CXR and ABG interpretation for RT

Pattabhi raman, Mahadevan & Arjun Srinivasan

Pulmonology AssociatesKMCH

Introduction

• Basic ideas about situations that RT would be facing with regards to CXR and ABG.

• Not going to be a comprehensive account of both.

• Might be too basic.• Speaker does not consider himself to be an

authority in both these topics .

CXR

Different tissues in our body absorb X-rays at different extents:

•Bone- high absorption (white)

•Tissue- somewhere in the middle absorption (grey)

•Air- low absorption (black)

Film Quality

• First determine is the film a PA or AP view.

PA- the x-rays penetrate through the back of the patient on to the film

AP-the x-rays penetrate through the front of the patient on to the film.

All x-rays in the ICU are portable and are AP view

Quality

• Is the film over or under penetrated if under penetrated you will not be able to see the thoracic vertebrae.

Quality (cont)

• Check for rotation

– Does the thoracic spine align in the center of the sternum and between the clavicles?

– Are the clavicles level?

NORMAL CHEST P/A

Abnormalities that RTs encounter

• White stuff on CXR-Collapse of lung / lobes and Consolidation

• The black stuff-Pneumothorax, Pneumomediastinum

• Displaced lines,tubes,Ryles Tubes.

White stuff - edema

Air bronchogram sign

• In a normal chest x-ray, the tracheobronchial tree is not visible beyond 4th order

• It becomes recognizable if the surrounding alveoli is filled, providing a contrast or if the bronchi get thickened

CAUSES

• Normal expiratory radiograph• Consolidation• Pulmonary edema• Nonobstructive pulmonary atelectasis-

RDS,compression atelectasis,Fibrotic scarring(radiation fibrosis,bronchiectatic lobe)

• Interstial disease-sarcoid,CFA• Neoplasms-BAC,lymphoma

White stuff - Collapse

• Important to recognise• May be lobar or segmental• Lower lobe collapse are more important to

recognise as they carry more volume• Usually positioning would help in a ventilated

patients and Bronchoscopy is done only if patient is hypoxemic ,suspected foreign body or failure of positioning.

Collapse LLL

LLL collapse

• Common in ICU• Slightly difficult to pick up clinically.• Look for the diaphragm.

RLL – LL PA

Upper lobes

Air beyond lungs

• Faulty ventilation strategy

• Iatrogenic

• Trauma patients

Air around lungs

• Pneumo mediastinum• Subcutaneous emphysema• Pneumothorax

Black stuff-Pneumomediastinum

• Continuous diaphragm sign• Ring around the artery sign

• Important to realise barotrauma during ventilation

Pneumomediastinum

Pneumomediastinumcontinuous diaphragm sign

Ring around the artery sign

Continuos diaphragm sign

Pneumopericardium

Pneumothorax

Hyperlucent hemithorax sign

Deep sulcus sign

• Air collects in the most superior portion.• In ventilated patient, it occupies anterior and

lateral portion of chest which is the most non dependant in supine lying.

Deep sulcus

Tubes and lines

• Important reason for taking an X-Ray• After ET /Trach or central lines, xrays give an

idea of the position of tubes and lines.• Need to rule out complications.

Importance of penetrated film

Hose goes where the nose goes

High ET

RMB intubation

ICD position

Very low Too high

NG tube

Twisted NG tube in airway

Central line

ABG

42

The Body and pH

• Homeostasis of pH is tightly controlled• Extracellular fluid = 7.4• Blood = 7.35 – 7.45• < 6.8 or > 8.0 death occurs• Acidosis (acidemia) below 7.35• Alkalosis (alkalemia) above 7.45

43

44

As required for ECG interpretation,

a systematic approach to ABGs enhances accuracy.

There are NO short-cuts!

A Systematic Approach

The Anatomy of a Blood Gas Report

----- XXXX Diagnostics ------

Blood Gas Report248 05:36 Jul 22 2000Pt ID 2570 / 00

Measured37.0o

CpH 7.463pCO2 44.4 mm HgpO2 113.2 mm Hg

Corrected38.6o

CpH 7.439pCO2 47.6 mm HgpO2 123.5 mm Hg

Calculated DataHCO3 act 31.1 mmol / LHCO3 std 30.5 mmol / LBE 6.6 mmol / LO2 CT 14.7 mL / dlO2 Sat 98.3 %ct CO2 32.4 mmol / LpO2 (A - a) 32.2 mm HgpO2 (a / A) 0.79

Entered DataTemp 38.6 oCct Hb 10.5 g/dlFiO2 30.0 %

Measured Values

Temperature Correction:Is there any value to it?

Calculated Data:Which are the useful ones?

Entered Data:Derived from other sources

----- XXXX Diagnostics ------

Blood Gas Report

Measured37.0o

CpH 7.463pCO2 44.4 mm HgpO2 113.2 mm Hg

Corrected38.6o

C

Calculated DataHCO3 act 31.1 mmol / LHCO3 std 30.5 mmol / LBE 6.6 mmol / LO2 CT 14.7 mL / dlO2 Sat 98.3 %t CO2 32.4 mmol / LpO2 (A - a) 32.2 mm HgpO2 (a / A) 0.79

Entered DataTemp 38.6 oCct Hb 10.5 g/dlFiO2 30.0 %

Oxygenation Parameters:O2 Content of blood:Hb x O2 Sat x Const. + Dissolved O2

Oxygen Saturation:

Alveolar / arterial gradient:

Arterial / alveolar ratio:

Oxygen Saturation

pO2

Satu

ratio

n

0 60 120

100% Most blood gasmachines estimate saturation from an idealized dissociation curve

Gold standard is co-oximetry

Errors may occur with abnormal haemoglobins.

Oxygen content is calculated from this.

Alveolar-arterial DifferenceInspired O2 = 21%= piO2 = (760-45) x .21=150 mmHg

O2

CO2

palvO2 = piO2 - pCO2 / RQ= 150 - 40/0.8= 150 – 50 = 100 mm Hg

partO2 = 90 mmHg

palvO2- partO2 = 10 mmHg

Alveolar-arterial Difference

O2

CO2

Oxygenation FailurepiO2 = 150

pCO2 = 40

palvO2= 150 – 40/.8=150-50 =100

pO2 = 45

D = 100-45 = 55

Ventilation FailurepiO2 = 150

pCO2 = 80

palvO2= 150-80/.8 =150-100

= 50

pO2 = 45

D = 50-45 = 5

----- XXXX Diagnostics ------

Blood Gas Report

Measured37.0o

CpH 7.463pCO2 44.4 mm HgpO2 113.2 mm Hg

Calculated DataHCO3 act 31.1 mmol / L

O2 Sat 98.3 %pO2 (A - a) 32.2 mm Hg

Entered DataFiO2 30.0 %

The Blood Gas Report:The essentials

pH 7.40 + 0.05PCO2 40 + 5mm HgPO2 80 - 100mm Hg

HCO3 24 + 4mmol/L

O2 Sat >95A-a D 2.5+(0.21 x Age) mm Hg

Technical Errors Glass vs. plastic syringe: Changes in pO2 are not clinically importantNo effect on pH or pCO2

Heparin (1000 u / ml):Need <0.1 ml / ml of bloodpH of heparin is 7.0; pCO2 trends downAvoided by heparin flushing & drawing 2-4 cc blood

Delay in measurement:Rate of changes in pH, pCO2 and pO2 can be reduced to 1/10 by cooling in ice slush(4o C)No major drifts up to 1 hour

Step 1Look at the pH

Is the patient acidemic pH < 7.35or alkalemic pH > 7.45

Step 2Is it a metabolic or respiratory disturbance ?

Acidemia: With HCO3 < 20 mmol/L = metabolicWith PCO2 >45 mm hg = respiratory

Alkalemia: With HCO3 >28 mmol/L = metabolicWith PCO2 <35 mm Hg = respiratory

Step 3If there is a primary respiratory disturbance, is it acute?

Expect D pH = 0.08 x D PCO2 / 10 (acute)Expect D pH = 0.03 x D PCO2 / 10 (chronic)

Step 4For a respiratory disorder is renal compensation OK?

Respiratory acidosis: <24 hrs: D [HCO3] = 1/10 D PCO2

>24 hrs: D [HCO3] = 4/10 D PCO2

Respiratory alkalosis: 1- 2 hrs: D [HCO3] = 2/10 D PCO2

>2 days: D [HCO3] = 5/10 D PCO2

Step 5If the disturbance is metabolic is the respiratorycompensation appropriate?

For metabolic acidosis:Expect PCO2 = (1.5 x [HCO3]) + 8 + 2(Winter’s equation)

For metabolic alkalosis:Expect PCO2 = (0.7 x [HCO3]) + 21 + 1.5

If not: actual PCO2 > expected : hidden respiratory acidosisactual PCO2 < expected : hidden respiratory alkalosis

Step 6If there is metabolic acidosis, is there an anion gap?

Na - (Cl-+ HCO3-) = Anion Gap usually <12

If >12, Anion Gap Acidosis : MethanolUremiaDiabetic KetoacidosisParaldehydeInfection (lactic acid)Ethylene GlycolSalicylate

Question: Should I calculate an anion gap when there is no acidemia?

Step 7Does the anion gap explain the change in bicarbonate?

D anion gap (Anion gap -12) ~ D [HCO3]

If D anion gap is greater; consider additional metabolic alkalosis

If D anion gap is less; consider a nonanion gap metabolic acidosis

To conclude

• Spend time with the patient and try and make sense of CXR and ABG .

• Continuous effort is required to master them.• Interpretation of both xrays and ABG have to

take the clinical context.

THANK YOU