42. Effect of Patient Position on Cardiovascular and Pulmonary Function

42. Effect of Patient Position on Cardiovascular and Pulmonary Function

Eric J. Hazebroek, M.D.

H. Jaap Bonjer, M.D., Ph.D.

The intraabdominal exposure required to perform laparoscopic surgery is most commonly provided by a carbon dioxide (CO

) pneumoperitoneum that ele- vates the abdominal wall, suppresses the viscera, and creates a working space in the abdominal cavity which permits the safe introduction of trocars and in- struments. The CO

pneumoperitoneum has a number of cardiopulmonary and physiologic effects. CO

, which is highly soluble, is readily absorbed through the visceral and parietal peritoneum and almost always results in hypercarbia and a respiratory acidosis. In addition, the increased intraabdominal pressure (IAP) pushes the diaphragms cephalad, which makes it more difficult to ventilate the patient. To compensate for these deleterious changes the minute ventilation is increased, by increasing either the tidal volume or the respiratory rate. Elevated peak inspiratory pressures are usually required to effect these changes. The increased IAP may also have an impact on cardiovascular function.

During laparoscopic procedures, the body position of the patient is often changed to expose the organ of interest and provide the best operative field. In general, more radical body positions are used in minimally invasive surgery than in open cases because retraction of the intestine and other mobile viscera is more difficult in minimally invasive cases. Although laparotomy pads and other packs are used in conjunction with retractors to create an operative field during an open procedure, gravity is the principal means by which retraction is accomplished during a laparoscopic case. The body positioning options include Trendelenburg (head down), reverse Trendelenburg (head up), lateral “airplaning” of the table (left or right side of patient up or down), or a combination of these. Radical patient positioning, by itself, will have an impact on cardiopulmonary function.

The effect, which varies depending on the position, may either exacerbate or

alleviate the pneumoperitoneum effects. Although for short laparoscopic pro-

cedures the cardiopulmonary changes are not problematic, during advanced

procedures and in patients with considerable cardiopulmonary disease clinically

significant cardiorespiratory changes may result. In this chapter the cardiovas-

cular and pulmonary changes associated with various patient positions during

laparoscopic procedures are discussed. A brief overview of the cardiovascular

and the pulmonary changes with the patient in the supine position is given, fol-

lowed by discussion of the cardiopulmonary impact of the specific positions.

A. Supine Position Cardiovascular Changes

Although not all reports agree about the impact of pneumoperitoneum on the cardiac output during laparoscopic procedures, the majority have noted increases in systemic vascular resistance (SVR) and mean arterial pressure (MAP). As mentioned, intraperitoneal insufflation with CO

causes hypercarbia and acidosis: hypercarbia stimulates the sympathetic nervous system, which may lead to an increase in blood pressure, heart rate, and vascular tone [1]. Increased IAP also compresses the inferior vena cava, which compromises venous return from the lower extremities. As a result cardiac preload will decrease. Afterload is increased as a result of the elevated increased SVR that is caused by com- pression of the abdominal vessels and increased sympathetic activity.

Reduced preload may cause a reduction of cardiac output and a compen- satory increase in heart rate. Therefore, if preload is markedly reduced, it may be critical to expand the intravascular volume. The effect of increased IAP on venous return is dependent on the intravascular volume status and central venous pressure (CVP). At low or normal right atrial pressure, venous return is reduced with increased IAP by compression of the inferior vena cava (IVC). In subjects with a high right atrial pressure, the IVC remains patent despite the IAP and, in fact, venous return is augmented. Several investigators have demonstrated that femoral vein blood flow decreases with increased IAP, which implies that instal- lation of pneumoperitoneum increases pooling of blood in the peripheral circu- lation [2] (Table 42.1).

Table 42.1. Effect of body position on cardiovascular and pulmonary changes during laparoscopic surgery.

Supine Head-down Head-up Lateral Cardiovascular changes:

MAP ≠ ≠≠ ≠≠ =/≠

SVR ≠ ≠ ≠ ≠

Venous return Ø ≠ ØØ =/Ø

Preload Ø ≠ ØØ =/Ø

Afterload ≠ ≠ ≠≠

Cardiac output Ø ≠ Ø

Blood pooling in legs ≠ Ø ≠≠ ≠

Pulmonary changes:

Diaphragmatic cephalad shift ≠ ≠≠ Ø ≠

FRC Ø ØØ ≠ Ø

Chest compliance Ø ØØ Ø Ø

Peak airway pressure ≠ ≠≠ ≠ ≠

MAP, mean arterial pressure; SVR, system vascular resistance; FRC, functional residual capacity.

a

Depends on right/left lateral decubitus position.

In general, the extent of the cardiovascular changes (including cardiac output) associated with creation of pneumoperitoneum depend on the IAP attained, the volume of CO

absorbed, the patient’s intravascular volume status, the ventilation technique, surgical conditions, and the anesthetic agents employed [3].

Pulmonary Changes

Anesthesia and both open and closed abdominal surgery may cause a pro- gressive cranial displacement of the diaphragm [4] (Figure 42.1). The sequence of events for an open procedure can be described as follows, assuming the patient is in the supine position: induction of anesthesia, causation of paralysis, and the placement of retractors and packs to provide exposure. During laparoscopic surgery, exposure is provided by the CO

pneumoperitoneum and fairly radical patient positioning. In both scenarios, the diaphragm is shifted in a cephalad direction, which results in a decreased functional residual capacity (FRC). With few exceptions, the alterations are significantly greater during laparoscopic surgery where the general anesthesia-related decrease in FRC is enhanced by intraperitoneal insufflation of CO

. Decreased FRC may result in the develop- ment of intraoperative atelectasis, intrapulmonary shunting, and hypoxemia. The patient’s position will influence the degree of the diaphragmatic shift and either enhance or lessen the effects of the CO

pneumoperitoneum.

In addition, increased IAP decreases chest compliance and thus increases peak airway pressure. As a result, the bronchial tree may expand, which increases anatomical deadspace. In patients with chronic obstructive pulmonary disease or bullous emphysema, airway pressures may be high, therefore, increased IAP during laparoscopy may pose an additional danger. Furthermore, pulmonary hypertension will increase right cardiac work. Together with the decrease of filling pressures and increased SVR, this may cause a fall in cardiac output.

FRC IAP

Figure 42.1. Cranial shift of the diaphragm.

B. Head-Down Position Cardiovascular Changes

The Trendelenburg or ‘head-down position’ was originally described and utilized to keep the small bowel or colon out of the pelvis during gynecologic and urologic operations. It is essential to distinguish between a moderate and extreme head-down position. During creation of pneumoperitoneum, the patient is commonly placed in a 10°–20° head-down tilt to minimize complications asso- ciated with blind trocar insertion. However, some laparoscopic procedures, such as sigmoid colon or rectal resection, require a steep Trendelenburg position to provide adequate exposure.

The physiologic effects of the Trendelenburg position in normovolemic patients have been reviewed in detail by Wilcox and Vandam [5]. As generally taught: “postures with the head lowered are more favorable to the circulation.”

The head-down position favors venous return and, thus, improves cardiac output [6]. In addition, the Trendelenburg position decreases pooling of blood in the lower body. In a study investigating the hemodynamic effects of body position during laparoscopic surgery, peritoneal insufflation caused a significant increase in MAP, CVP, and pulmonary capillary wedge pressure (PCWP) [7]. Placement of patients in the Trendelenburg position further increased these hemodynamic changes, demonstrating an additive effect of elevated IAP and head-down posi- tioning on hemodynamic parameters. The cardiovascular changes associated with the Trendelenburg position may be influenced by the degree of head-down tilt, the patient’s age, associated cardiac disease, anesthetic drugs, and ventila- tion techniques.

Respiratory Changes

The Trendelenburg position reduces vital capacity because of the increased weight of the shifted abdominal viscera on the diaphragm [8]. This exacerbates the pneumoperitoneum-related cranial displacement of the diaphragm and its associated changes. Therefore, an even greater reduction of FRC is noted when pneumoperitoneum and the head-down position are applied simultaneously.

Steep Trendelenburg positioning further decreases chest compliance, which may

result in a decrease in the ventilation perfusion ratio and accentuated hypercar-

bia and hypoxemia. Studies that have assessed pulmonary function have not, in

general, shown significant compromise resulting from head-down positioning

itself. However, the potential for compromise exists. There may be a predilec-

tion to develop pulmonary atelectasis if deep breaths are not intermittently sup-

plied when mechanical ventilation is employed. Increased airway pressure and

the use of positive end-expiratory pressure (PEEP) may minimize this compli-

cation. The position-related changes in pulmonary function are more marked in

obese, elderly, or debilitated patients and are enhanced by the placement of

certain laparoscopic instruments such as liver retractors in the upper abdomen.

C. Head-Up Position Cardiovascular changes

The reverse Trendelenburg or head-up position is commonly used in proce- dures such as laparoscopic cholecystectomy and Nissen fundoplication to facil- itate adequate exposure of the upper abdomen and to prevent inadvertent bowel injury. Positioning in reverse Trendelenburg accentuates the CO

pneumoperi- toneum-related decrease in venous return and may also be associated with decreased cardiac preload and cardiac index [9]. Furthermore, the head-up posi- tion may increase afterload via increases in MAP, SVR, and pulmonary vascu- lar resistance. The decreased venous return also facilitates the pooling of blood in the lower extremities. Pneumatic compression stockings and bandages improve the venous return from the legs and thus counter the effects of both the reverse Trendelenburg position and the pneumoperitoneum. These devices should be routinely employed.

Pulmonary Changes

During laparoscopy, the change in position to reverse Trendelenburg should improve diaphragmatic function and respiratory status. Shifting the abdominal viscera away from the diaphragm toward the pelvis will improve diaphragmatic excursion, increase chest wall compliance, increase FRC, and lower the peak inspiratory pressures.

D. Lateral Position Cardiovascular Changes

The lateral position is commonly used for laparoscopic renal and adrenal

operations. In most cases, a flexed lateral decubitus position is used. Flexion

enlarges the operative field by increasing the distance between the costal margin

and the anterior superior iliac fossa on the exposed side and may decrease venous

bleeding by decreasing venous pressure. However, this position facilitates the

pooling of blood in the legs, which are dependent. During laparoscopic adrenal

or renal surgery, CO

absorption is increased because retroperitoneal dissection

often results in insufflation of the various soft tissue planes [10]. The increased

PaCO

causes vasodilatation and stimulates the sympathetic nervous system,

resulting in tachycardia and hypertension [11]. Few investigators have studied

the hemodynamic changes associated with pneumoperitoneum and the lateral

position [12]. In patients undergoing minimally invasive urologic surgery, fewer

marked hemodynamic effects were noted in the left lateral position when com-

pared to results with the patient in the right lateral position [12]. There are a few

theoretical explanations for these differences. First, because of the right-sided anatomic position of the right atrium, venous return via the inferior and supe- rior vena cava to the right atrium may be improved when the patient is in the right lateral position. Second, the heart, located in the left hemithorax, will shift to the dependent side to a different extent as a result of gravity when a patient is placed in the lateral position.

Pulmonary Changes

As in the supine position, the lateral position itself may displace the diaphragm cranially, reducing the FRC of the dependent lung and inducing a ventilation and perfusion mismatch [13]. A concomitant pneumoperitoneum is likely to further displace the diaphragm cranially and reduce FRC. Although data on pulmonary consequences following laparoscopic surgery in the lateral posi- tion are scarce, it appears that intraperitoneal insufflation of CO

with retroperi- toneal exposure in the lateral position does not clinically affect oxygenation in healthy patients. However, ventilation is inhibited somewhat by the cephalad dis- placement of the dependent diaphragm and the attendant reduction in lung com- pliance. Furthermore, the ventilatory requirements of the patient in the lateral decubitus position are increased because of the elevated PaCO

that results from the increased absorption of CO

.

E. Conclusion

During laparoscopic surgery, cardiopulmonary function is influenced by the CO

used for insufflation, elevated IAP, and body position. It is difficult to deter- mine which of these three factors has the greatest influence on cardiovascular and respiratory function. The patient’s body position can either lessen or enhance the pressure and CO

-gas related effects. The Trendelenburg position, in general, increases venous return, cardiac output, CVP, and MAP. Pulmonary wise, the head-down position decreases chest wall compliance, inhibits ventilation, decreases the FRC, and increases PaCO

. The reverse Trendelenburg position, in general, decreases venous return, cardiac preload, and cardiac index. The head-up position, however, improves pulmonary function via increased chest wall compliance and FRC in addition to lower peak inspiratory pressures.

During short laparoscopic procedures in healthy patients, alterations in body

position seldom are associated with clinically significant cardiopulmonary

effects beyond those noted with CO

insufflation in general. However, during

long procedures problems may arise, most typically hypercarbia and very high

peak inspiratory pressures. It is essential that the surgeon and anesthetist com-

municate with each other frequently during the operation in regard to insuffla-

tion pressures being used, body position, and patient respiratory status (end-tidal

CO

, PaCO

, and oxygen saturation). If the end-tidal CO

or PaCO

becomes

prohibitive, surgery should be halted, the patient placed in the neutral or reverse

Trendelenburg position, and the pneumoperitoneum released for a 5- to 10- minute period. In most cases the end-tidal CO

and the PaCO

will fall back into the normal range within 5 to 10 minutes, after which the pneumoperitoneum can be reestablished and the procedure resumed. The lowest acceptable peak insuf- flation pressure should be utilized and, if required, as little Trendelenburg as possible.

In patients with significant cardiopulmonary problems, even a relatively brief laparoscopic procedure may be poorly tolerated. An attempt should be made to use low insufflation pressures and to avoid radical positions in these patients.

Further, meticulous attention must be paid to both cardiovascular and pulmonary function during the case in these patients. In patients with severe pulmonary disease, the Trendelenburg position should be avoided, whereas in those with marginal cardiac function the reverse Trendelenburg position is best avoided. In patients with the most severe cardiopulmonary disease it may not prove feasible or safe to carry out the procedure in question using a CO

pneumoperitoneum and minimally invasive method.

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