cat, so if it's low EXTRAcellular in the serum, patient is quite low. Most attendings and pharmacists will want to give 2grams a day and recheck labs, which is stupid, but it will probably be what will happen anyway.

Drunks are always low on mag. The heart patients need it to prevent arrhythmias. You also need mag to keep potassium in normal range, so if mag is low, you can chase K but might not catch it.

INR stays elevated while the already formed clotting factors clear.

 

[45408]

Warfarin inhibits the synthesis of future factors, so you have to wait until the existing factors are used up/break down before the INR starts to rise.

Most attendings and pharmacists will want to give 2grams a day and recheck labs, which is stupid, but it will probably be what will happen anyway.

It's what the protocol sheet I found recommends. Why is it stupid? If someone is 1.6 and you give 2g, it'll probably be 1.8-1.9 the next day. 4g would put them over 2, if you want. Or is it the re-checking that is stupid? I almost never re-check it until the next day.

INR stays elevated while the already formed clotting factors clear.

Or the other way around....

 

[jdh71]

It's what the protocol sheet I found recommends. Why is it stupid? If someone is 1.6 and you give 2g, it'll probably be 1.8-1.9 the next day. 4g would put them over 2, if you want. Or is it the re-checking that is stupid? I almost never re-check it until the next day.

It's stupid because Mg is an intracellular cation. If it's low in the serum, it's low everywhere. The serum level is a very poor predictor of total body magnesium, though you can reasonably infer that if low in the serum, low everywhere (if anything serum mag levels tend to over estimate mag levels) - nice recent review here

Why give so much? Because they likely need it. Rechecking serum levels won't be a good indicator of where you are in the replacement scheme. 2grams Q8 for 3 days makes sure they are tanked up. There is no real downside - you're not going to give anyone symptomatic hypermag with this, provided they have reasonably functioning beans (remember how much the OBs give to pre-term labor, or how much they'll dump into asthma kids in the PICU??)

 

[jdh71]

Warfarin inhibits the synthesis of future factors, so you have to wait until the existing factors are used up/break down before the INR starts to rise.

Right. That's what I was saying.

 

[45408]

It's stupid because Mg is an intracellular cation. If it's low in the serum, it's low everywhere. The serum level is a very poor predictor of total body magnesium, though you can reasonably infer that if low in the serum, low everywhere (if anything serum mag levels tend to over estimate mag levels) - nice recent review here

They don't discuss the acute post-operative state, which is usually when we check it. Probably half of our bariatric patients have a low serum magnesium level post-operatively, but judging from your review and the overall over-nourished status of that patient population, I doubt their total body magnesium stores are depleted.

Why give so much? Because they likely need it. Rechecking serum levels won't be a good indicator of where you are in the replacement scheme. 2grams Q8 for 3 days makes sure they are tanked up. There is no real downside

Our bariatric patients don't even stay for three days. They usually leave within 48 hours of a Roux-Y. I also doubt they really need 18 grams of Mg.

We do replace it because of the concern about cardiac dysrhythmias as well as the effect on potassium, and hypokalemia can prolong ileus.

Now, the interesting point would be if it's worth checking their Mg levels or just replacing it. I've been told that it's more expensive to check a Mg level than it is for a bag of mag sulfate. Anyone know?

 

[jdh71]

They don't discuss the acute post-operative state, which is usually when we check it. Probably half of our bariatric patients have a low serum magnesium level post-operatively, but judging from your review and the overall over-nourished status of that patient population, I doubt their total body magnesium stores are depleted.

Our bariatric patients don't even stay for three days. They usually leave within 48 hours of a Roux-Y. I also doubt they really need 18 grams of Mg.

We do replace it because of the concern about cardiac dysrhythmias as well as the effect on potassium, and hypokalemia can prolong ileus.

Now, the interesting point would be if it's worth checking their Mg levels or just replacing it. I've been told that it's more expensive to check a Mg level than it is for a bag of mag sulfate. Anyone know?

Surgeons . . .