Transcript Slide 1
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? C-D alveolar gas, high CO2 with upward slope due to continuing CO2 production and emptying of all alveolar units A-B exhalation begins anatomical and ETT deadspace, no CO2 D end-tidal CO2. The highest CO2 value at endexpiration B-C exhalation continues - anatomical deadspace mixed with alveolar gas, increasing CO2 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) • CO2 production continues throughout expiration, resulting in a higher CO2 at the end of the breath www.capnography.com Bhavani Shankar Kodali MD 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 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, 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