please Help! understand interpreting Blood Gas from "the ICU BOOK"

This forum made possible through the generous support of SDN members, donors, and sponsors. Thank you.

ketap

Full Member
10+ Year Member
Joined
Jun 10, 2009
Messages
171
Reaction score
1
hello..i want to ask you all..i have been reading Marino's "The ICU BOOK" for a while now. and i am now reading how to interpret Blood Gas from that book.
now, my confusion is:
1. in stage 1 : rule 3, Marino tells us that " if either the pH or PCO2 is normal, there is mixed metabolic and respiratory acid base disorder"
and...
"if the PCO2 is normal, the direction of change in pH identifies the metabolic disorder"

i don't really understand this...how can he come up with that statement?
for example, pCO2 = 40 ,pH=7.6 , how can he be so sure that it is an alkalosis metabolic (without looking at HCO3 value) combined with respiratory acidosis (as u can see, the pCO2 is still normal)?

2. i noticed that Marino doesn't look HCO3 at all in the first stage (he only use pCO2 and pH)..why?

please help me to understand this..thank you

best regards,Ketap

Members don't see this ad.
 
hello..i want to ask you all..i have been reading Marino's "The ICU BOOK" for a while now. and i am now reading how to interpret Blood Gas from that book.
now, my confusion is:
1. in stage 1 : rule 3, Marino tells us that " if either the pH or PCO2 is normal, there is mixed metabolic and respiratory acid base disorder"
and...
"if the PCO2 is normal, the direction of change in pH identifies the metabolic disorder"

i don't really understand this...how can he come up with that statement?
for example, pCO2 = 40 ,pH=7.6 , how can he be so sure that it is an alkalosis metabolic (without looking at HCO3 value) combined with respiratory acidosis (as u can see, the pCO2 is still normal)?

2. i noticed that Marino doesn't look HCO3 at all in the first stage (he only use pCO2 and pH)..why?

please help me to understand this..thank you

best regards,Ketap

Because you can't have a RESPIRATORY acidosis or alkalosis if the pCO2 is normal. That's why.

You could conceivably have a mixed metabolic disorder but your primary metabolic disorder should be as Marino says.
 
Because you can't have a RESPIRATORY acidosis or alkalosis if the pCO2 is normal. That's why.

hi,Jdh71...Thx for the reply..i appreciate it..but isn't it possible to have a normal PCO2 level but the patient actually has respiratory alkalosis or acidosis...for example, when someone has acidosis metabolic...the compensation should be respiratory alkalosis (low PCO2) but if the blood gas show normal PCO2 level, doesn't it indicate that there is something holding the PCO2 (a concurrent respiratory acidosis)?

thank you..
regards,Ketap
 
Members don't see this ad :)
hi,Jdh71...Thx for the reply..i appreciate it..but isn't it possible to have a normal PCO2 level but the patient actually has respiratory alkalosis or acidosis...for example, when someone has acidosis metabolic...the compensation should be respiratory alkalosis (low PCO2) but if the blood gas show normal PCO2 level, doesn't it indicate that there is something holding the PCO2 (a concurrent respiratory acidosis)?

thank you..
regards,Ketap

You don't compensate to normal. And there is no such thing as mixed resp acidosis/alkalosis. You are either ventilating more or less not both.
 
hi,Jdh71...Thx for the reply..i appreciate it..but isn't it possible to have a normal PCO2 level but the patient actually has respiratory alkalosis or acidosis...for example, when someone has acidosis metabolic...the compensation should be respiratory alkalosis (low PCO2) but if the blood gas show normal PCO2 level, doesn't it indicate that there is something holding the PCO2 (a concurrent respiratory acidosis)?

thank you..
regards,Ketap
You can have a mixed deficit where the pCO2 is normal and yet the patient has a respiratory acidosis. Example take a severe DKA with HCO3 of 6. By Winter’s formula his expected CO2 would be (1.5❎PCO2+8(+/-2)=15-19. By the Henderson Hasselbach equation his pH would be 7.17.

But turns out this DKA guy vomited and aspirated and is obtunded and now has a pCO2 of 40 with his HCO3 of 6. The H-H equation shows his pH to be in the toilet at 6.8 . He has a profound metabolic acidosis as well as an inappropriate respiratory acidosis. He needs a stat ET tube followed by hyperventilation to a pCO2 of 20 with a rate of at least 30 to prevent him from coding from acidemia.
 
  • Like
Reactions: 1 user
You don't compensate to normal. And there is no such thing as mixed resp acidosis/alkalosis. You are either ventilating more or less not both.
hi,jdh71..thx for the quick reply...but i think you misunderstood me..i am not saying that the patient can have both resp acidosis and resp.alkalosis at the same time...
what i mean is just like what nephrocritical care said below

You can have a mixed deficit where the pCO2 is normal and yet the patient has a respiratory acidosis. Example take a severe DKA with HCO3 of 6. By Winter’s formula his expected CO2 would be (1.5❎PCO2+8(+/-2)=15-19. By the Henderson Hasselbach equation his pH would be 7.17.

But turns out this DKA guy vomited and aspirated and is obtunded and now has a pCO2 of 40 with his HCO3 of 6. The H-H equation shows his pH to be in the toilet at 6.8 . He has a profound metabolic acidosis as well as an inappropriate respiratory acidosis. He needs a stat ET tube followed by hyperventilation to a pCO2 of 20 with a rate of at least 30 to prevent him from coding from acidemia.
that is exactly what i mean by having a normal PCO2 but he actually has respiratory acidosis...in other words: something is preventing the appropriate compensation (= hyperventilation) for the metabolic acidosis..

so, in that condition (normal PCO2 with low pH) how can u decide that it is a combination of acidosis metabolic and concurrent respiratory acidosis?thank you
 
Last edited:
hi,jdh71..thx for the quick reply...but i think you misunderstood me..i am not saying that the patient can have both resp acidosis and resp.alkalosis at the same time...
what i mean is just like what nephrocritical care said below


that is exactly what i mean by having a normal PCO2 but he actually has respiratory acidosis...in other words: something is preventing the appropriate compensation (= hyperventilation) for the metabolic acidosis..

so, in that condition (normal PCO2 with low pH) how can u decide that it is a combination of acidosis metabolic and concurrent respiratory acidosis?thank you

And I'm telling you that doesn't happen. The respiratory "compensation" never goes back to normal. You either have it if you don't. If your pCO2 is normal, it's normal. You don't have a respiratory disorder.

Just because a patient isn't compensating doesn't mean they have a "respiratory disorder". That's just asinine. And. Wrong.
 
I disagree with jdh. When you fail to compensate for a metabolic acidosis you have a respiratory disorder that is superimposed upon the metabolic disorder and is further worsening your acidosis. These are going to be type 2 reps failure whether it is suffocation / airway obstruction from aspiration, CNS depression due diabetic coma or neuromuscular from muscle fatigue. So a normal pCO2 in a condition expected to give resp alkalosis should always provoke alarm and a search for the etiology of the type 2 resp failure. Otherwise there will be bad consequences and I will give you an example.

Type 1 diabetic “who we will call Humpty Dumpty because of his bald head “comes with severe DKA. HCO3 6 , pH 7.17, pCO2 17. Comes to me who is moonlighting at rural hospital who notes appropriate resp compensation. Gives 10 units insulin and 2 L saline and asks for tx to higher center.
Nurse doesn’t have IV access and tx pt to ivory tower without insulin/saline or notifying me. Pt arrives at ivory tower ED where now BG is 800, HCO3 is 1. His expected pCO2 is 8-12 but it is 42. By the Henderson Hasselbach equation now pH is 6.48. Pt is nearly comatose. He is met by ivory tower physician who doesn’t believe that a “normal” pCO2 could ever be abnormal. He sneers at the rural hospital and proceeds with fem access.
10 minutes later as access is obtained and nurse is pushing insulin and saline pt totally occludes his airway from secretions and coma and PCO2 climbs to 90. HCO3 is 1 now . By H-H equation now pH is 5.67. Poof ! Assytole.
Now all the king’s horses and all the kings men can’t put Humpty Dumpty together again.
 
  • Like
Reactions: 1 user
I disagree with jdh. When you fail to compensate for a metabolic acidosis you have a respiratory disorder that is superimposed upon the metabolic disorder and is further worsening your acidosis. These are going to be type 2 reps failure whether it is suffocation / airway obstruction from aspiration, CNS depression due diabetic coma or neuromuscular from muscle fatigue. So a normal pCO2 in a condition expected to give resp alkalosis should always provoke alarm and a search for the etiology of the type 2 resp failure. Otherwise there will be bad consequences and I will give you an example.

Type 1 diabetic “who we will call Humpty Dumpty because of his bald head “comes with severe DKA. HCO3 6 , pH 7.17, pCO2 17. Comes to me who is moonlighting at rural hospital who notes appropriate resp compensation. Gives 10 units insulin and 2 L saline and asks for tx to higher center.
Nurse doesn’t have IV access and tx pt to ivory tower without insulin/saline or notifying me. Pt arrives at ivory tower ED where now BG is 800, HCO3 is 1. His expected pCO2 is 8-12 but it is 42. By the Henderson Hasselbach equation now pH is 6.48. Pt is nearly comatose. He is met by ivory tower physician who doesn’t believe that a “normal” pCO2 could ever be abnormal. He sneers at the rural hospital and proceeds with fem access.
10 minutes later as access is obtained and nurse is pushing insulin and saline pt totally occludes his airway from secretions and coma and PCO2 climbs to 90. HCO3 is 1 now . By H-H equation now pH is 5.67. Poof ! Assytole.
Now all the king’s horses and all the kings men can’t put Humpty Dumpty together again.

There isn't a resp acidosis or alkalosis until there is one.

In a patient with multiple metabolic derangements with a pCO2 of 40, the resp component is contributing exactly ZERO to the pH and therefore there cannot be a resp acidosis or alkalosis. The mere fact that a patient with profound resp failure will develop a resp acidosis isn't a resp acidosis until it is.

It's like definitions are meaningless.
 
There isn't a resp acidosis or alkalosis until there is one.

In a patient with multiple metabolic derangements with a pCO2 of 40, the resp component is contributing exactly ZERO to the pH and therefore there cannot be a resp acidosis or alkalosis. The mere fact that a patient with profound resp failure will develop a resp acidosis isn't a resp acidosis until it is.

It's like definitions are meaningless.

Just curious to know. If a patient comes with respiratory acidosis ( PCO2 of 80, Ph: 7.0 ) but a bicarb of 24, could there be a component of metabolic alkalosis or acidosis ?
 
Just curious to know. If a patient comes with respiratory acidosis ( PCO2 of 80, Ph: 7.0 ) but a bicarb of 24, could there be a component of metabolic alkalosis or acidosis ?
HCO3 24 and pCO2 80 adds to a pH of 7.1. But yes there are patients with a respiratory acidosis who can have a metabolic alkalosis and metabolic acidosis as well.
Example would be an morbidly obese middle aged guy with BMI 60, obesity hyperventilation syndrome and CKD stage 3 with creat 1.7 and pronounced type 4 RTA. He presents to you with mild lethargy . His ABG shows pH 7.0,pCO2 110 HCO3 of 26. He hasn’t received any benzos or narcs recently to precipitate an acute respiratory acidosis so we can’t postulate that his HCO3 is just hanging behind. No he has a type 4 RTA. For him if his kidneys were healthy we would expect a 3.5 meq correction in HCO3 for 10 rise in CO2. For him with the pCO2 OF 110 the respiratory compensation should be 24.5 so our HCO3 should be 48-49 which would give him a pH 7.27. So while his HCO3 is 26 he is failing to compensate for his resp acidosis.
So he has a respiratory acidosis a metabolic alkalosis and a metabolic acidosis.
 
Last edited:
  • Like
Reactions: 1 user
There isn't a resp acidosis or alkalosis until there is one.

In a patient with multiple metabolic derangements with a pCO2 of 40, the resp component is contributing exactly ZERO to the pH and therefore there cannot be a resp acidosis or alkalosis. The mere fact that a patient with profound resp failure will develop a resp acidosis isn't a resp acidosis until it is.

It's like definitions are meaningless.

So someone with a profound metabolic acidosis who is not performing the respiratory compensation that they should, therefore allowing the pH to be lower than it would otherwise be, should not be treated like they have a relative respiratory acidosis?
 
So someone with a profound metabolic acidosis who is not performing the respiratory compensation that they should, therefore allowing the pH to be lower than it would otherwise be, should not be treated like they have a relative respiratory acidosis?

You mean by increasing their minute ventilation on the vent?
 
Top