Physiology
Carbon dioxide is found in the blood of all living humans as it is a major by-product of metabolism. With a normally functioning cardiopulmonary system, that carbon dioxide is carried out of the body and eliminated by lung exhalation. If this carbon dioxide is not eliminated, it can cause respiratory acidemia with a myriad of complications to the patient including cardia arrhythmias. The creation of a pneumoperitoneum increases the amount of carbon dioxide in the body. Carbon dioxide can be absorbed when coming into contact with the peritoneum or can be inadvertently introduced into an injured blood vessel. Carbon dioxide gas can also unintentionally be introduced into the subcutaneous tissue causing subcutaneous emphysema. It is thought this occurs in up to 2% of laparoscopies leaving another reservoir of carbon dioxide that must be ventilated off7. One study found that peak serum carbon dioxide levels occur 10 minutes after insufflation3,8and persist for 45 minutes after the pneumoperitoneum is released3. Most patients, especially when ventilated, can easily clear this excess carbon dioxide. Anesthesiologists often adjust minute ventilation to account for this increase in carbon dioxide uptake, allowing a normal acid-base status to be maintained during surgery.
The act of creating the pneumoperitoneum does create some boundaries to clearing that carbon dioxide. The pneumoperitoneum places upwards pressure on the abdominal wall to increase visualization but also increases pressure on the diaphragm and inferior vena cava. Pressure on the diaphragm decreases lung volume and therefore gas exchange making it harder for carbon dioxide to be eliminated. Trendelenburg position is frequently used in laparoscopy and it also causes pressure on the diaphragm by shifting the abdominal consents against the diaphragm. All of these changes serve to worsen lung compliance, lowering tidal volumes and increasing the risk of barotrauma with mechanical ventilation. While well-tolerated in young and healthy patients, this ventilator impairment can pose a challenge for older or morbidly-obese patients. Pressure on the inferior vena cava will also decrease blood return to the heart and hence cardiac output. Therefore, less carbon dioxide makes it to the lungs to be eliminated and heart perfusion is decreased. Decrease in myocardial oxygen delivery also leads to arrhythmia. One study found that when the pneumoperitoneum exceeds 15 mmHg cardiac output can be decreased by 28%4. Again, these physiological changes can be compensated for in a healthy ventilated patient. Obese patients and patients with pre-existing heart and lung disease will need closer monitoring to compensate for their pathophysiology7. The speed at which the pneumoperitoneum is created also has effects on the sympathetic nervous system. The rapid stretching of the peritoneum with insufflation greatly increases vagal tone8. This rush of sympathetic response leads to hypotension and bradycardia even to the point of asystole.