Capnography in ICU Shari Mc Keown RRT Overview

Capnography in ICU Shari Mc. Keown, RRT

Overview • Mainstream sensor displays real-time, continuous carbon dioxide level throughout the respiratory cycle by measuring absorption of infrared light by CO 2 molecules

What does the waveform mean? C-D alveolar gas, high CO 2 with upward slope due to continuing CO 2 production and emptying of all alveolar units A-B exhalation begins anatomical and ETT deadspace, no CO 2 D end-tidal CO 2. The highest CO 2 value at endexpiration B-C exhalation continues - anatomical deadspace mixed with alveolar gas, increasing CO 2 D-E inspiration begins, CO 2 rapidly drops E-A inspired gas contains no CO 2

Why does the CO 2 level always slope upwards to end-tidal? • As expiration progresses, basal lung units empty last – these contain a higher CO 2 level (lower V/Q ratio) • CO 2 production continues throughout expiration, resulting in a higher CO 2 at the end of the breath www. capnography. com Bhavani Shankar Kodali MD

What increases PETCO 2? • Increased CO 2 Production – Increased metabolic rate • • Fever Seizures Shivering Pain – Bicarbonate infusion • Increased delivery of CO 2 to lungs – Increased cardiac output – Hypertension • Reduced clearance of CO 2 from lungs – Hypoventilation – Mainstem bronchus intubation (ETT in one lung) – Partial airway obstruction

What decreases PETCO 2? • Decreased CO 2 Production – Decreased metabolic rate • Hypothermia • Analgesia • Sedation • Decreased delivery of CO 2 to lungs – – – Decreased cardiac output Hypotension Hypovolemia Pulmonary Embolism Cardiac Arrest • Rapid clearance of CO 2 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 ETCO 2 values and higher Pa-ETCO 2 gradient. • Under conditions of constant lung ventilation, ETCO 2 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 PETCO 2 is low • If the blood flow to the lungs improves, more alveoli are perfused and PETCO 2 will increase • C 02 presentation to the lungs is the major limiting determinant of PETCO 2 and it has been found that PETCO 2 correlates well with measured cardiac output during resuscitation • Therefore PETCO 2 can be used to judge the effectiveness of resuscitative attempts • PETCO 2 has a prognostic significance. It has been observed that non-survivors had lower PETCO 2 during CPR than survivors.

How does PETCO 2 correlate with Pa. CO 2? • Normal gradient of (a-ET)PCO 2 is 2 -5 mm. Hg, 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 CO 2; when exhalation occurs, this results in a higher Pa-ETCO 2 gradient • Pa-ETCO 2 gradient will decrease in pregnancy reflecting the higher cardiac output and pulmonary perfusion in the pregnant patient • PETCO 2 should always be recorded when ABG’s are taken to trend the Pa-ETCO 2 gradient Record hourly Record when ABG drawn

How can you use Pa-ETCO 2 gradient for PEEP titration? • Pa-ETCO 2 gradient is a good reflection of alveolar deadspace • When V/Q is at its best (optimum PEEP) the Pa-ETCO 2 gradient is low. Oxygenation should be optimal. • As the level of PEEP is increased beyond this, alveolar deadspace increases, the Pa-ETPC 02 increases, and oxygenation worsens. • Pa-ETC 02 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 waveform www. 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. singlelung transplant) will have different empyting rates, CO 2 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 ETCO 2 over time (normal Pa-ETCO 2) • 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 ETCO 2 over time (normal Pa-ETCO 2) • This trend may also be caused by a patient with autopeep – incomplete exhalation results in alveolar gas not reaching airway (increased Pa-ETCO 2) www. capnography. com Bhavani Shankar Kodali MD

Clinical applications • • • Estimate Pa. Co 2 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 Mc. Keown, RRT