8/10/2019 Acio Base

1/3

Intens Ive care

Acidbase andgas analysissheena M a Hubble

blood+alveolar ventilation. The higher the concentration of H the more

CO 2 (volatile acid) is ex ired from the lungs. This is a ra id and o!erful com ensator" s"stem.

# $enal bicarbonate control and excretion of metabolic (nonvolatile) acids. This is a relativel" slo! s"stem (hours or da"s).# %uffering b" bicarbonate& sul hate and haemoglobin. Thisminimi'es acute changes.

Traditional approach to altering pH

The traditional a roach to acid base control focuses on theHenderson Hasselbalch e uation. This e uation describes thecarbonic acid buffer s"stem& !hich is fundamental to res irator"and renal control of H. Carbon dioxide reacts !ith !ater to formcarbonic acid& !hich dissociates to form bicarbonate and H + *

Abstractt he concentration of h"drogen ions is one of the most tightl" controlleds"stems in the bod". efence of normal H is thought to be from thr ee

basic mechanisms* res irator" control of carbon dioxide& renal excre,tion of acids& and lasma buffering s"stems. t he traditional a roachto acid base control centres on the Henderson Hasselbalch e uation& in!hich H can be defined as the ratio of bicarbonate to carbon dio xide.alterations in H result from changes in carbon dioxide (res irator") or

bicarbonate (metabolic). Most H disturbances can be classified into oneof four main t" es* res irator" acidosis- res irator" al alosis- metabolicacidosis- metabolic al alosis. t he s te !art h" othesis is an alternativ ea roach to acid base anal"sis. It challenges the conce t that changesin bicarbonate concentration can alter H. t his theor"& based on math ,ematical solution& is that onl" three things& alone or in combination& candetermine the h"drogen ion concentration* strong ion difference (netcharge balance of dissociated ions in lasma)- artial ressure of carbondioxide- and the sum of acids r esent.

+ /CO 2 + H 2O 0 H 2CO 1 0 H + HCO 1

If b" the la! of mass action* H + 3 HCO / 34 H CO 3 5 ( con sta n t)1 2 1

Then b" rearranging and ta ing logs of both sides* H 5 6a + log ( HCO 1 34 7 .71 8 CO 2 mm Hg3) !here the 6avalue at 19:C is ;.tandard bicarbonate is thecalculated bicarbonate value that !ould be resent if a bloodsam le !as ad?usted to a CO 2 of @7 mm Hg (A.1 Ba)& andtherefore removes the res irator" com onent of the acid baseabnormalit" to reveal an" metabolic derangement. >tandard baseexcess or deficit uantifies the amount of acid in mmol4litre thatmust be added or subtracted from the same blood sam le !itha haemoglobin set at A.Ag4dl to regain a normal H at a CO 2of @7 mm Hg. ote that the more negative the standard baseexcess& the more acidic the blood sam le. Table < summari'es

/

The concentration of h"drogen ions in mammalian s"stems isver" tightl" regulated and& in contrast to most other ion concentrations& is maintained in the nanomolar (1; @1 nmol4litre)rather than the millimolar range. This is because the high chargedensit" and large electrical field surrounding the h"drogen ioninfluences nearl" all biochemical rocesses& including roteinstructure and function& ionic dissociation and movement& andchemical or drug reactions.

The H is the negative log atholog" or drug intoxication. Other causes include excess CO 2 roduction in h" ercatabolic states&inade uate ventilator minute volume or administration of exogenous CO 2 (e.g. eritoneal insufflation during la arosco ").$enal com ensation b" increased H+ excretion and bicarbonate

Sheena M A Hubble, FRCA, is Consultant in Intensive Care Medicine at the Royal Devon and Exeter Hospital. She qualified from the Universityof ristol in !""! and trained in medicine# anaesthesia and intensive care in the South$%est. Her research interests include themicrocirculation# lactate# and intensive care patient follo&$up.

@9

8/10/2019 Acio Base

2/3

Intens Ive care

aceta'olamide thera " and& most commonl"& the administrationof large volumes of intravenous normal saline.

Batients !ith lo! albumin and hos hate must have their anion ga corrected to avoid missing high anion ga acidoses.The normal range of anion ga can be ad?usted according to the

atient s albumin and hos hate concentration as follo!s*Corrected anion ga 5 ( a + 6)/(Cl + HCO 1 )3/(7.2

albumin g4G + I

#

#

the sum of cations 5 the sum of anions a + + 6 + + unmeasured cations 5 Cl / + HCO 1 +

unmeasured anions

/

# anion ga 5( a + +6 + ) (Cl / +HCO 1 )5I +&strong ion d if f e r e nce

D2O

+ ,pected changes in primary acidbaseabnor malities

pCO ' tanda rdbase e ,c ess(mm ol-litre!

Metabolic acidosis Metabolic al alosis acuteres irator" acidosis chronicres irator" acidosis acuteres irator" al alosis chronicres irator" al alosis

cO & artial ressure of carbon dio xide

8/10/2019 Acio Base

3/3

Intens Ive care

pCO 2" The effects of changes in res irator" CO 2 on H are !ellunderstood and roduce the ex ected alterations in H+ according to Henderson Hasselbalch e uation*

ATOT is the total lasma concentration of the !ea non volatileacids& inorganic hos hate& serum roteins and albumin.

Clinical applications

The ne! a roach does not change the measurement or classification of acid base disorders or challenge the rimar" role of CO 2

in determining H. The standard base excess can also be usedto uantif" the amount of change in >I that has occurred from

baseline& or the amount of strong anion that must be removedor strong cation added to restore the H to 9.@& given a CO 2of @7 mmHg. =or exam le& to change the standard base excessfrom /27 to /I >I e(effective) should e ual 'ero. Dn" difference bet!een >I e a nda arent >I (>I a ) is the strong ion ga (>I ) and r ese n tsunmeasured anions. D, re resents dissociated !ea a cids(mainl" albumin and ho s hate s )

>I a

>I

a +

>I eD

HCO1

a ctate

Cl

http://www.acid-base.com/http://www.acid-base.com/http://www.acid-base.com/