Thread by @mahrukhrizvi_MD

Tweetorial: An amp of 8.4% NaBicarb in the ICU 💉. Panacea or poison?

#NephTwitter #askrenal #medtwitter @askrenal

Normally, at a pH of 7.4, what is the concentration of H+ in the arterial plasma?

Let’s begin with a mind excercise 🧮!

How efficient is Bicarbonate as a buffer? 🧪

Remember:
[H+] = 24 x PCO2/[HCO3-]

pCO2 = 40
[HCO3-] = 24

Hence, normally:
[H+] = 24 x 40/24 = 40 nanoeq/L

If [H+] exceeds 160 nanoeq/L ➡️ pH = 6.8 ➡️ DEATH 💀

Say a healthy individual presents after attempting suicide 🚫 by ingesting an acidic substance 💊, adding 12 MILLION nanoeQ/L (12 meQ/L) of H+ to circulating plasma (300,000 times normal [H+])- what pH do you expect at presentation, assuming NO respiratory compensation?

💊 Adding 12 MILLION nanoeq/L (12 meq/L) of H+ to the circulating plasma 🩸will result in the consumption of 12 meQ/L of HCO3 to buffer the 12meQ/L of H+

Now [H+] = 24 x 40/12 = 80 nanoeq/L ➡️ pH = 7.1

Hence, increasing [H+] by 300,000x only moderately decreases pH!

WOW! ✨

That speaks to the remarkable efficacy of bicarbonate in buffering acid and improving acidemia! 🌟

Should we give acidemic pts in ICU IV NaHCO3💉?

In an 84 kg male, administering 1 amp (50 meQ) of NaHCO3 will increase serum bicarbonate concentration by:

Nl serum [HCO3]-: 24 meq/L

1 amp 8.4% NaHCO3 injection 💉 has 50 meq bicarbonate

Administered IV NaHCO3 distributes in total body water 💦 (TBW=60% body weight)

Let’s go through a typical ICU case:

84 kg male ➡️ TBW 💦 = 50L

Severe ARDS, on LTV ventilation ⚡️

Shock, on multiple pressors 🤒

Serum Bicarb = 10 ; pCO2 = 35

[H+] = 24 x 35/10 = 84 nanoeq/L ➡️ pH = 7.10

Let’s administer 1 amp (50meQ) of NaHCO3 💉

TBW= 84kg x 0.6 = 50L

50meq bicarb 💉 distributed over 50L 💦 will increase serum bicarb by 1meQ/L

Serum bicarb will increase from 10Meq/L to 11meq/L

[H+] = 24 x 35/11= 76.3 nanoeq/L➡️ pH = 7.11

pH increased by 0.01☹️

Well that’s rather dissatisfying 😕

But...

Clinically, when I administer an amp of NaHCO3 it works! 😀

I have see an amp of bicarb instantly improve hemodynamics... probably because it improves acidosis...win win... ✅. Agree?

Let’s do another mind excercise! 🧮
How much Na is in an amp of NaHCO3? 🧂

50 ml amp of 8.4% NaBicarb is profoundly concentrated —> 2000 mosm/L = thats 2x hypertonic saline

1 meQ of Na = 23 mg of Na

50meQ of Na = 1150 mg Na

That’s how much Na is in 1/2 teaspoon of salt!

So... don’t be fooled! 🧠

The amp of NaBicarb instantly, BUT transiently, improves BP NOT due to improvement in acidemia...🚫

... but rather due to the injection of 1/2 teaspoon of salt 🧂 into the patients IV 🤯

Now lend thought to those soaked ARDS inflicted lungs 😰

Note, we are assuming:

1.No ongoing acid production
2.Constant pCO2

In reality, minor increments in Bicarb/pH via injection are defeated by ongoing acid production

Na🧂load leads to uninvited volume retention➡️ worse ARDS➡️ pCO2 retention➡️ worse acidemia

In summary, with an amp of bicarb💉:

🔴The improvement in pH is negligible and likely quickly negated by ongoing acid production (continued shock/ARDS)⌛️

🔴We have subjected an ARDS patient to a load of Na🧂which may lead to stiffer lungs

🔴In this patient, harm>>benefit

What if we use an isotonic bicarbonate drip?

Suppose we administer an isotonic NaHCO3 gtt with 3 amps bicarb (150 meq) in 1 L D5W 💧@ 100cc/hr

150meQ/50L=3meQ/L

Serum bicarb: 10Meq/L ➡️ 13meQ/L

[H+] = 24 x 35/13= 64 nanoeq/L ➡️ pH = 7.2

We have increased pH by 0.1 in 10 hrs

Key points 🔑:
🟢An amp of NaBicarb contains 50meQ bicarbonate that distributes in TBW
🟣An amp of Nabicarb contains 1150 mg of Na
🟢An isotonic drip of Nabicarb has 150meQ of bicarbonate that distributes in TBW
🟣This drip contains 3450 mg of sodium equal to 1.5 tsp salt🧂

Key points 🔑:
🟣Na load is what causes instant improvement in BP when an amp of Nabicarb is administered
🟢Na load may prove harmful in overloaded pts
🟣Weigh risks/benefits when administering bicarb in volume overloaded pts w acidemia
🟢 🔑 Treat underlying etiology of acidosis

🟣Most benifit from an amp of Bicarb can be derived once ongoing acid production has decreased
🟢Volume depleted patients will not suffer the harm of a Na load
🟣Measuring serum Bicarbonate within 2 hrs of administration may be misleading as it has not yet distributed in TBW.

An amp of NaBicarb has the following amounts of Na(mg) and Bicarb(meQ), respectively:

No drug is a panacea (except fish oil?) 🧚‍♀️
Every drug can be a poison ☠️

Tweetorial inspired by @tony_breu

@nephondemand @NephJC @CCJournalClub @ansakhuja @pkguru10