Arterial blood gases – - PowerPoint Presentation

Slide1

Arterial blood gases – acid base balance 4

Dr. S.

Parthasarathy

MD., DA., DNB, PhD (

physiology

),

MD (Acu), Diploma in Software

based

statistics

,

Dip

. Diabetes, IDRA , FICA, Certifícate in USGRA

Associate

editor IJA

Slide2

Indications

To document respiratory failure and

assess

its severity ( any

tachypnea

,

dyspnea

, distress undue) define respiratory failure !!

To monitor patients on ventilators

and

assist

in weaning

To

assess acid base

( mixed) imbalance

in

critical illness

To

assess response to

therapeutic

interventions and

mechanical ventilation

when venous blood is not found for biochemistry

Slide3

Indications

6. Quantification

of the levels of

dyshemoglobins

(

eg

, carboxyhemoglobin and methemoglobin

)

any shock, sepsis, drug overdose

Renal or liver failure

9

. COPD – home oxygen need ??

Slide4

Contraindications

poor collateral circulation

(

allens

) /

peripheral vascular disease in the limb / cellulitis surrounding the site / arteriovenous

fistula/ grafts

Functional arterial line !!

Relative

 

–

 impaired coagulation (e.g. 

anticoagulation therapy / liver disease / low platelets <50

)

Burns and distorted anatomy

Slide5

But in sepsis !!

Although patients with severe coagulopathy are at higher risk for bleeding complications, no clear evidence on the safety of arterial puncture in the setting of coagulopathy exists. In patients with coagulopathy, careful evaluation of the need for ABG sampling is recommended

.

USG useful

Slide6

Modified allen’s test

1. Ask the patient to clench their fist

2. Apply pressure over both the radial and ulnar artery to obstruct blood supply to the hand

3. Ask the patient to open their hand, which should now appear blanched (if not you have not completely occluded the arteries with your fingers)

4. Remove pressure from the ulnar artery whilst maintaining pressure over the radial artery

5. If there is adequate blood supply from the ulnar artery,

colour

should return to the entire hand within 5-15 sec

Slide7

Challenges

Uncooperative patient

Moving patient

Joint diseases

Tremors

Obesity

Rigid walls – may be easy to palpate – positioning for aspiration ?

Slide8

Challenges

ABGs measure gas partial pressures (tensions)

Remember:

PO

2

is not the same as content!

A

severely anemic patient may have an oxygen content reduced by half while maintaining perfectly acceptable gas exchange and therefore maintaining

pO

2

PaO2 is 95 but

Hb

is 3 - think

Slide9

Patient education

Consent

Slide10

Which site ?

Puncture of the

radial artery

- preferred accessibility

of the vessel

,

the presence of collateral circulation

,

the

artery's superficial course

hold

local pressure after the procedure is finished.

Slide11

Why not more – pulse may disappear

Slide12

Slide13

Problems with other sites

may be harder to locate, because they are less superficial than the radial artery;

have

poor collateral circulation;

are surrounded by structures that could be damaged by faulty technique.

Slide14

Technique

Area swab

3 ml syringe with heparin flush

1 % local without adrenaline

Hold with left hand

Palpate for maximum feel

Insert the needle

Look for gush

Slide15

So many hiccups

In patients with hemodynamic disturbance

-- pull back to get blood from artery

-- ? Venous -- SaO2

Get your left index and thumb to action !!

Press the

edema

fluid outside

2 – 3 ml

Remove air

Shift within 10 minutes ( mechanical ventilation but spontaneous – 20 minutes !!?)

Place in cut ice - otherwise

Slide16

Go slow - Otherwise spasm

Automatic filling – otherwise aspirate

Next in other limb

Press for five minutes

Slide17

Analyses of ABG !!

Patient

Time

Fi02

Temperature

Ventilation

Steward s approach !!

Utilizes ion difference instead of

henderson

hasselach

Example if bicarb comes down , it argues that chloride may move up !!

Slide18

Six step analyses

Respiratory or metabolic

Anion gap and delta gaps

Predicted ?

Slide19

Step 1

Look at pH

7.35 to 7.45

< 7.35 =

acidemia

> 7.45 =

alkalemia

Acidosis is the process and

acidemia

is the effect !!

Slide20

Step2: Analyze the CO2

The second step is to examine the pCO2.  Normal pCO2 levels are 35-45mmHg. 

Below

35 is

alkalotic

,

above

45 is acidic

Slide21

Step 2 inference

If pH <

7.35

and the PCO2 is high – then it is a RESPIRATORY ACIDOSIS

If pH <

7.35

and the PCO2 is

normal

– then it is a METABOLIC ACIOSIS

If pH >

7.45

and the PCO2 is low – then it is a RESPIRATORY ALKALOSIS

If pH >

7.45

and the PCO2 is

normal

– then it is a METABOLIC ALKALOSIS

Slide22

Step 3 : Analyze the HCO3

The third step is to look at the HCO3 level

.

A normal HCO3 level is 22-26

mEq

/L

.

If the HCO3 is below 22, the patient is

acidotic

.

If

the HCO3 is above 26, the patient is

alkalotic

.

Slide23

Step 4 - Match either the PaCO2 or HCO3 with the

pH.

pH

acidotic

– Paco2 is more 60 –

pH is

acidotic

- HCO3 is 12

pH is

alkalotic

-- Paco2 is 18

pH is

alkalotic

– HCO3 is 32

Slide24

Step 5 – direction ??

pH

acidotic

– Paco2 is more 60 –

Respiratory acidosis

HCO3 should be normal but if its high , then

There is compensation

Movement opposite ??

Slide25

Does PCO2 and HCO3 match ?

Winters

Rule: PCO2 = 1.5 (HCO3) + 8 (+/- 2).

If the PCO2 is higher

;

there is a assoc

resp

acidosis and if the PCO2 is

lower; there

is an assoc

resp

alkalosis

PCO2 = Metabolic alkalosis

(0.7 * HCO3) + 20

Slide26

Acute

resp. acidosis - -HCO3 = 24+{(actual PaCO2-40)/10}

Chronic respiratory acidosis

HCO3 = 24+ 4 {(actual PaCO2-40)/10}

Acute respiratory alkalosis –

HCO3 ---- 24 – 5 { (40 – actual PaCO2)/10}

Slide27

Another metabolic acidosis situation

1.5 * 15 = 22.5 + 8 =

30.5 ±2

ABG result is showing - PaCO2 = 32

No primary respiratory problem

1.5 * 15 = 22.5 + 8 =

30.5 ±2

ABG result is showing - PaCO2 = 42

Mixed respiratory problem

- yes

Slide28

Delta gap

Na = 145

K = 4

Cl = 101

HCO3 =

15 ?acidosis

Anion gap = ( 145 + 4) – (105 +15) = 29

Delta gap = anion gap(29) – 12 = 17

17 + 15 ( HCO3) = 32

Additional metabolic alkalosis

Slide29

Step 6 = analysing oxygenation

Hypoxemia – less than 60 mmHg or SaO2 < 90

The PaO2 rises with increasing FiO2. Inadequate or decreased oxygen exchange decreases the ratio.

Normal PaO2/FiO2 is >400 mmHg

Approximate PaO2 by multiplying FiO2 by 5

(

eg

, FiO2 = 21%, then PaO2 = 100 mmHg)

Slide30

The 

Strong Ion Difference

 (SID) is the 

difference

 between the positively- and negatively-charged 

strong ions

 in plasma.

In man the renal and respiratory systems regulate acid-base homeostasis by modifying the bicarbonate buffer pair (i.e. PCO2 and HCO3 - ), with all other body buffer systems adjusting to alterations in this pair.

To maintain electrical neutrality there is a change in

cation

concentration commensurate with the change in bicarbonate concentration.

Slide31

The Venous blood Gas - The differences are :

has

a close approximation to the arterial blood gases. They will

tend to

differ more when the patient is in shock.

1. pH – will be about 0.04-0.07 lower with venous gases. Therefore, add this

amount to the measured level and that will be a close approximation of the arterial blood gas

2. PCO2- the PCO2 will be about 6-7 mm HG greater than with an ABG.

Therefore, subtract this amount to get a close approximation of the arterial PCO2.

3. HCO3- the HCO3 – will be about 1-2 mm higher on the venous blood gas.

Slide32

The Capillary blood Gases will also have an approximation to Arterial Blood Gases

It will differ slightly in shock states

pH – will be about 0.04 lower than arterial gases

PCO2 – will be very similar to arterial gases. This can help determine the ventilation rate

HCO3- will be about 1-2 mm higher that an arterial gas

Slide33

Some examples

pH = 7.3

HCO3 = 18

PCO2 =

38

Na = 141

Cl

= 105

K = 4

PO2 = 58

Acidemia

Metabolic

Winters - some resp.

Increased anion gap

Hypoxemic

Slide34

Example 2 - Hypotensive shocked patient Conscious

Ph 7.22

Pco2 -

27

Hco3 – 11

Na 131 cl 90 glucose 135

Po2

– 78 – FiO2 =

30

Acidemia

Metabolic

compensation – no extra

Increased AG

Hypoxemia

But ??

Slide35

Step 1 –

acidemia

Step 2 PCO2 – low HCO3 low – metabolic acidosis

Step 3 anion gap = 29 Increased AG acidosis

Step 4 – respiratory alkalosis – compensation

Delta gap – (29-12) 17 + 11 (HCO3) = 28

Some metabolic alkalosis

30 * 5 = 150 - hypoxia

Slide36

Hypoxic

elevated anion gap metabolic acidosis with incomplete respiratory compensation with concomitant metabolic alkalosis

Slide37

60 years aspiration pneumoniaenteritis hypotension

pH = 7.22

PCO2 = 25

PO2 = 89 -- FIO2 = 21

HCO3 = 10

Na 129

Cl

= 99

Glucose 135

Slide38

Acidemia

-step 1

PCO2 – low – HCO3 – low - step 2 metabolic

Metabolic acidosis

Anion gap – 20 Increased AG acidosis

Delta gap 8 + 10 = 18 -- N AG MA

Mild hypoxia

pH = 7.22

PCO2 = 25

PO2 = 89 -- FIO2 = 21

HCO3 = 10

Na 129

Cl = 99

Glucose 135

Slide39

Increased AG Metabolic acidosis

Insufficient respiratory compensation

But a different NAGMA –

diarhea

and bicarb loss

Anion gaps will come into play in metabolic acidosis

Slide40

A case of CRF with dyspnea

pH 7.28

PCO2 – 29

PO2 – 85

HCO3 – 16

Na – 131

CL 105

Slide41

Step 1 –

acidemia

Step 2 – metabolic

Step 3 AG = 11

Respiratory alkalosis is compensating

Delta gap =

-1

-1 +15 = 14

Same acidosis

pH 7.28

PCO2 – 29

PO2 – 85

HCO3 – 16

Na – 131

CL 105

-1 + 22 = 21

Slide42

Severe acute asthma

pH = 7.29

pCO2 = 60

HCO3 =

25

Na = 138

Cl

= 103

PO2 = 58

Step 1 =

acidemia

Step 2 – respiratory

Step 3 = compensation - nil

AG = 13

Delta gap = 1 +

25

=

26

no other acid base disturbance

3 – 5 mm

mEq

rise in bicarbonate expected – 10 mmHg of PaCO2 rise

Slide43

Hypoxemic uncompensated respiratory

acidosis

Acute resp. acidosis -

-expected HCO3

= 24+{(actual PaCO2-40)/10}

24 + 60-40/10 = 24 + 2 = 26 = HCO3

Here we have 25.

Slide44

A case of persistent vomiting

pH = 7.4

HCO3 = 31

PCO2 =

42

Po2 = 90

Na = 139

Cl

=90

Neutral pH

Alkalosis

- metaboli

c

Compensation

Nomoxia

AG = 18

(18-12) + 31 = 37

Metabolic alkalosis

Compensated metabolic

alkalosis

Expected PCO2 in Metabolic

alkalosis

(0.7 * HCO3) + 20

Slide45

Enough

Thank you all