The monitoring of end-expiratory CO2 concentration or partial pressure (ETCO2) can reflect pulmonary ventilation, and qi can also reflect pulmonary blood flow. When there is no obvious cardiopulmonary disease and the V/Q ratio is normal. ETCO2 can reflect PaCO2 _ 2, and 5% of normal ETCO 2 is equivalent to 5KPa(38mmHg).
Indications for monitoring: 1. Safe use of anesthesia machine and ventilator. 2, all kinds of respiratory insufficiency. 3. Cardiopulmonary resuscitation. 4. Severe shock. 5. Heart failure and pulmonary infarction. 6. Determine the position of tracheal intubation under general anesthesia.
Second, the clinical evaluation
When using ventilator and anesthesia, adjust ventilation according to ETCO2 measurement to make ETCO2 close to preoperative level. Monitoring and its waveform can also determine whether the endotracheal tube is in the airway. For patients undergoing mechanical ventilation, such as air leakage, catheter distortion, tracheal obstruction and other faults, the digital and morphological changes of ETCO2 can immediately appear and alarm, and be found and handled in time. Continuous monitoring provides a basis for the safe evacuation of mechanical ventilation. However, malignant high fever, elevated body temperature and intravenous injection of a large amount of sodium bicarbonate can increase the output of CO2 and increase the range of ETCO2. In shock, cardiac arrest, pulmonary air embolism or thromboembolism, the reduction of pulmonary blood flow can immediately reduce the CO2 depth to zero. ETCO2 also helps to judge the effectiveness of cardiopulmonary resuscitation. If ETCO2 is too low, factors such as hyperventilation should be excluded.
(a) the determination principle of ETCO2:
The advent of exhaled carbon dioxide monitoring curve is another great progress in noninvasive monitoring of lung function, especially pulmonary ventilation function, which makes it possible to continuously and quantitatively monitor patients by bedside, especially providing clear indicators for respiratory support and respiratory management of anesthesia patients, ICU and respiratory department.
The so-called carbon dioxide curve can be obtained by tracking the measured carbon dioxide concentration one-to-one with the corresponding time during breathing. The standard curve is divided into four parts, namely ascending branch, alveolar plateau, descending branch and baseline. Exhale starts from point P of ascending branch and passes through point Q to point R. QR stands for alveolar plateau period (also called peak period), and point R stands for carbon dioxide concentration at the end of exhalation (also called tidal end). The beginning of descending branch means the beginning of inhalation. With the inhalation of fresh gas, the concentration of carbon dioxide gradually returns to the baseline. Therefore, P.Q.R is the expiratory phase and R.S.P is the inspiratory phase. The area between the curve and the baseline can be compared with the carbon dioxide emissions.
The most commonly used method is infrared absorption spectrometry, the principle of which is that when infrared light passes through a gas sample, its absorption rate is related to the concentration of carbon dioxide (carbon dioxide mainly absorbs infrared light with a wavelength of 4260nm), and the reaction is rapid and the determination is convenient. At the same time, there are other methods such as mass spectrometry, Roman spectrometry, photoacoustic spectrometry, carbon dioxide chemical electrode method and so on.
According to the position of the sensor in the airflow, the common sampling methods are mainstream sampling and side hole sampling. Mainstream sampling is to connect the sensor in the patient's airway, which has the advantages of direct contact with airflow and fast recognition response. The secretion or water vapor in the airway has little effect on the monitoring effect; No gas loss. The disadvantage is that the sensor is heavy; Add extra dead space (about 20 ml); Not suitable for patients without endotracheal tube. Side hole sampling is to continuously suck some gas from the airway through the sampling tube, and the sensor is not directly connected to the ventilation loop, which does not increase the dead zone of the loop; Will not increase the weight of parts; For patients without endotracheal tube, the improved sampling tube can still make accurate determination through nasal cavity. The disadvantage is that the recognition response is a bit slow; Sampling is affected by water vapor or secretions in the airway; During low-flow anesthesia or pediatric anesthesia, attention should be paid to supplementing the amount of gas lost due to sampling. At present, most monitors adopt side hole sampling method.
(2) Interpretation of common clinical carbon dioxide curves:
1. Excessive end-expiratory carbon dioxide: Its important physiological significance is insufficient alveolar ventilation or increased CO2 input into alveoli. There are often the following four situations, and the curves and graphs are different. ① The respiratory frequency and peak phase are normal, but the value of ETCO2 is higher than normal. It is common in patients with artificial ventilation, and their scheduled breathing frequency can be normal, but the minute ventilation is too low, or CO2 production increases due to changes in illness, such as malignant high fever. ② Breathing is slow, the peaks are long, and ETCO2 is higher than normal. It can be seen as follows: the increase of intracranial pressure and the inhibitory effect of narcotic analgesics such as pethidine and fentanyl on breathing; When the breathing frequency and ventilation per minute are too low. ③ Hyperrespiration, short peak period and higher ETCO2 than normal. In shallow and rapid breathing, try to compensate for the inhibition of breathing by increasing the breathing frequency, such as patients breathing spontaneously by inhaling a volatile anesthetic; During mechanical ventilation, the breathing frequency is faster, but the tidal volume is insufficient. (4) Severe hypoventilation worthy of vigilance, characterized by shortness of breath, extremely low tidal volume, and most of the peak values are abnormal. The true CO2 value can only be seen after pressing the chest or exhaling forcefully once. This can be seen in the spontaneous breathing of patients with severe respiratory muscle paralysis; Ventilator failure or air leakage in loop system during mechanical ventilation.
2. Low end-expiratory carbon dioxide: mainly due to alveolar hyperventilation or decreased CO2 input into alveoli. There are three situations.
① Respiratory frequency and peak phase are normal, but ETCO2 is too low. It is found in mechanical ventilation with excessive tidal volume; Patients with shock and hypothermia; It can also be seen in patients with metabolic acidosis who spontaneously breathe during compensatory period. ② Bradycardia, long peak period and low ETCO2 value. Such as artificial ventilation, the frequency is too slow and the tidal volume is too large; Central hyperventilation can occur in patients with central nervous system diseases, and similar symptoms can occur when hypothermia occurs. ③ Hyperrespiration, short peak period and low ETCO2 value. The artificial ventilation frequency and tidal volume are too high; The patient suffers from pain, metabolic acidosis, hypoxemia, severe shock or hyperventilation of the central nervous system.
3. Ginger-like residual effect: it is more common in the initial stage of patients' spontaneous breathing and ventilator confrontation; Dyskinesia of intercostal muscle and diaphragm; Cervical nerve damage. The main feature is that ETCO2 is slightly higher, and there is a crack on the right side of the peak phase 1/3, and its depth is inversely proportional to the degree of muscle paralysis. If the patient is recovering from anesthesia or receiving respiratory support treatment, it is necessary to wait until the fissure disappears before pulling out the tracheal intubation, because this indicates that there is ventilation disorder.
4. Cardiogenic vibration wave: it is caused by central respiratory depression or slow ventilator frequency, which is caused by beating the lungs with heartbeat. The breathing curve of low frequency and small tidal volume appears after a long period of end-expiratory, which is synchronous with the heartbeat and can be slightly higher.
5. iceberg curve: it is more common in the recovery period after using muscle relaxants and narcotic analgesics. The spontaneous breathing frequency is low, the peak phase is discontinuous, like the melting of an iceberg, and the ETCO2 value is higher than the normal value.
6. Other forms of curves: such as hump curves, mostly appear when the patient lies on his side. When there is air leakage in the breathing circuit, the carbon dioxide curve is irregular, which can increase the carbon dioxide value due to the decrease of ventilation or decrease due to air mixing.
(3) Interpretation of CO2 curve trend chart:
The trend chart of carbon dioxide curve can also provide us with many reference materials.
1. Respiratory rhythm disorder: only seen in patients with spontaneous breathing. ① Chen's breathing, all groups have cardiogenic vibration (arrow) after breathing, which can be seen in severe cerebral arteriosclerosis, brain injury, alcoholism or critical patients; ② Breathing: The breathing frequency is slow (2-6 beats/min), exceeding the normal CO2 value, and cardiogenic vibration often appears after each CO2 curve (arrow). It is seen in patients with very severe respiratory depression or dying; (3) Sigh-like breathing: The curves and waveforms are regular, interrupted by regular intermittent deep sighs, and the CO2 value can be higher or lower than normal or normal. This mode can also be obtained if the intermittent deep breathing function is used during artificial ventilation. The CO2 value of normal lung is lower than the average value when breathing deeply (Figure C-a), and that of patients with obstructive pulmonary disease is higher than the average value when breathing deeply (Figure C-b). Young people or the elderly sigh deeply 1 time or more within 5 minutes, which should be considered as pathological and indicate brain injury. Under physiological conditions, babies, or the elderly can also have deep sighs under sleep or anesthesia; ④ Irregular breathing: In patients with severe brain injury, the wave size, shape and height of each curve are irregular, and the average value of CO2 is higher than the normal value.
2. Confrontation between human and ventilator: When the patient resumes spontaneous breathing, it is easy to confront the ventilator, which is characterized by the regular interruption of CO2 curve and spontaneous breathing curve. With the rapid increase of patients' respiratory movement, the uncoordinated activities of respiratory muscles increase the metabolic rate of the body, and at this time, the end-tidal CO2 increases slightly (Figure A). In the figure, B is the CO2 curve generated by the ventilator, and C is the CO2 curve of the patient's spontaneous breathing. When this pattern occurs during anesthesia, it indicates that muscle relaxants need to be added (Figure B.C.).
3.CO2 curve drops: ①CO2 suddenly drops to zero or extremely low level, indicating technical failure. Such as distortion of sampling tube, failure of CO2 instrument, detachment of tracheal catheter from trachea or respiratory circuit, failure of ventilator, etc. ②CO2 suddenly drops, but it is less than zero. It is common that the breathing tube leaks and the airway pressure drops; Or when the airway is blocked, the peak phase transition is very small and there is no flat top, and the airway pressure increases at this time; ③CO2 gradually decreases in a short time (1 ~ 2 min), which often indicates a sudden change in pulmonary circulation or ventilation. Such as cardiac arrest, pulmonary infarction, severe blood pressure reduction and severe hyperventilation. ④CO2 decreased gradually and the curve was normal. It is more common when ventilation increases gradually, body temperature decreases, and systemic or pulmonary perfusion decreases.
4.CO2 curve rises: ①CO2 curve rises gradually. It can be seen in situations such as insufficient ventilation, gradual absorption of CO2 injected during laparoscopic examination or operation, and unexpected increase of body temperature. ② ② The ②②CO2 curve suddenly rises. After rapid injection of sodium bicarbonate, it can temporarily rise, when the tourniquet of the limb is suddenly loosened or the blood pressure suddenly rises; ③ The baseline and top line of CO2 move up gradually, which is common in the technical calibration error of CO2 analyzer and the failure of CO2 absorbent, leading to repeated breathing (Figure 85- 17A. BC).
5. Evaluation of cardiopulmonary resuscitation: In the process of cardiopulmonary resuscitation, CO2 curve can reflect the effectiveness of cardiopulmonary perfusion, on the one hand, it can monitor whether the patient's exhaled CO2 meets the physiological requirements, on the other hand, it can predict the effect of resuscitation.
(four) the relationship between carbon dioxide curve and other monitoring parameters:
CO2 curve, sometimes with other parameters, such as heart rate, blood pressure, pulse volume curve (Pleth), body temperature, PaCO2 _ 2, PaO2 _ 2 or SaO2 _ 2, airway pressure, CVP, pH, etc. The correct explanation and differential diagnosis can be obtained by comprehensive analysis. Especially for patients in anesthesia or ICU, multi-parameter monitoring is beneficial to the correct judgment of the condition. Commonly used and intuitive comprehensive parameters include electrocardiogram, peripheral pulse volume diagram, blood pressure and carbon dioxide curve. The following introduces several common clinical differential diagnosis.
1. When the heart rate increases, blood pressure increases, volume flow amplitude changes, and ETCO2 increases, it is possible that ① large doses of adrenaline drugs (such as local anesthesia) have been used; ② The surgical procedure of pheochromocytoma; ③ The stimulation of pain may be an indication of adding narcotic drugs; ④ Patients who wake up after anesthesia.
2. Tachycardia or bradycardia, ST segment decreased, accompanied by obvious hypotension, amplitude of plethysmography curve decreased, and ETCO2 curve also decreased. The above situation is common in patients with severe circulatory dysfunction. Such as: ① severe blood loss; ② Anaphylactic shock; ③ Cardiac insufficiency; ④ Inhibitory effect of drugs on myocardium. At this time, it is often suggested that the patient is in a dangerous state.
No obvious abnormality was found in 3.3 cases. Electrocardiogram or bradycardia, the amplitude of volume wave increased rapidly, blood pressure remained at a low level, and ETCO2 did not change significantly. This may be the result of α receptor blocker, ganglion blocker or anesthetic. For example, during the induction period of anesthesia, due to vasodilation, peripheral perfusion improved and blood pressure decreased slightly. As long as there is no obvious change in ETCO2 curve, the patient is not in danger.
4. When pulmonary embolism occurs, frequent ventricular premature beats or hypoxic changes may occur in ECG within a few minutes. Pleth amplitude first widened, then almost became a straight line, blood pressure decreased obviously, and ETCO2 curve decreased sharply within 1 min. This kind of typical signs of pulmonary infarction often appear in surgery, such as air embolism, fat embolism, amniotic fluid embolism and cardiovascular embolus falling off. And patients are often in extreme danger. Even for small embolism that is not life-threatening, these changes will take 5 ~ 10 minutes to return to the original level.
CO2 curve of pulmonary embolism (air embolism, fat embolism, amniotic fluid embolism, thrombosis, etc.). )
5. When the heart stops beating, the typical manifestations are as follows: ① ECG shows that ventricular premature beats gradually stop; ② ② The amplitude of ②②Pleth decreases and becomes a straight line; ③ Blood pressure drops to zero; (4) (4) CO2 curve is a flushing curve, which cannot be returned to zero; If there is no sign of improvement after rescue measures, it indicates that the patient is on the verge of death.
CO2 curve of cardiac arrest
6. ECG graph suddenly disappears, Pleth becomes a straight line, blood pressure drops to zero rapidly, but ETCO2 curve is normal. This is the fault of the monitoring instrument, and the patient's life is not in danger.
Instrument failure (hemodynamic monitor) CO2 curve is normal.
7. ECG is normal, plethysmography is normal, blood pressure is normal, and ETCO2 curve drops rapidly. In case of the above situation, immediately check whether the patient's ventilation loop or tracheal catheter falls off or leaks, and then find out whether the monitoring device is faulty, which may be the fault of the ETCO2 monitor (poor line contact, sensor needs to be recalibrated, etc.). ).
ETCO2 monitor fails when the ventilation loop or endotracheal tube falls off.
ETCO2 curve monitoring is intuitive and fast, which is not only an index of pulmonary ventilation efficiency, but also provides a reference for circulatory function and the relationship between them. It has become one of the important monitoring indicators for patients undergoing anesthesia surgery and critically ill patients, and the health departments in some countries have listed this monitoring technology as one of the basic working conditions for carrying out anesthesia surgery.