Capnography in ICU

Shari McKeown, RRT

Overview

• Mainstream sensor displays real-time, continuous carbon dioxide level throughout the respiratory cycle by measuring absorption of infrared light by CO2 molecules

What does the waveform mean?

A-B exhalation begins - anatomical and ETT deadspace, no CO2

B-C exhalation continues - anatomical deadspace mixed with alveolar gas, increasing CO2

C-D alveolar gas, high CO2 with upward slope due to continuing CO2 production and emptying of all alveolar units

D end-tidal CO2. The highest CO2 value at end-expiration

D-E inspiration begins, CO2 rapidly drops

E-A inspired gas contains no CO2

Why does the CO2 level always slope upwards to end-tidal?• As expiration progresses, basal lung units empty last –

these contain a higher CO2 level (lower V/Q ratio)

www.capnography.com Bhavani Shankar Kodali MD

• CO2 production continues throughout expiration, resulting in a higher CO2 at the end of the breath

What increases PETCO2?

• Increased CO2 Production– Increased metabolic rate

• Fever• Seizures• Shivering• Pain

– Bicarbonate infusion

• Increased delivery of CO2 to lungs– Increased cardiac output– Hypertension

• Reduced clearance of CO2 from lungs– Hypoventilation– Mainstem bronchus intubation (ETT in one lung)– Partial airway obstruction

What decreases PETCO2?• Decreased CO2 Production

– Decreased metabolic rate• Hypothermia• Analgesia• Sedation

• Decreased delivery of CO2 to lungs– Decreased cardiac output– Hypotension– Hypovolemia– Pulmonary Embolism– Cardiac Arrest

• Rapid clearance of CO2 from lungs– Hyperventilation

• No communication with alveolar gas– Total airway obstruction– Accidental tracheal extubation– Apnea

• Increased alveolar deadspace– High PEEP

• Technical Errors– Circuit disconnection– Leaks

Cardiac Output

• Decreasing cardiac output will reduce pulmonary blood flow, causing a decrease in alveolar perfusion and increased alveolar deadspace

• A higher alveolar deadspace will result in lower ETCO2 values and higher Pa-ETCO2 gradient.

• Under conditions of constant lung ventilation, ETCO2 can be used as a monitor of pulmonary blood flow.

www.capnography.com Bhavani Shankar Kodali MD

CPR• During CPR, blood flow to the lungs is low and few

alveoli are perfused• Tidal volumes delivered with a resuscitation bag tend to

be large, high deadspace results in PETCO2 is low• If the blood flow to the lungs improves, more alveoli are

perfused and PETCO2 will increase• C02 presentation to the lungs is the major limiting

determinant of PETCO2 and it has been found that PETCO2 correlates well with measured cardiac output during resuscitation

• Therefore PETCO2 can be used to judge the effectiveness of resuscitative attempts

• PETCO2 has a prognostic significance. It has been observed that non-survivors had lower PETCO2 during CPR than survivors.

How does PETCO2 correlate with PaCO2?

• Normal gradient of (a-ET)PCO2 is 2-5 mmHg, and will increase with age

• This is due to normal ventilation/perfusion (V/Q) mismatching throughout the lung

• An increased gradient reflects increased deadspace - alveoli that are ventilated but not perfused will have low CO2; when exhalation occurs, this results in a higher Pa-ETCO2 gradient

• Pa-ETCO2 gradient will decrease in pregnancy reflecting the higher cardiac output and pulmonary perfusion in the pregnant patient

• PETCO2 should always be recorded when ABG’s are taken to trend the Pa-ETCO2 gradient

Record hourly

Record when ABG drawn

How can you use Pa-ETCO2 gradient for PEEP titration?

• Pa-ETCO2 gradient is a good reflection of alveolar deadspace

• When V/Q is at its best (optimum PEEP) the Pa-ETCO2 gradient is low. Oxygenation should be optimal.

• As the level of PEEP is increased beyond this, alveolar deadspace increases, the Pa-ETPC02 increases, and oxygenation worsens.

• Pa-ETC02 can be used as a sensitive indicator to titrate PEEP in patients with early ARDS or with alveolar edema

What information can you get by looking at the waveform?

• The shape of a capnogram is identical in all humans with healthy lungs. Any deviations in shape must be investigated to determine a physiological or a pathological cause of the abnormality

Normal waveformwww.capnography.com Bhavani Shankar Kodali MD

Slanting of upstroke

• Occurs when there is obstruction to expiratory gas flow • e.g. asthma, bronchospasm, obstructive pulmonary

disease, and kinked endotracheal tube

Normal Airway obstruction

www.capnography.com Bhavani Shankar Kodali MD

Patient Efforts

• A sudden decrease during expiratory phase indicates spontaneous patient effort

• Waveform can be used to identify missed ventilator triggers that lead to patient-ventilator asynchrony

Normal Patient Effort

www.capnography.com Bhavani Shankar Kodali MD

Cardiac Oscillations

• Ripple during expiratory phase indicate small movements in alveolar gas

• Caused by cardiac or aortic pulsations against alveoli

Normal Cardiac Oscillations

www.capnography.com Bhavani Shankar Kodali MD

Heterogeneous Lung Pathology

• Lungs with differing compliance/resistances (e.g. single-lung transplant) will have different empyting rates, CO2 clearance times, and V/Q ratios

• May result in dual-peak or dual-slope waveforms

Normal Heterogenous V/Q ratios

www.capnography.com Bhavani Shankar Kodali MD

Waveform Trends• Hypoventilation or patient fatigue (e.g. during CPAP

trials) may result in gradual increase in ETCO2 over time (normal Pa-ETCO2)

• Sweep speed can be decreased to illustrate gradual trending

www.capnography.com Bhavani Shankar Kodali MD

Waveform Trends• Hyperventilation may result in gradual decrease in

ETCO2 over time (normal Pa-ETCO2)• This trend may also be caused by a patient with

autopeep – incomplete exhalation results in alveolar gas not reaching airway

(increased Pa-ETCO2)

www.capnography.com Bhavani Shankar Kodali MD

Clinical applications

• Estimate PaCo2• Estimate alveolar deadspace• Optimal PEEP setting• Verify ETT placement• Monitor adequacy of ventilation• Evaluate weaning trial• Monitor effectiveness of CPR• Assess pulm blood flow• Assess effectiveness of bronchodilators• Detect patient/ventilator asynchrony• Immediate alert to accidental extubation, large

pulmonary embolism, apnea, circuit disconnection, leaks• Trend metabolic rate

Capnography in ICU

Shari McKeown, RRT