Why is
carbon dioxide (CO2) the gas used for insufflation?
Many gases have been utilized to create the
pneumo-peritoneum to facilitate the exposure necessary for surgical laparoscopy.
These gases include helium, argon, nitrous oxide (N2O), and CO2.
Each gas has problems associated with its use. Nitrous oxide supports
combustion. Helium and argon are insoluble and are more likely to be associated
with adverse events following embolic phenomena. CO2 pro-vides the
safest profile. It is safe during electrocautery and laser surgery and can
easily be eliminated through the lungs. Additionally, CO2 is rapidly
absorbed into the blood-stream. Its absorption is greater during extraperitoneal
insufflation (laparoscopic inguinal hernia repair) com-pared with
intraperitoneal insufflation. Extraperitoneal insufflation occurs when gas
accumulates accidentally in the subcutaneous tissues or in the potential space
between the fascia and the peritoneum, as is the goal for inguinal hernia
repair. This absorption proves beneficial if moderate hypercapnia is maintained
because the resulting cardiovas-cular stimulation helps to offset some of the
hemodynamic burden imposed by the pneumoperitoneum. However, intraoperative
hypercapnia is not completely benign. In patients with poor reserve (i.e.,
ischemic heart disease), whose arterial CO2 (PaCO2)
levels approach 55–65 mmHg, there is a significant increase in systolic blood
pressure, heart rate, and cardiac output. Hypercarbia causes sympa-thetic
nervous system stimulation as manifest by a 2- to 3-fold increase in plasma
catecholamine concentrations. When the PaCO2 is >65 mmHg,
cardiodepressive effects predominate with possible cardiovascular collapse or fatal
dysrhythmias. Hypercapnia also causes pulmonary vaso-constriction that may
worsen right ventricular ischemia or pulmonary hypertension. Patients with
increased intra-cranial pressure may also be adversely affected by increases in
PaCO2.
End-tidal CO2 (ETCO2)
provides an estimation of PaCO2 levels. In relatively healthy
individuals, the PaCO2–ETCO2 gradient approximates 3–5
mmHg, and is not affected dur-ing short laparoscopic procedures. However, in
patients with severe cardiopulmonary disease or in prolonged operations, the
PaCO2–ETCO2 gradient increases in an unpredictable manner
and the usual PaCO2–ETCO2 relationship may be lost.
Not all increases in PaCO2 result
from increased absorp-tion. Absorption is responsible for the increases in PaCO2
that occur initially, until a plateau level is reached 15–30 minutes after the
onset of gas insufflation. Any sig-nificant increases thereafter require a
search for the cause. The differential diagnosis of hypercarbia is:
· Absorption of CO2
· Hypoventilation
· Increased dead space
· CO2 embolism
· Pneumothorax, pneumomediastinum,
pneumoperi-cardium
· Subcutaneous emphysema
· Exhausted CO2 absorber
· Unidirectional valve dysfunction
·
Malignant
hyperthermia
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