Capnography for the intensivist

Sarah Philipson

THE END.

THE END.Questions?

Questions?CO2

physiologyWhat is

capnography?

Questions?What is

capnography?CO2

physiology

Questions?What is

capnography?CO2

physiology

How is it used?

Questions?What is

capnography?CO2

physiology

How is it used? Do I care?

What are the

problems with it?

Capnography

• Measurement of CO2 vs time• Infrared spectroscopy measures the fraction

of energy absorbed and converts this to a percentage of CO2 exhaled

• Normal capnogramI – inspiratory baselineII – expiratory upstrokeIII – alveolar plateauIV – inspiratory downstroke

Capnography

• Measurement of CO2 vs time• Infrared spectroscopy measures the fraction

of energy absorbed and converts this to a percentage of CO2 exhaled

• Normal capnogramI – inspiratory baselineII – expiratory upstrokeIII – alveolar plateauIV – inspiratory downstroke

Normal EtCO2 = 38-40mmHg

Capnography

• Measurement of CO2 vs time• Infrared spectroscopy measures the fraction

of energy absorbed and converts this to a percentage of CO2 exhaled

• Normal capnogramI – inspiratory baselineII – expiratory upstrokeIII – alveolar plateauIV – inspiratory downstroke

A-B: Dead space

B-C: Dead space + alveoli

C-D: Alveoli

D: ETCO2

D-E: Inspiration

How we measure CO2

Physiology – “ICU is easy!”

Carbon Dioxide

PRODUCTION AT TISSUES

Carbon Dioxide

PRODUCTION AT TISSUES

TRANSPORT IN BLOOD

Carbon Dioxide

PRODUCTION AT TISSUES

TRANSPORT IN BLOOD

REMOVAL VIA VENTILATION

Carbon Dioxide

PRODUCTION AT TISSUES

TRANSPORT IN BLOOD

REMOVAL VIA VENTILATION

Carbon Dioxide

PRODUCTION AT TISSUES

TRANSPORT IN BLOOD

REMOVAL VIA VENTILATION

Carbon Dioxide

PRODUCTION AT TISSUES

TRANSPORT IN BLOOD

REMOVAL VIA VENTILATION

CO2 production

• Produced in tissues through cellular respiration – glycolysis, Krebs cycle, phosphorylation

CO2 up CO2 down

Increased metabolic rate-Sepsis- Hyperthermia-Burns-Trauma-Hyperthyroidism-Shivering-Malignant hyperthermia-Neuroleptic malignant syndrome

Decreased metabolic rate-Hypothermia-Starvation-Drugs for hyperthyroidismMetabolic acidosis

CO2 production

CO2 transport• Diffuses across capillary membranes and is transported to lungs through

the venous system– ~7% transported dissolved in blood– ~20% as carbaminohaemoglobin (reaction between carbon dioxide

and the amine radicals of the haemoglobin molecule)– ~70% as bicarbonate and hydrogen ions from dissociation of carbonic

acid

CO2 transport i.e. cardiac output• Diffuses across capillary membranes and is transported to lungs through

the venous system– ~7% transported dissolved in blood– ~20% as carbaminohaemoglobin (reaction between carbon dioxide

and the amine radicals of the haemoglobin molecule)– ~70% as bicarbonate and hydrogen ions from dissociation of carbonic

acid

CO = SV x HR

CO2 transportCO2 up CO2 down

Tourniquet release Arrest

Shock

Drugs- Carbonic anhydrase inhibitor (acetazolamide) – prevents CO2 transportShunting eg. PE

CO2 removal i.e. ventilation

• Ventilation = rate, volume, diffusion

CO2 removal i.e. ventilationCO2 up CO2 down

Low RR- Drugs – sedatives, opiates- Neurological causes

High RR-Acidotic-Psychological-Ventilator settings

Diffusion impaired -Chronic lung disease-Inflammation – infection/inflammatory process

Impaired ventilation-APO-Intrapulmonary shunt: atelectasis, collapse, haemo/pneumothorax, effusion

Low volumes-Ventilator settings-Poor compliance-Equipment – leak, tube placement

High volumes- Ventilator settings eg. PS too high

ETCO2 - Why is it useful?Reflects changes in:- Ventilation

- Can predict impending respiratory failure- Provides early warning of airway compromise

- Transport- Can be used as a predictor of fluid responsiveness – found to

be proportional to CI in measuring response to passive leg raise in patients with stable metabolic and respiratory conditions

- Production- Metabolism

Have I convinced you?

Problems with capnography- Only reliable(?) in patients with stable metabolic

and respiratory states- Abnormal Aa gradients make EtCO2 not a good

predictor of PaCO2, but can still use trend- Needs to be a trend, not a one-off measure- Detects, does not diagnose – more tests!- Difficulty with equipment – easily clogged with

water droplets- Normal capnogram can occur with glottic

intubation – still need an XR

Convinced?

• Capnography• CO2 physiology– Production– Transport– Ventilation

• The capnography curve and what it can tell us• Problems with capnography

KEEP CALMAND

WATCH THE CO2