EXCERPT FROM NEW TEXTBOOK PREVENTION AND MANAGEMENT OF LAPAROENDOSCOPIC SURGICAL COMPLICATIONS, 2ND EDITION
W. BOSSEAU MURRAY, MB, ChB, MD
This chapter will emphasize the differences in monitors required and anesthesia techniques used (compared with those of open abdominal surgery), based on the physiological challenges presented by laparoscopic surgery.
CHOICE OF ANESTHETIC TECHNIQUE
For laparoscopic surgery, 3 types of anesthesia are available: local, regional, and general anesthesia.
Local anesthesia involves infiltrating the surgical sites with a local anesthetic agent (bupivacaine, lidocaine, and others) to enable a pain-free incision. The abdominal cavity is not anesthetized, and the surgeon needs to minimize manipulation and pneumoperitoneum pressure to avoid patient discomfort. Intravenous agents, such as narcotics (fentanyl and morphine) and sedatives (typically midazolam), can be used for patient comfort.
Regional anesthesia involves injecting a local anesthetic near the spinal cord (epidural space) or the cauda equina (lumbar subarachnoid space.) By manipulation of the amount of local anesthetic and patient positioning, the height of the block can be varied. Typically, the block needs to extend to a thoracic level (T4 or nipple level) for laparoscopic surgery.
During local and regional anesthesia, intravenous agents may be used to keep the patient calm and comfortable: this is called monitored anesthesia care (MAC) or local standby (LOSTBY.)
It is useful to be aware that laparoscopic surgery can be performed with the patient under local or regional anesthesia. For instance, a patient with a recent lung transplant (or pneumonia or lung abscess) may need a laparoscopic appendectomy.
The vast majority of laparoscopic surgeries are performed with the patient under general anesthesia. The classical triad of general anesthesia was originally described as unconsciousness, analgesia, and muscle relaxation. Further goals include lack of awareness (lack of patient remembering events during surgery), and attenuation of sympathetic as well as parasympathetic nervous system hyperreactivity.
CHOICE OF MONITORS
The monitors used by anesthesiologists for laparoscopic surgery are similar to the monitors used for any surgery and anesthesia. However, the implications of monitor variations, and the interpretation of such changes are often different during laparoscopic surgery.
Standards of Monitoring
The American Society of Anesthesiologists (ASA) has developed standards for monitoring of a patient who is under any form of anesthesia: local, regional, or general anesthesia. These standards include:
1. a vigilant anesthesiologist. This is the most
important monitor;
2. monitors of a patient's ventilation, oxygena-
tion, circulation, and temperature;
3. automated noninvasive blood pressure mon-
itor (NIBP);
4. end-tidal carbon dioxide analyzer (capno-
graph); and
5. a temperature probe (esophageal or skin.)
Due to the many erroneous measurements and false alarms caused by electrical cautery (diathermy) and movement, continued observation of the patient's skin color is advisable.
The required monitors of anesthesia machine function include an analyzer of delivered gaseous inspired oxygen concentration and a ventilator-disconnect alarm. Most manufacturers add several machine-specific monitors of anesthesia machine function.
Capnography
Laparoscopic surgery necessitates a special understanding of the interaction of physiology and the monitors. For instance, the capnograph may be the earliest of the standard 5 monitors to detect a venous gas embolism [4]. Due to the obstruction, the gas embolism occludes pulmonary arterial blood flow to a number of alveolar-capillary lung units, leading to the development of alveolar (parallel) dead space. (Physiological [total] dead space consists of the anatomical [serial] dead space [eg, trachea, nonperfused conducting airways] and the alveolar [parallel] dead space [poorly or nonperfused alveoli].) The specific alveolar units are not perfused and therefore do not receive carbon dioxide. During exhalation, these physiological dead-space alveolar units “dilute” the carbon dioxide exiting from normally perfused alveoli, leading to a decrease in end-tidal carbon dioxide tension (and an increase in the arterial to end-tidal carbon dioxide gradient.) A biphasic response (first increased expired carbon dioxide followed by decreased concentration) has been described [5]. However, the brief increase may not be present or go unnoticed. Prompt therapy of a gas embolism is necessary [6] (Table 1).
The capnograph can also give a false, low reading when high inspiratory ventilation pressures are required. The change from spontaneous to controlled ventilation leads to a change in the distribution of ventilation and perfusion (Va/Q ratio) within the lung: more ventilation goes to the upper lung with controlled ventilation. The upper lung regions have less perfusion and therefore are part of West lung zone I (high Va/Q ratio). The result is an increase in the total physiological (anatomic and alveolar) dead space. The effects of the increased dead space can be minimized by increasing the tidal volume, thereby forcing more ventilation to the lower (better perfused) alveoli. The decrease of lung compliance due to the pneumoperitoneum and consequent requirement for higher inspiratory pressures aggravates this dead space effect, necessitating even greater increases in tidal volume and minute ventilation. However, the increases in minute ventilation required to maintain stable arterial carbon dioxide (PaCO2) values vary from 12% to 55% and may require unacceptably high ventilatory pressures. Accepting an increased carbon dioxide level (“permissive hypercapnia”) might be the prudent choice.
The typical desired “normal” end-tidal carbon dioxide concentration with the patient under general anesthesia is 32 to 36 mm Hg. During laparoscopic surgery, carbon dioxide is absorbed from the pneumoperitoneum and end-tidal carbon dioxide values of 40 to 50 mm Hg are not uncommon. The elevated (higher) levels of carbon dioxide may lead to increased venous oozing as carbon dioxide has a direct vasodilatory effect on the microvasculature. Although it is possible to increase minute ventilation using higher than normal ventilatory pressures, these higher pressures increase the risk of barotrauma. The relative danger of barotrauma versus the risk of increased bleeding needs to be evaluated and discussed by the operating room team.
The capnograph is usually considered a “ventilation” monitor, but, given a relatively constant tidal volume as with a ventilator (in volume controlled mode), the capnograph can be used as a cardiac output monitor. The constant tidal volume eliminates only the carbon dioxide brought to the lungs by the cardiac output in each time unit. Any sudden decrease in cardiac output leads to a decrease in end-tidal carbon dioxide concentration.
Pulsatile Oxygen Saturation Monitor (SpO2)
The anesthesiologist should be aware of the temporal relationships between changes in monitor readings and the progress of the laparoscopic surgery. For instance, a sudden decrease in oxygen saturation during initial insufflation of gas into the peritoneal space might be due to a gas embolism (late sign), while a decrease in saturation at a later stage might be due to atelectasis of basal lung units (due to an excessively high setting of the laparoscopic pneumoperitoneum pressure or obstruction of the outflow) (Table 2).
A continual slowly decreasing oxygen saturation is regularly encountered during laparoscopic surgery, and it might be treated with an increased inspired oxygen concentration. However, it is advisable to try to diagnose the cause. For instance, a more physiologically appropriate approach might be to test the effect of an increased level of positive end-expiratory pressure (PEEP) after a few large breaths to recruit alveoli. Another remedy to consider is to lengthen the inspiratory pause of the ventilator to allow the overventilated “fast” alveoli (with short time constants) to equilibrate with the “slow alveoli” (with long time constants).
Ventilation Monitoring
By monitoring both the peak inspiratory pressure (PIP) as well as the inspiratory plateau pressure during the inspiratory pause, the anesthesiologist gains information about:
1. the inspiratory flow resistance as indicated by the difference between the peak and plateau pressures. The resistance may be increased due to equipment factors, such as a kinked or obstructed (blood, secretions) endotracheal tube or by patient factors like bronchospasm;
2. the compliance of the lung can be approximated by the following: tidal volume divided by the plateau pressure. The compliance might be decreasing due to barotrauma or due to fluid shifts (eg, Shire's Third Space [7]) during prolonged surgery. A decreasing compliance might also be an early indicator of a relative fluid overload. The compliance might also be used as one of the factors in the decision to extubate the patient after prolonged surgery.
The decreased compliance of the lung (due to the pneumoperitoneum) influences the ventilator parameters monitored by the anesthesiologist:
1. for volume-controlled mechanical ventilation, with a preset tidal volume, the parameters to be monitored are peak and plateau pressures. Excessively high values may indicate the need for adjustment (decrease) of the preset tidal volume;
2. for pressure controlled ventilation (PCV), the maximum inspiratory pressure is set (fixed). The parameter to be monitored is the tidal volume, which will decrease as the compliance decreases. Adjustments of the “set pressure,” respiratory rate (both upwards), or both, might be required in order to avoid excessive hypercapnia.
Neuromuscular Blockade Monitor
Monitoring of the neuromuscular blockade throughout the surgery gives the anesthesiologist an idea of how rapidly this specific patient reverses a given dose of muscle relaxant. Due to the requirement for adequate muscle relaxation during laparoscopic surgery until deflation of the pneumoperitoneum and the rapid closure of the small laparoscopic incisions, it is necessary to know the state of muscle relaxation with the aim of rapid reversal of the neuromuscular blockade.
Urine Output
The urine output may be used as a monitor of fluid balance and adequacy of intravolume volume. The pneumoperitoneum decreases the cortical and medullary renal perfusion and hence the total renal perfusion is also decreased [8]. This leads to a decreased glomerular filtration rate (GFR), decreased sodium excretion, and decreased creatinine clearance. The monitor affected is the urine output. This “normal” or typical decrease in urine production needs to be considered when calculating intravenous fluid requirements. Accumulation of gas in the urinary catheter bag during insufflation may indicate a bladder injury.
Temperature
A temperature monitor is necessary, as all but the shortest laparoscopic surgeries require some form of external heating (eg, a forced warm air blanket.) The evaporation of liquid carbon dioxide in the gas cylinder cools the insufflating gas. Further cooling occurs due to the expansion of the compressed gas (Joule-Thompson effect.) The dry insufflated gas is humidified in the abdominal cavity, and the most cooling is caused by the latent heat of vaporization of water, which far exceeds the energy needed to heat the gas. (One liter of gas requires ±24 cal to be fully humidified (44 mg water vapor per liter at 37°C); one liter of gas requires ±7 cal to be heated from 20°C to 37°C)
Precordial (Esophageal) Stethoscope
The precordial or esophageal stethoscope may be one of the first monitors to give an indication of surgical emphysema (mediastinal, subcutaneous, neck) by the development of audible “crackles.” Palpation of the patient may also reveal the presence of surgical emphysema. Due to the possibility of airway obstruction by the surgical emphysema, the endotracheal tube may be left in situ until the surgical emphysema has started to dissipate.
W. Bosseau Murray, MD, is Professor, Department of Anesthesiology, and Associate Director, Simulation Development and Cognitive Science Laboratory and works within the Departments of Anesthesiology and Nursing and Surgery at Pennsylvania State University College of Medicine at the Milton S. Hershey Medical Center, Hershey, Pennsylvania. Dr. Murray has over two-dozen areas of expertise including laparoscopy, anesthesiology, physiologic monitoring, computer simulation, hemodynamic processes, and teaching.
References
4. Brooks PG. Venous air embolism during operative hysteroscopy. J Am Assoc Gynec Laparosc. 1997;4:399-402.
5. Shulman D, Aronson HB. Capnography in the early diagnosis of carbon dioxide embolism during laparoscopy. Can Anaes Soc J. 1984;31(4):455-459.
6. Durant TM, Long J, Oppenheimer MJ. Pulmonary (venous) air embolism. Am Heart J. 1947;33:269-281.
7. Shires T, Williams J, Brown F. Acute changes in extracellular fluids associated with major surgical procedures. Ann Surg. 1961;154:803-810.
8. Chiu AW, Chang LS, Birkett DH, Babayan RK. The impact of pneumoperitoneum, pneumoretroperitoneum, and gasless laparoscopy on systemic and renal hemodynamics. J Am Coll Surg. 1995;181:397-406.
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