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.