Metabolic/Respiratory - Acidosis/Alkalosis

[firemedic0227]

I am having troubles remembering the difference between these and how the body comes to these two things. Anyone have any thing to help me remember these better?

 

[systemet]

I am having troubles remembering the difference between these and how the body comes to these two things. Anyone have any thing to help me remember these better?

To put this simply:

The respiratory system controls the amount of carbon dioxide dissolved in the plasma. With a greater alveolar ventilation, you remove more CO2. With hypoventilation, you retain more CO2.

Carbon dioxide dissolves in the blood stream to carbonic acid, which dissociates into hydrogen ions and bicarbonate, i.e. H20 + CO2 <-> H2CO3 <-> H+ + HCO3-.

So, if we hypoventilate, we retain CO2 (i.e. our pCO2 increases), and our amount of hydrogen ions increases, causing a respiratory acidosis (i.e. pH < 7.35, pCO2 > 45 mmHg). This is a rapid process.

Alternatively, if we hyperventilate, we reduce the CO2 levels, the amount of dissolves CO2 decreases, and so does the amount of hydrogen ions (i.e. pH > 7.45, pCO2 < 35 mmHg).

We can also produce too much acid metabolically, e.g. DKA, lactic acidosis, certain drug overdoses, resulting in an decreased bicarbonate levels, and producing a state of metabolic acidosis (pH < 7.35, HCO3- < 22 mmol). Rarely, we can lose acid, e.g. prolonged vomiting, resulting in a high bicarbonate and pH, e.g. metabolic alkalosis.

Do you get this? Because once you understand that you can't start looking at ideas like full and partial compensation, or even anion and delta gaps.

 

[ThadeusJ]

Respiratory
1) Normal arterial pH is between 7.35 and 7.45: The lower the number the more acidic, the higher the number, the more alkaline;

2) the primary regulator is through CO2 (normal levels 35-45 mmHg) and therefore breathing. Simply put, CO2 mixes with H2O to make H2CO3 (carbonic acid, which quickly breaks down to HCO3 and H+). Therefore, the more CO2, the more acid.

3) We regulate CO2 by breathing: Increase breathing (i.e. minute volume), you blow off CO2 = less acid = higher pH (alkalosis)and when you decrease breathing, you build up CO2 = more acid = lower pH (acidosis); Some people think of a seesaw with CO2 on one side and pH on the other (when one goes down, the other comes up).

Metabolic
1) usually regulated by kidneys, but other things can mess it up. Kidneys regulate bicarb levels: Normal is between 23-27 mmol. If bicarb falls, so does the pH. If it rises, so does the pH. Some people think of an elevator, when one goes down, so does the other.

2) bicarb levels change slowly and can be corrected in the short term via the respiratory mechanism (i.e. bicarb falls in acute renal failure or DM to create a metabolioc acidosis, pH is stabilized by breathing faster, creating a compensating effect of boosting the pH.

There's a lot more to it, but once you understand the basic concepts, you can better understand some of the scenarios and presentations that are out there. There's a ton of PowerPoints and other resources available through search engines. Hopefully the above explanation can get you started.

 

[Carlos Danger]

I am having troubles remembering the difference between these and how the body comes to these two things. Anyone have any thing to help me remember these better?

Here's the down and dirty of the respiratory side of it:

Carbon dioxide = acid (high levels of acid = low pH; low levels of acid = high pH)

One of the primary reasons you breath is to get rid of acid.

Breathe too slow = you don't get rid of acid fast enough and it accumulates, causing your pH to drop (acidemia)

Breath too fast = you get rid of too much acid and your pH rises (alkalemia)

 

[firemedic0227]

Thanks guys, this really helps.

 

[NomadicMedic]

Here's a great place to start: http://www.acid-base.com/

 

[MSDeltaFlt]

Remember this: R.O.M.E. Respiratory Opposite Metabolic Equal.

The key is balance. "pH" balance, that is. The human body works best when its pH is within a very narrow range. Preferably 7.35-7.45. It doesn't like extremes of either. Infact nothing will work in an acid medium or alkali medium. For instance vasopressors. When a pt's pH falls below say somewhere around 7.2 so will the blood pressure... and the cardiac output... and the pulse.

Look at the pH first. Then look at the PaCO2. Then the HCO3.

Also, PaCO2 and PaO2 are not related. Oxygenation and ventilation are not related whatsoever. Never have been. Never will be.

 

[ThadeusJ]

Speaking of acid/base imbalances, can anyone explain why firing in boluses of saline (or whatever) doesn't affect the pH balance? The pH of normal saline is around 5.5 or 6, yet one can administer huge amounts directly into the bloodstream. One would think that's asking a lot from the buffering agents.

 

[MSDeltaFlt]

Speaking of acid/base imbalances, can anyone explain why firing in boluses of saline (or whatever) doesn't affect the pH balance? The pH of normal saline is around 5.5 or 6, yet one can administer huge amounts directly into the bloodstream. One would think that's asking a lot from the buffering agents.

It does. If you infuse large volumes of hypertonic solutions (blood, blood products, albumin), as in your Parland formula beyond 24hrs then you'll see HCO3 changes within hours (<2). But the shere volume of isotonic solution needed to change pH would turn your blood volume into very thin koolaid.