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Before one can interpret Arterial Blood Gases, the RCP must know the normal arterial blood gas values:

                                                                             

Because mistakes can be made and the RCP might stick a vein by accident, it is helpful to learn the normal venous blood gas values, too. These are mixed venous samples because the value of any other vein will reflect only the status of that limb.

Any values that drift out of the normal gas range are considered abnormal.

·  pH above normal levels are alkalotic

·  pH below normal levels are considered acidotic

• PaC0₂ above normal are hypercapnic and will create  acidosis
• PaC0₂ below normal will result in alkalosis

 

• HC0₃- above normal are considered alkalotic
• HC0₃- below normal are considered acidotic

 

• Pa0₂ above normal are considered hyper-oxygenation or over-correction
• Pa0₂ below normal ranges are considered hypoxemic

 

One looks to the pH to find the acid base status. Is the pH out of range? If so, then look to the PC0₂ and to the HC0₃ to see which of them is also out of range and in what direction.

Say your pH is 7.28; this is acidic. You look to the PaC0₂ which is 54 mmHg and to the HC0₃- which is 24. The PaC0₂ is out of range and because it is higher than normal, it is the cause of the acidotic pH. The HC0₃- is in range so it is not responsible for the pH derangement.

‘Newbies’ [that’s you] never expect the acid base to be normal, but to this clinician, is seems that the single most common arterial blood gas seen in the hospital is the normal acid-base balance with some degree of hypoxemia. The doctor ordered a blood gas to assess the hypoxemia. If everything is normal there cannot be acidosis or alkalosis. 

 

 

If both values are out of range, there are two possibilities: one is that there are two problems  both bicarb and C0₂ are out of range and acid or both out of range and alkalotic.... this situation is a mixed problem.

 

Example: A pregnant woman could have a mixed alkalosis, because she is losing HC0₃- by vomiting & because she is hypocapnic [has low PaC0₂]  due to breathing rapidly and shallowly due to the fetus pressing up on the diaphragm.

 

Another possibility:

If both values are out of range, but one is acid and the other is alkalotic, there has been an effort by the body to correct some problem by throwing other values out of range. This is called compensation.

If the body has a derangement, the attempt to correct the situation by throwing something else out of range to return the pH to normal is an attempt to compensate for the problem.

5. After you find the criminal; you need to see if there was a cover up---- this is compensation:

The human body attempts to keep the body at a normal state of acid-base balance. So--- if there is a problem, the body will try to fix the pH by throwing another parameter out of range. If the problem is acidic due to hypercapnia, the kidneys may keep HC0₃- so that the pH will go back to normal. If the problem is a metabolic acidosis, the patient may increase his minute ventilation to drop the PC0₂.

Because of these efforts to compensate for primary problems by throwing other parameters out of range, the blood gas interpretation needs to address the concept of compensation.

B + N = uncompensated B

A + N = uncompensated A

but

B + A = compensation

However, as we said earlier, if both parameters are out of range:  both C0₂ and HC0₃- are acidic or both are alkalotic, we have a mixture of problems. This is not compensation. It is mixed acidosis or mixed alkalosis

A +  A = mixed A [ not compensation]

B + B = mixed B [not compensated]

6. Answer this only after one has established that there is compensation

• Is it fully compensated?
• or is it partially compensated?

• If the pH is normal, it's "fully compensated"
• if the pH is still out of range, it's only "partial compensated "

 

Why would an ABG be only partially compensated?

The difference between partial and fully compensated is sometimes due to time. There just wasn’t time for the compensation to complete. The renal system requires 24 hours to change the pH. 

Or the difference between partial and fully compensated is due to limitations. Remember that you cannot stop breathing just to compensate for metabolic alkalosis. Your peripheral chemoreceptors will not allow you to kill yourself to get the pH back to normal. There will be an incomplete attempt  which we call partial compensation.

A + N = Uncompensated A

B + N = Uncompensated B

A + A = mixed A

B + B = mixed B

B + A = compensation

N + N    = Normal Acid/base balance

7. Is this gas possible?

• Another interpretation frequently missed is the interpretation that the DATA IS BAD.

• There was a collection error OR a reporting error.

A + A = pH B or N …. This is an example of bad data

B + B = pH A or N…. this is bad data, too

N + N = pH  A or B…. is bad data

At this point, you may need to look deeper... to make sure the data is bad or that the problem is subtle

1. move from 2 SD to 1 SD

or

2. check the validity of the gas with the Henderson/Hasselbalch equation

Standard deviation

When you have a gas like this one:

 pH          7.34       A
 PC0₂        45        N
 HC0₃-       25       N

you would like to call it ‘normal’ but the pH is out of range. Is this bad data?  Based on the Henderson Hasselbalch equation, the Ph should be 7.36. So, yes this is bad data.

To interpret other ABGs, you might need to tighten your standards of what is normal by looking at 1 standard deviation rather than 2 SD.

pH          7.36      N
PC0₂        46       A
HC0₃-       25       N

 

When we do the H/H equation, we see the parameters are valid, so what’s going on? We need to look at it more closely

When they were figuring out who was normal, various datum was placed on a graph and it created a bell curve, 95% of the persons tested had normal ph between 7.35- 7.45, but the middle 68% had a normal pH between 7.38 -7.42.

1 ST

XX

XXXXXXXXX

XXXXXXXXXXXXXXXX

XXXXXXXXXXXXXXXXXXXXXXX

XXXXXXXXXXXXXXXXXXXXXXXXXXXX

 

A standard deviation of 1 [1 SD] is more “picky” than 2 standard deviations [2 SD.]  where the data is farther from Ideal.

It might be bad data, but FIRST look closer at the gas based on 1 SD
 

* values accepted by NBRC

After you move from 2 SD to 1 SD, look at this gas again:

pH   7.36   ---was N & is now A

 

PC0₂    46 ---   was N, but now it's A

HC0₃-   25---    was N, still N

Now we see this gas is an uncompensated respiratory acidosis

Rules for ABG interpretation

1. Compensation implies there was first a problem to be overcome

2. It is possible to have 2 unrelated problems in the same direction. This is mixed

3. It is possible to have 2 unrelated problems in the opposite direction. While strictly speaking; it's a compensation and you would interpret it as compensation.... it's actually a lucky coincidence

4. It is possible to have a normal acid/ base. In fact, most of us have normal acid-base balances, even if we are hypoxemic.

5. The body never over-compensates… The body will get the pH to normal and not overshoot*

6. Look to the pH first to find the derangement

7. "Bad data; we need to run the sample again" is a valid interpretation.. highly unpopular--- but valid

8. Once the patient is placed on mechanical ventilation, rule # 5 is easily over-ruled.*

Using the Henderson/ Hasselbalch

As stated earlier, the pH is the result of the ratio of the bicarb [HC0₃-] to the carbonic acid [PaC0₂] and your blood gas result MUST conform to the Henderson/ Hasselbalch formula. If it doesn't, you may have one or more bits of data recorded wrong.

The formula is the H/H formula:

 

 

as the HC0₃- rises the pH rises

as the PaC0₂ rises the pH drops

in compensation situations, both HC0₃- and PaC0₂ rise or both drop to keep the pH WNL

Example:

 

pH = x
6.1 is a factor
HC0₃- & PaC0₂ are taken from the ABG
the log can be calculated with a scientific calculator
                               or
use the modified logarithm table for the H/H found below

Example #1

pH = X
HC0₃-  25
PaC0₂   47

x =    6.1 + log of 17.7 [now look up 18]
x =  6.1 + 1.26
x = 7.36
pH = 7.36

Example #2

pH = X
HC0₃-  18
PaC0₂   40

x = 6.1 + log of 15 [look up log of 15]
x = 6.1 + 1.18
pH = 7.28

The sequence to writing the blood gas interpretation

Start on the right side of the page & answer each of these questions:
1.    is the pH acidic or alkalotic or normal
2.    is the ph caused by  metabolic? Or respiratory problem?
3.    is it compensated? Or mixed?
4.    if compensated, is it fully? Or partially?

Example:
fully [4] compensated [3] metabolic [2]  acidosis [1]

Lastly, the RCP must address the 0xygenation level of the patient:

for a person under 60 years of age hypoxemia is stated as:

normal oxygen level                80-100 mmHg
mild hypoxemia                        60- 79 mmHg
moderate hypoxemia              40-59 mmHg
severe hypoxemia                    less than 40 mmHg

If the patient's Pa0₂ is higher than 100 mmHg, this is interpreted as hyper-oxygenation or over correction because one may need to consider weaning the level of 0₂ to avoid the many hazards of 0₂ therapy.

[reference Wilkin's Clinical Assessment in Respiratory Care]

if the patient is over 60 years of age, we adjust his normal values to fit the less effective  aging lung:

Predicted Pa0₂ = 103.5 - [0.42 x age]    this is -/+ 4 mmHg

[reference Wilkin's Clinical Assessment in Respiratory Care]

For the purposes of arterial blood gas interpretation, we generally, don't make exceptions for persons with chronic hypercapnia, nor for infants whose normal values are different from adults and older children.

We will interpret the ABG the same way----however we could note that this is a "baseline ABG for this person [mention disordered state], so no corrections are needed."

 

Using the H/H equation at the bedside:

To completely control your patients ABG,  You need 1 blood gas and 3 formulae:

 

·  Pa0₂¹ [you have] : Fi0₂¹[you have]  as Pa0₂² [you want] :  Fi0₂² [you need]

·          Henderson/Hasselbalch

·          Ideal Ve formula

  Ideal Ve formula is used to correct the C0₂, which [if the ABG is an uncompensated respiratory problem] will also correct the pH:

Example:
Your patient is on mechanical ventilation at a rate 10 bpm, with a Vt 800 ml

            f x Vt = VE = 10 x .8 Liters = [Ve 8 liters] 

and an Fi0₂ 30%
Your ABG on these settings:

 

pH	7.46
CO2	32
HCO3-	23
PaO2	155
SaO2	99

Obviously, the patient is over-ventilated, as well as over-oxygenated.

 to decrease his PaC0₂ to 40 mmHg :

if the Ve is decreased from 8L to 6.4 Liters, we predict that the PaC0₂ will return to normal [40 mmHg]

Use H/H to predict the pH
if the PaC0₂ returns to 40 torr [WNL], we can expect the pH to also return to normal because the bicarb is already normal--- but we can prove this via the H/H equation.

x  = 6.1 + log of 19.1 =

x =  6.1 +1.28 = pH = 7.38

We aren't done yet, because the patient's Fi0₂ needs adjusting.

 

Pa0₂¹ [you have] : Fi0₂¹[you have]  as Pa0₂² [you want] :  Fi0₂² [you need]

Example:

Pa0₂ = 155
Fi0₂ =  50%
Pa0₂ that I want is 90 mmHg
Fi0₂ = x

Pa0₂¹: Fi0₂¹ as Pa0₂²: Fi0₂²

155 torr:  .50   as  90 torr : X

155 X    =     .50 [90]

   =  . 29  or Fi0₂ is 29%

After 1 blood gas, you can recommend *

1.         Decrease the Fi0₂ to .29%, to achieve a predicted Pa0₂ of 80-100 mmHg

2.         Decrease the Vt from 800 to 650 mL, with a predicted PaC0₂ of 40*

3.         This will result in a predicted pH of 7.38

* number 2 and 3 only work  if the RCP completely controls the VE.

Limitations to arterial blood gas analysis

ABG interpretation can answer a lot of questions, but like all tools, the analyzers have some limitations.

• ABG machines do not measure HC0₃- while they measure the pH and the PaC0₂, they merely calculate the HC0₃- based on the Henderson/Hasselbalch equation

• ABG machines do not measure the Sa0₂, they calculate the Sa0₂ based on the 0xyhemoglobin curve

• they will not recognize abnormal states of hemoglobin such as methemoglobin or co-oxyhemoglobin
• in cases of suspected carbon monoxide poisoning, and smoke inhalation patients, the clinician would need to run the sample through both the co-oximeter as well as the ABG to  get a true clinical picture.
• ABG machines do not measure the hemoglobin. If your ABG machine displays a Ca0₂ , this figure will be based on a hemoglobin value that the clinician must dial into the machine. To get the true hemoglobin one must look to the Hemoglobin/hematocrit reading or to the co-oximetry reading.

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Acid base relationships

pH is the measurement of the H+ which results in the balance of acids to bases.

• Increasing the number of H+ will decrease the pH, because there is more acid

• Decreasing the number of H+ will increase the pH, because there is less acid

 

pH changes back and forth in all parts of the body.

• It happens in the blood

• in the cerebral spinal fluid [CSF]

• and in the urine

As acid collects, the pH drops in that part of the body

 

Because blood is mostly water the pH is close to water [7.0]

• The blood pH is WNL between 7.35-7.45

• Anything above 7.45 is considered basic or alkalotic

• Anything below 7.35 is considered acidic

 

Based on the Henderson Hasselbalch equation

The pH of the blood reflects the relationship between the bicarbonate to the carbonic acid levels of the blood. Because the carbonic acid and the PaC0₂ have a stable relationship one can multiply the PaC0₂ by .03 to get the value of the carbonic acid.

 

Remember! In any formula in which you have   

 
if Y goes up, X goes up
if R goes up, X goes down

 

 

so if

then as HC0₃- goes up, the pH goes up
as
PaC0₂ goes up, the pH goes down

 

The body tends to make acids

So we are generally trying to get rid of acids.  Acids are sent out of the body in two methods:

• Exhaled as volatile acids such as C0₂
• Excreted as fixed acids in the urine

 

Exhaled C0₂ [respiratory ]

• As excessive PaC0₂ rises in the blood stream, the blood pH drops
• As the minute ventilation [Ve] increases to blow off excessive C0₂, the pH rises
• These changes can happen within minutes

 

Fixed acids [metabolic]

• As fixed acids are excreted in the urine, the blood’s pH rises

• It would be no surprise at this point to understand that the urine pH is very low because it is so acidotic

• It would also not surprise you to find out that the person who is in renal failure not only has fluid building up but acids and other waste products

 

Buffering

Another way the body uses to deal with excessive acids is to buffer an acid.

This buffering captures free H+ so that the pH of the blood rises a bit.

If the proper substances are available in the blood stream, the free H+ will get collected and the pH will rise

 

Buffers

• Substances that capture free H+ are called buffers
• Some work better than others and are called strong buffers
• Many of the plasma proteins, including hemoglobin, are weak buffers

The most important buffer Is HC0₃- [called bicarbonate or  ‘bicarb’]

The pH of the blood is determined by the balance between ‘bicarb’ and the acid carbonic acid [Henderson/Hasselbalch]

 

pH of the blood is altered by:

• The kidneys alter the pH by excreting fixed acids into the urine to leave the blood

• The lung excretes volatile acid in the form of C0₂

• The use of buffers

 

Other situations that alter the pH of the blood 

The rate of C0₂ production [H+] in cellular respiration can be altered as the rate or mode of C0₂ production is altered by diet

pH can be altered by outside influences such as illness, ingestion of toxins or by mechanical ventilation

 

Metabolism

Metabolism is the sum of all the body’s various functions and the result of metabolism includes waste products

The body happens to create more acids than it creates bases….so we are left with having to get rid of the these acids or with having to buffer them

Cellular metabolism creates energy as it consumes 0₂ and produces C0₂ as waste:

 RQ = VC0₂  /  V0₂

The ratio of C0₂ produced to 0₂ consumed was felt for years to be fixed at .8 this is called the respiratory quotient [RQ.]

 

                                        200 ml/minute of C0₂ produced    = .8 which is the RQ
250 ml/minute of 0₂ consumed

Once indirect calorimetry measurements were devised, this RQ turned out to be a mutable number. It can be changed by alterations in wellness, diet and in lifestyle.

 

Events that change the RQ

Events that affect the way the cell metabolisms changes the respiratory quotient.

• Alternations in diet
• Body temperature
• Various illnesses

 

Alterations in diet

• If most of your calories come from carbohydrates, you will create a lot of C0₂

• If most of your calories come from fats, you will create much less C0₂

• A high protein diet will increase acids so that the ventilatory drive increases to get rid of C0₂

• If you stop eating [diet or starvation] your entire rate of metabolism decreases. This is one of the problems with diets for weight loss

• If you eat a lot  [hyperalimentation] you will crank the metabolism back up.

• In the early stages of starvation,  keto-acids result from fat utilization, but in the later stages as the body uses its own muscle mass for energy, the breakdown of proteins increases the acid production

 

 

Body temperature

• As the body temperature rises, the rate of metabolism rises above normal so that both the C0₂ production and the 0₂ consumption rise

• Fever is a common cause of not only tachypnea,  but of sinus tachycardia for this reason

• When a person’s temperature drops very low, his rate of metabolism drops. Hypothermia is used during open heart surgery to lessen the effect of decreased CO on the body during the surgery 

 

The effect of illness on the metabolism

Many pathological processes alter the metabolism:

• The act of wound healing can raise the metabolic rate by 5%

• Renal failure will effect urine production which prevents the excretion of fixed acids, so that the lung must work harder to get ride of volatile acids such as C0₂

• Diarrhea increases the elimination of bases so that the pH will drop

• Vomiting will dump stomach acids, raising the pH

• Diuretic drugs will increase urine production which alters the pH

• Tissue hypoxia results in the creation of lactic acids from anaerobic metabolism

 

Anion Gap

There is a normal balance between the serum electrolytes and a normal Anion gap is 8-16 mEq/L. Higher than 16 mEq/L is a sign of lactic acid. To find out if one has anaerobic metabolism or lactic acid, the Anion Gap is calculated:

 

Na - (HC03- + Cl)

 

The kidneys

• The role played by the kidneys in pH balance of the blood is a pivotal one.

• The major job of the kidneys is to excrete the excessive H+ that life in the fast lane creates

• as well as to conserve all the HC0₃- buffer that it can.

• It is no wonder that the pH of urine is only 6

• It has all the volatile acids and little of the HC03-

 

The renal system: Slow steady worker

It takes the kidneys a full 24 hours to change the acid base balance

It will attempt to get the pH as close to normal as possible

It can return the body to complete acid base homeostasis within 24 hours

Once the blood is back to normal pH, the kidneys will stop dumping acids

If the condition that resulted in acid production is resolved, it will take another 24 hours to get back to normal again
 

 

however, ventilation can change the pH in minutes

The normal control over ventilation is primarily to control the pH of the cerebral spinal fluid which is affected as C0₂ diffuses from the blood into the CSF & undergoes hydrolysis.

if there is  a rise in CSF levels of H+, then there must be an increase of Ve to blow off more C0₂ to get the pH back to normal.

 

How can CSF H+ increase?

• As metabolic acids rise, there are more free H+ which easily pass the blood/brain barrier so that the pH drops in the CSF

• And as C0₂ rises in the bloodstream , this gas passes from the blood plasma into the cerebral spinal fluid [CSF] that bathes the surface of the brain . The C0₂ increases the carbonic acid in the CSF so the H+ rises and the pH drops.

 

Increasing ventilation

• In the face of an acidic CSF, the brain sends a message via the phrenic nerve to the diaphragm to increase the rate and depth of ventilation

• This increased minute ventilation [Ve] can alter the C0₂ in less than 30 minutes

• As the C0₂ decreases, the H+ decrease and the pH rises back to normal

• As the pH gets normalized, the Ve returns to its baseline

• There is a limit to how low we can get the PaC0₂, due to limits on the lungs, the lowest C0₂ is generally in the teens.

 

We breathe to get rid of C0₂

The normal PaC0₂ will vary only 3-4 mmHg all day long because our Ve varies as the C02 rises. Even in deep sleep, the PaC0₂ will not rise much more than 3-4 mmHg from the daytime baseline.

There is a fail-safe mechanism in which we will respond to severe hypoxemia [less than 60 mmHg] If we don’t respond to acidosis, our Ve rises due to hypoxemia noted by the peripheral chemoreceptors.

The combination of hypoxemia and hypercapnia is a massive reaction in which not only the diaphragm, but the accessory muscles of ventilation in the chest wall will get the message to breath deeper and faster

 

The relationship between C0₂ and H+ is a chemical equilibrium. The C0₂ hydrolysis into water will yield H₂C0₃ [carbonic acid]. This is an unstable relationship: the C0₂ can go into carbonic acid or can return to the gas. If there is too much C0₂ on one side there will be a swing to the carbonic acid side. If the carbonic acid builds up there will be a swing to the C0₂ side of the equation

this is not the complete hydrolysis of  C0₂. The carbonic acid can also go into HC0₃- and H+
complete would be:

water + carbon dioxide ==== carbonic acid  =====  HC0₃-   +  H+