Arterial Blood Gas AnalysisArterial Blood Gas AnalysisWirral Respiratory Unit Teaching Pack

Maria ParsonageAdvanced Nurse Practitioner

Wirral University Teaching Hospitals Foundation Trust

Housekeeping

• Introduction• Plan for session• Fire Procedure• Facilities• Mobile Phones

Learning OutcomesLearning Outcomes• To have an understanding of the relevance of Arterial

Blood Gas analysis in the Acute setting

• To have an understanding of normal arterial blood gas parameters

• To have an understanding of acid/base balance and be able to recognise metabolic and respiratory differences

• To understand when to alert medical staff

What is an ABG?What is an ABG?• It is an invasive procedure

• Arterial blood is drawn from an artery– Radial– Femoral– Brachial

• It can be a painful test

• It provides us with information on oxygenation and acid base abnormalities

• It should be performed by a clinician who has been appropriately trained

When and Why To Check When and Why To Check ABGABG’’ss

• ABG’s can provide us with valuable clinical information in acutely ill patients with both respiratory and metabolic compromise

• ABG’s are indicated if your patient has a sudden deterioration in their condition such as– ↓ SaO2– ↑ RR– Cyanosis– Confusion– ↓ GCS– Rising MEWS

ParametersParameters

pH ~ H+ ionspO2 ~ partial pressure O2

pCO2 ~ partial pressure pCO2

HCO3 ~ BicarbonateBE ~ Base excessSaO2 ~ Oxygen saturation

pHpH

• pH is directly proportional to hydrogen ion concentration and is the measurement of the acidity or alkalinity of a substance

• The pH scale is a continuum from 0 (strongly acidic) to 14 (strongly alkaline)

• The pH of blood is 7.35 – 7.45– Relatively small changes in pH can be dangerous for

the patient

POPO2 2 & PCO& PCO22

• This stands for partial pressure of O2 and CO2• They reflect the measurements of the partial

pressures which oxygen and carbon dioxide exert in the blood

• PO2 and PCO2 are usually measured in kilopascals (kPa)– some blood gas analysers measure them in millimetres

of mercury (mmHg)– If you need to convert kPa to mmHg, multiply the kPa

by 7.5 – Vice versa, divide mmHg by 7.5

Bicarbonate (HCOBicarbonate (HCO33 ))

• Bicarbonate is referred to as the metabolic component

• We need to know the amount of HCO3 in the blood as 70% of CO2 is carried in this form from the tissues to the lungs for excretion

Base Excess (BE)Base Excess (BE)

• Rising levels of bicarbonate make the blood more alkaline and a depletion of bicarbonate makes it more acidic

• Base excess refers to the amount of base (alkali) which needs to be added or taken away from the blood to return the pH to 7.4

Oxygen Saturation (SaOOxygen Saturation (SaO22 ))• This refers to the amount of

oxygen being carried by the haemoglobin (Hb) molecules

• The Hb molecule is divided into two portions– Globin - made of protein– Haem - made of iron– There are 4 groups of haem on

each molecule of Hb– Each haem group can bind 1 O2

molecule

Oxygen Dissociation CurveOxygen Dissociation Curve• The curve highlights the

affinity of oxygen to haemoglobin

• When PO2 is high, oxygen is strongly affiliated to haemoglobin, so oxygen saturation will be high

• When PO2 is low there is less affinity of oxygen to haemoglobin, so oxygen saturation drops

Oxygen Dissociation CurveOxygen Dissociation Curve• It can be seen from the shape of the oxygen

dissociation curve that initially there is a slight drop in SaO2 when there is a reduced PO2

• However, at a certain point there is a sudden drop in saturation as indicated by the steep decline in the curve

• Therefore, SaO2 normally only drops sharply if the PO2 is at a very low level– 8 kPa is the point at which the patient is hypoxaemic

and is in respiratory failure

Normal Arterial ValuesNormal Arterial Values

pH 7.36 – 7.44 pO2 11 – 13 kPapCO2 4.7 – 5.9 kPaHCO3 21- 28

mmols/lBE ± 2SaO2 >95%

Acid/ Base RelationshipAcid/ Base Relationship

• The relationship is critical for homeostasis

• Significant derivations of the normal pH is poorly tolerated and can be life threatening

• Homeostasis is maintained by the respiratory and renal systems

Buffers Buffers

• There are 2 buffers that work in pairs to maintain homeostasis

• H2 CO3– Carbonic Acid

• NaHCO3– Base Bicarbonate

• The buffers kick in in minutes

• Respiratory acid/ base compensation is rapid and starts within minutes and completes within 24 hours

• Renal acid/base compensation takes hours and up to 5 days

CompensationCompensation• If there is respiratory acidosis (↓pH/ ↑PCO2

) the metabolic component (HCO3 ) will increase– This will increase alkalinity to compensate and

bring pH back towards normal

• If there is metabolic acidosis (↓HCO3 ) the respiratory component (PCO2 ) will decrease– Hyperventillation to blow off CO2 to compensate

and bring the pH back towards normal

CompensationCompensation

• If there is sufficient compensation to bring pH back to normal limits this is full compensation

• If there is some compensation, but not fully normalised pH then this is partial compensation

Compensatory MechanismsCompensatory MechanismsNORMAL VALUE

PH 7.35 – 7.45 pCO2 4.5 – 6 kPa

HCO3 22 – 28 kPa

COMPENSATION

RESP ALKALOSIS

↑ ↓ Normal until compensation

Kidneys will ↓

HCO3 (SLOW)

RESP ACIDOSIS

↓ ↑ Normal until compensation

Kidneys will ↑HCO3 (SLOW)

METABOLIC ALKALOSIS

↑ Normal until compensation

↑ Lungs will try to↑CO2 (QUICK)

METABOLIC ACIDOSIS

↓ Normal until compensation

↓ Lungs will try to ↓CO2 (QUICK)

Respiratory AcidosisRespiratory Acidosis

• Think of CO2 as an acid– A failure of the lungs to

exhale adequate CO2

pH <7.35PCO2 >5.9 kPa

• Causes– Hypoventillation – drug

overdose, narcosis, airway obstruction, respiratory arrest

– Impaired gas exchange – Asthma, COPD

Type I Respiratory FailureType I Respiratory Failure• Type I RF is defined as hypoxaemia (PO2 < 8 kPa)

without hypercapnia (PCO2 > 6 kPa)• It is typically caused by a ventilation/perfusion VQ

mismatch– The air flowing in and out of the lungs is not

matched with the flow of blood to the lungs– Parenchymal disease - CAP/ consolidation– Diseases of vasculature - PE

• There may be acidaemia– pH < 7.35

• There may be compensation– pH 7.35 - 7.45

Type II Respiratory FailureType II Respiratory Failure• Type II RF is defined as the build up of carbon

dioxide (PCO2 > 6 kPa) that has been generated by the body

• The underlying causes include– Reduced respiratory effort - fatigued patient– Increased resistance to breathing - Asthma– A decrease in the area of the lung available for

gas exchange - COPD • There may be acidaemia

– pH < 7.35• There may be compensation

– pH 7.35 - 7.45

Non Invasive VentilationNon Invasive Ventilation• NIV is indicated for patients

with COPD with Type II respiratory failure– pH <7.35– PCO2 > 6 kPa– +/- Hypoxaemia (PO2 < 8 kPa)

• ACTRITE criteria accepts a PO2 > 7 kPa for patients with COPD and a PO2 > 8 kPa for patients with Cor Pulmonale (COPD and Right Heart Failure)

Respiratory AlkalosisRespiratory Alkalosis

• Think of CO2 as an acid– Too much exhaled CO2

pH >7.45PCO2 <4.7 kPa

• Causes • Hyperventilation

– Hypoxemia– Metabolic acidosis– Anxiety

(hyperventillation)• Neurological

– SOL– Trauma– Infection

• Other– Acute anaemia– Salicylate overdose

Metabolic AcidosisMetabolic Acidosis

• Failure of kidney function– Too much H+ in the blood

pH <7.35HCO3 <22 kPa

• Causes• Ketoacidosis

– Type I DM

• Renal tubular acidosis– Renal failure

• Lactic acidosis– Decreased tissue

perfusion– Severe hypoxemia– Cardiac arrest

• Excessive diarrhoea

Metabolic AlkalosisMetabolic Alkalosis

• ↑ plasma bicarbonate

pH >7.45HCO3 >28 kPa

• Causes – Hypokalaemia– Gastric suction– Vomiting– Excessive alkali intake

4 Step Guide to Interpretation4 Step Guide to Interpretation1. Is the pH normal, acidotic or alkalotic?

2. Are the pCO2 or HCO3 abnormal?• Which one appears to influence the pH

3. If both the pCO2 and HCO3 are abnormal, the one which deviates most from the norm is most likely causing an abnormal pH

4. Check the pO2• Is the patient hypoxaemic?

Respiratory Case Study Respiratory Case Study • 35 year old presents with acute

onset of DIB, wheeze and cough

• BP 100/60, HR 120bpm, Temp 37.1°c, RR 36 min

• ABG’s fiO2 60%– pH – 7.30 – PO2 – 8.9 kPa– PCO2 – 5.2 kPa– HCO3 – 24 kPa

• What is the abnormality?

Respiratory Case Study Respiratory Case Study • 71 year old presents with 1/52

increasing shortness of breath, wheeze and cough

• BP 170/80, HR 100bpm, Temp 36.1°c, RR 32 min

• ABG’s fiO2 35% – pH – 7.31 – PO2 – 11.1 kPa– PCO2 – 9.2 kPa– HCO3 – 37 kPa

• What is the abnormality?

Discussion/ Questions?Discussion/ Questions?

Arterial Blood Gas SamplingArterial Blood Gas Sampling

Learning OutcomesLearning Outcomes

• To have an understanding of the relevance of Arterial Blood Gas analysis in the Acute setting

• To demonstrate an understanding of the anatomy of arteries

• To perform and be aware of dangers and complications of this invasive technique

• To safely work towards competence

Arterial SamplingArterial Sampling

• There are two methods – Arterial puncture– Taking sample from an indwelling arterial line

• Samples are usually taken from the radial artery because it is easily accessible, although the brachial and femoral arteries are sometimes used

Arterial PunctureArterial Puncture

• In radial cannulation, a modified Allen’s test should be carried out to ensure the collateral circulation to the hand is adequate to maintain perfusion

Modified AllenModified Allen’’s Tests Test• The radial and ulnar

arteries are occluded by firm pressure while the fist is clenched until the hand blanches

• The hand is opened and the pressure is released from the ulnar artery

• Colour should return within 15 seconds to imply adequate arterial circulation

Equipment Equipment

• Personal protective equipment• Alcohol wipe• Gauze• Tape• Name label and request form• Arterial Blood Gas pre-heparinised syringe• Sharps bin• Blood bag with ice for transport

Components of the ProcedureComponents of the Procedure• Identification of a suitable artery to sample

• Universal precautions, cleaning and preparation of the sample site

• Arterial puncture and aspiration of arterial blood sample

• Occlusion of sample site with dressing and pressure applied to promote clotting and avoid bleeding or haematoma formation

ProcedureProcedure

• Wash your hands and put on PPE

• Locate the approximate position of the artery by slowly rolling your index finger from side to side

• Identify again the point of maximal pulsation of the radial artery

• Clean the skin over the proposed site of puncture

ProcedureProcedure

• Insert into the artery at 45° angle to the skin with bevel uppermost

• Guide the needle slowly toward the point of maximum pulsation

• When you hit the artery there will be a sudden gush of arterial blood into the hub of the needle

Post ProcedurePost Procedure

• Apply direct pressure over the site for 3-5 minutes

• Expel all air bubbles from the sample holding the syringe upright and allowing the bubbles to collect near the needle hub– Then evacuate it by pushing on the plunger

• Carefully cap the needle with a rubber stopper

Post ProcedurePost Procedure

• Don't forget to label the tube with patient's name

• Place the sample in the bag containing ice and send it to the lab

• It is very important to return about 10 minutes later to check for adequate perfusion of the hand and for possible haematoma formation

ResultsResults• You should record FiO2 and temperature accurately on

the ABG request

• If you have sent ABG to the lab for analysis you must review the results

• All ABG’s should be interpreted in relation to the patient’s physical condition

• You must have an understanding of ABG parameters and know when to call for help

Discussion/ Questions?Discussion/ Questions?