Blood gases

What is most likely to be wrong with the patient and how will

you manage them?

The Rules

All units in mmo/L and mmHg

Anion Gap = Na - (Cl + HCO3)? N < 15, > 30 = DKA or lactic

acidosis

Rule of thumb Cl normal = HAGMA

A-a gradient N = age/4+4

A = FIO2 x 713 (at sea level) - pCO2 x 1.25

= 150 - pCO2 x 1.25 at sea level on room air

Predicted pCO2 for metabolic acidosis = 1.5 x HCO3 + 8 +/-

2

Metabolic alkalosis: Expected PCO2 = 0.9 [HCO3] + 9

Compensation for respiratory acidosis

* Acute:? HCO3 incr 1 for each 10mmHg rise pCO2 above 40

* Chronic: HCO3 incr 4 for each 10mmHg rise pCO2 above 40

* To a maximum HCO3 of 45

Respiratory alkalosis predicted HCO3 drop

* acute 2 for each 10mmHg pCO2 below 40

* chronic 5 for each 10mmHg pCO2 below 40 minimum HCO3 12-15 after 2-3 days.

Estimated osmolality = 2 Na + urea + glucose

Corrected sodium = glucose/3.5 + measured Na

HAGMALactate or DKA

The restL CAT MUDPILES

CO, CN

Alcoholic ketoacidosis

Toluene

Methanol

Uraemia

DKA

Phenformin, Paracetamol, Pyroglutamic metabolic acidosis, Paraldehyde

Iron, isoniazid

Lactate

Ethylene glycol, ethanol

Salicylates

NAGMAGI or renal loss / decr production of HCO3

HARDUP

Hyperchloraemia

Acetazolamide, Addison’s disease

Renal tubular acidosis

Diarrhoea, vomiting, ileostomies, fistulae

Ureteroenterostomies

Pancreatoenterostomies

Raised Osmolar Gap

MIME ELK

Methanol/mannitol

Isopropyl alcohol

Methylene glycol

Ethylene glycol

Ethanol

Lactate

Ketones

Case 1: 75F

Arterial

FIO2 0.5

pH 7.289 7.35-7.43

pO2 147 69-116

pCO2 26.4 32-45

BXS 13.1 -2 - +2

HCO3 12.3 22-26

Na 155 136-146

K 4.4 3.5-5.0

Cl 120 99-106

Glucose 46 3.9-5.8

Lactate 4.3 0.5-1.6

Hb 163 130-180

Case 1: 75F

Arterial

FIO2 0.5

pH 7.289 7.35-7.43

pO2 147 69-116

pCO2 26.4 32-45

BXS 13.1 -2 - +2

HCO3 12.3 22-26

Na 155 136-146

K 4.4 3.5-5.0

Cl 120 99-106

Glucose 46 3.9-5.8

Lactate 4.3 0.5-1.6

Hb 163 130-180

Before we do any calculations look at the numbers, give me a

couple of diagnoses and tell me your management

Case 1: 75F

Arterial

FIO2 0.5

pH 7.289 7.35-7.43

pO2 147 69-116

pCO2 26.4 32-45

BXS 13.1 -2 - +2

HCO3 12.3 22-26

Na 155 136-146

K 4.4 3.5-5.0

Cl 120 99-106

Glucose 46 3.9-5.8

Lactate 4.3 0.5-1.6

Hb 163 130-180

DKA or HHS + relative

hypoxia + hypoperfusion ?

septic

Rehydrate slowly

Add insulin and K+

replacement once K+

known

Seek and treat source

CXR

Anion Gap

Anion Gap

Anion Gap = Na - (HCO3 + Cl)

= 155 - (12.3 + 120)

= 22.7 (N < 15)

Causes of a raised anion gap metabolic acidosis

●DKA

●Renal failure

●Lactic acidosis

olocalised ischaemia

oshock

ohypoxia

●Toxins

A-a Gradient

A-a Gradient

A = FIO2 x Dry atmospheric pressure - pCO2/0.8

= FI02 x 713 (at sea level) - pCO2 x 1.25

= 0.5 x 713 - 26.4 x 1.25

= 356 - 33

= 323

A-a = 323 - 147

= 176 (N = age/4 +4

= 23)

So he’s got something going on in his lungs

Respiratory compensation for metabolic acidosis

Respiratory compensation for metabolic acidosis

Predicted pCO2 = 1.5 x HCO3 + 8 (+/- 2) at 24 - 48 hours

= 26.45

Which is what is measured.

This would suggest there is no respiratory component to this

patients presentation, which, given the A-a gradient, is not

correct

So ? partially compensated metabolic acidosis which would

cause a CO2 greater than 26.45 + a respiratory alkalosis

driving the CO2 down to the measured 26.4

Corrected Sodium

Corrected Sodium

Corrected Na = glucose/3.5 + measured Na

= 46/3.5 + 155

= 171

Na > 160 has 75% mortality high incidence of neurological

deficits in survivors.

Estimated Osmolality

Estimated Osmolality

Osmolality = 2Na + glucose + urea

= 310 + 46 + urea - which is likely to be high

= 356 + urea (278-305)

Conclusion

Raised anion gap metabolic acidaemia, extreme

hyperglycaemia, hyperosmolality + lactic acidosis.

At this age most likely to be HHS rather than DKA. With lactic

acidosis the acidosis, though likely to be a component of

renal failure

K normal but patient likely to be whole body K deficient and

likely to become hypoK with treatment.

Marked A-a gradient suggesting a respiratory component to

the presentation, with hypoxia prehospital causing the

lactic acidosis

Source of deterioration needs to be sought and corrected eg

pneumonia, CCF, MI.

The patient needs volume replacement initially with isotonic

crystalloid until perfusion is normalised then with hypotonic saline

to correct hyperosmolality (patient is relatively more deficient in

free water than sodium). There is a risk of cerebral and

pulmonary oedema if osmolality and dehydration is corrected

too quickly. If there is pulmonary oedema before resuscitation the

patient will need ventilatory support as volume is replaced.

1L NS/1H till intravascularly euvolamic then 1/2NS at 250ml/hr

(maximum of 50ml/kg over first 4 hours). Add K 20mmol/hour

when passing urine. Change to 1/2NS + 5% dextrose + 40mmol KCL

when BSL < 16.5

Use of insulin is controversial as may cause too rapid correction of

hyperosmolality. I would use low dose insulin at 0.1U/kg and

monitor BSL and osmolality Q2H. BSL should not be reduced

below 14mmol/L until mental state and hyperosmolarity

corrected.

Reference for HHS: UpToDate

Case 2 85F

Arterial

FIO2 1.0

pH 7.186 7.35-7.43

pO2 87.5 69-116

pCO2 37.8 32-45

BXS -12.9 -2 - +2

HCO3 13.7 22-26

Na 139 136-146

K 9.4 3.5-5.0

Cl 117 99-106

Glucose 8.6 3.9-5.8

Lactate 0.6 0.5-1.6

Hb 124 130-180

Case 2 85F

Arterial

FIO2 1.0

pH 7.186 7.35-7.43

pO2 87.5 69-116

pCO2 37.8 32-45

BXS -12.9 -2 - +2

HCO3 13.7 22-26

Na 139 136-146

K 9.4 3.5-5.0

Cl 117 99-106

Glucose 8.6 3.9-5.8

Lactate 0.6 0.5-1.6

Hb 124 130-180

Before we do any calculations look at the numbers,

give me a couple of diagnoses and tell me your

management

Case 2 85F

Arterial

FIO2 1.0

pH 7.186 7.35-7.43

pO2 87.5 69-116

pCO2 37.8 32-45

BXS -12.9 -2 - +2

HCO3 13.7 22-26

Na 139 136-146

K 9.4 3.5-5.0

Cl 117 99-106

Glucose 8.6 3.9-5.8

Lactate 0.6 0.5-1.6

Hb 124 130-180

Life threatening

hyperkalaemia

Hypoxia

Severe acidosis with poor

respiratory compensation ?

Decr LOC

Salbutamol

Calcium gluconate

ECG

Repeat K+ sample

Most important thing

Life threatening hyperK: 9.4

Needs to be confirmed and simultaneously treated

●safe to start a salbutamol neb while ECG and repeat gas is

being done

Other treatment options? What would you do?

Anion Gap

Anion Gap

Anion Gap = Na - (HCO3 + Cl)

= 139 - (13.7 + 117)

= 8.3 (N < 15)

NAGMA

Hyperchloraemic metabolic acidosis

●Diuretics

●Renal disease

oclassically Renal Tubular Acidosis

orenal failure

●GI losses

Predicted Respiratory Compensation

Predicted Respiratory Compensation

pCO2 = 1.5 x HCO3 + 8

= 28.5

Measured CO2 is 37.8 so some respiratory acidosis (eg

pneumonia, decr LOC) (driving the CO2 up) and/or

metabolic alkalosis (eg vomiting) (driving the HCO3 down)

A-a Gradient

A = 713 x FIO2 - pCO2 x 1.25

= 713 - 47.25

= 665.75

A-a = 665.75 - 87.5

= 578 (N = age/4 +4

= 25)

= bad!

Despite a SaO2 of 96%

Conclusion

Conclusion

Life threatening hyperkalaemia

Marked NAGMA / hyperchloraemic metabolic acidosis

Acidosis and hyperK likely to be on the basis of renal failure

The hyperK needs urgent treatment and confirmation

Find cause of renal failure: Hx, examination and further

examination

If appears volume depleted she needs replacement with K

free crystaloid till euvolaemic

Likely to need urgent dialysis if she is a candidate

Significant pulmonary pathology needs to be addressed based

on further assessment.

Prognosis: poor.

Case 3: 3 year old with tachypnoea

Venous

FIO2 0.21

pH 7.22 7.35-7.43

pO2 48 69-116

pCO2 24 32-45

BXS -2 - +2

HCO3 10 22-26

Na 139 136-146

K 1.9 3.5-5.0

Cl 118 99-106

Glucose 4.1 3.9-5.8

Lactate 2.5 0.5-1.6

Hb 107 130-180

Case 3: 3 year old with tachypnoea

Venous

FIO2 0.21

pH 7.22 7.35-7.43

pO2 48 69-116

pCO2 24 32-45

BXS -2 - +2

HCO3 10 22-26

Na 139 136-146

K 1.9 3.5-5.0

Cl 118 99-106

Glucose 4.1 3.9-5.8

Lactate 2.5 0.5-1.6

Hb 107 130-180

NB venous

Hypokalaemia ? Secondary to B2 agonist

Or diarrhoeal illness

Needs oxygen

Most important finding

Life threatening hypoK

Needs to be immediately confirmed and treated

IV KCL max 0.4mg/kg/hour with ECG monitoring

Estimated weight 14kg (2(age+4))

KCL 5.6mmol/hour

Will get onto how to do that.

Anion gap

Anion gap

AG = Na - (HCO3 + Cl)

= 139 - (10 + 118)

= 11

= N

NAGMA / Hyperchloraemic metabolic acidois

●Diuretics

●GIT loss

●Renal disease

What's going on and what are you going to do?

What's going on and what are you going to do?

Probable gastroenteritis with life threatening hypokalaemia.

Mx (lots of ways to do it)

If really shocked IV boluses eg Hartmann's (K = 5mmol/L) 20mmol/kg boluses

till clinically intravascularly euvolaemic.

Then

Eg 20mmolKCL in 1/2NS + 5% dextrose run at double maintenance

(Maintenance = 4ml/kg for 1st 10kg + 2ml/kg for next 10kg + 1ml/kg

thereafter, (but in Alice they use 5ml/kg if under ) 40 + 8 ml/hour. Double

maintenance = 96ml/hour

Alternatively (RCH): 250ml/hour NS + KCL 20mmol/L for first 6 hours

if severely dehydrated (=5mmolKCL/hour), or 150ml/kg thereafter

Alternatively (CC) 280ml boluses of Hartmann's if shocked. Then or

otherwise: 250 0r 150ml/hour (depending on severity) NS + 5%

Dextrose + KCL 20mmol/L. Offer ORS. Stop IV fluid when taking >

10ml/kg/hour ORS.

ORS (liquid or icy pole) contains KCL 20mmol/kg.

Ondansetron 2mg SL wafer or IV

See http://sites.google.com/a/emergency-medicine-

tutorials.org/www/Home/medical-

3/gastroenterology/gastroenteritis

Also note anaemia - probably nutrition related, needs further

assessment and mx.

Case 4 78F SOB

Arterial

FIO2 0.21

pH 7.421 7.35-7.43

pO2 41.9 69-116

pCO2 50.8 32-45

BXS 7.8 -2 - +2

HCO3 32.4 22-26

Na 140 136-146

K 2.7 3.5-5.0

Cl 99 99-106

Glucose 6.0 3.9-5.8

Lactate 0.9 0.5-1.6

Hb 104 130-180

Is there an acid base disorder present? If so what?

No acidaemia or alkalaemia but probably respiratory acidosis (low HCO3, high CO2 and

hypoxia (or could be metabolic alkalosis with full compensation - but hypoxia

suggests respiratory process and don't usually get full compensation for metabolic

acidosis).

Compensation for respiratory acidosis:

●acute: HCO3 1mmol/10mmHg pCO2 above 40

●chronic: HCO3 4mmol/10mmHg pCO2 above 40

pO2 50.8 so max HCO3 should be 24 +4 = 28

Measured (actually, calculated) HCO3 = 32.4

So some other process driving HCO3 higher or pCO2 lower

●metabolic alkalosis eg vomiting, diuretics

●acute respiratory alkalosis eg acute respiratory compensation on back ground of

chronic respiratory disease

Other disorder?K = 2.7

Most common causes diuretics and GIT loss

● Decreased intake

o rare

● Redistribution

o Alkalaemia (think of cells pumping out H in exchange for K to correct alkalaemia)

o Insulin

o Beta agonists - eg salbutamol - quite likely in this patient

o Thyrotoxic periodic paralysis

o Familial hypokalaemic periodic paralysis

o Aldosterone excess

● Loss

oGIT

especially vomiting to give alkalosis

o Renal

Diuretics

Hyperaldosteronism

Primary

Secondary

Renal hypoperfusion

CCF

Renal artery stenosis

CRF

Liver disease (splanchnic shunting through varices)

Post ATN

Renal tubular acidosis

Bartters

hypoMag

Leukaemias

HypoK

Most likley secondary to any or all of:

●Alkalosis

●Salbutamol

●Diuretics

●Vomiting

A-a gradient

Patient on room air oops!

A = 150 - 50.8 x 1.25

= 86.5

A-a = 86.5 - 41.9

= 44.6 N = age/4 +4

= 23

Conclusion

Severely unwell patient who requires oxygen.

She has hypoxic and hypercapnic respiratory failure.

She is not acidaemic or alkalaemic probably on the basis of a compensated

chronic respiratory acidosis + another process eg

●metabolic alkalosis from vomiting or diuretics (also causing hypo K)

●acute respiratory acidosis

She is hypokalaemic from eg

●vomiting

●diuretics

●alkalosis

●salbutamol

She needs urgent correction of her hypoxia as first line therapy. She is likely

to require ventilatory support. Her K needs to be corrected.

Case 5. 71F with persistent vomiting

Arterial

FIO2 0.21

pH 7.671 7.35-7.43

pO2 28.2 69-116

pCO2 43 32-45

BXS 25.7 -2 - +2

HCO3 49.1 22-26

Na 129 136-146

K 1.9 3.5-5.0

Cl 71 99-106

Glucose 6.0 3.9-5.8

Lactate 1.8 0.5-1.6

Hb 136 130-180

A-a gradient

A = 150 - 43 x 1.25

= 96.25

A - a = 96.25 - 28.8

= 67.45 (N = age/4 + 4 = 21.75)

Severe A-a gradient

Acid base disorder

Metabolic alkalosis

Causes

Most common GIT loss (vomiting) or diuretics

Chloride responsive

● GI loss

●Diuretics

●Chloride wasting disease

o cystic fibrosis

Non-chloride responsive

●hyperaldosteronism

oExpect hyperNa and hypoK

oeg renal artery stenosis, CCF, liver failure, nephrotic syndrome

Compensation for metabolic alkalosis

Expected PCO2 = 0.9 [HCO3] + 9

= 0.9 x 49.1 + 9

= 53

Measured 43

So probable respiratory alkalosis also, especially as severely

hypoxic.

Severe hypoK: 1.9

Likely to be as a result of vomiting

●Kidneys retain H at expense of K

Needs urgent correction and cardiac monitoring

Hyponatraemia: 129

Likely to be due to vomiting

Usually classified as

●Spurious

●Hypertonic

oHyperglycaemia, Mannitol

●Hypotonic

oHypovolaemic

Decr intake or loss eg GI loss, 3rd spacing, burns

oEuvolaemic

SIADH, Drugs, Glucocorticoid deficiency, Excess water

intake (psych, potomania, novice athletes)

oHypervolaemic

renal failure, cirrhosis, nephrotic syndrome, CCF

Hyponatraemia does NOT need urgent correction

Criteria for urgent correction

●Na ~< 115 and

oSeizure or

oComa or

ofocal neurological deficit

Estimated Osmolality

2Na + glucose + urea = 158 + 6.0 + urea

= 164 + urea

Conclusion

Severe hypoxaemia requiring oxygen and probably ventilatory

support.

Hypoxaemia likley to be as a result of aspiration from

vomiting and exacerbated by respiratory compensation for

metabolic alkalosis

Given Hx of persistent vomiting, vomiting is probable cause of

metabolic alkalosis.

Needs volume, Na, Cl and K replacement eg NS + 40mmolKCL/

1 hour with cardiac monitoring

Case 6 74M SOB

Arterial

FIO2 0.21

pH 7.49 7.35-7.43

pO2 38.4 69-116

pCO2 26.7 32-45

BXS -2.8 -2 - +2

HCO3 20.1 22-26

Na 139 136-146

K 4.2 3.5-5.0

Cl 108 99-106

Glucose 6.2 3.9-5.8

Lactate 1.4 0.5-1.6

Hb 121 130-180

Respiratory alkalosis

Predicted HCO3 drop

●acute 2 for each 10mmHg pCO2 below 40

●chronic 5 for each 10mmHg pCO2 below 40 minimum HCO3

12-15 after 2-3 days.

Predicted HCO3 drop 1.5 - 6.5 -> 22.5-17.5

Measured = 20.1 So probably full compensation and single

process depending on chronicity

A-a gradient

A = 150 - 26.7 x 1.25

= 116.6

A-a = 116.6 - 38.4

= 78 (N = age/4 +4 = 22.5)

Anion Gap

139 - (20.1 + 108) = 10.9

= N

Conclusion

Severe hypoxic respiratory failure.

Patient requires oxygen and treatment of underlying process