Ventilation-perfusion relationships during epidural analgesia

Advantages of Epidural Analgesia on Pulmonary Functions in Liver Transplant Donors

OBJECTIVE

Epidural analgesia (EA) has positive effects on anesthetic requirement, blood loss, postoperative analgesia, and pulmonary function tests (PFTs). The purpose of the present study was to investigate the effect of EA on postoperative PFTs in liver transplant donors (LTDs).

METHODS

In the present study, 66 LTDs were classified as total intravenous anesthesia (TIVA) and TIVA+EA groups. Patient's age, sex, body mass index, induction and maintenance dose of propofol (IDP and MDP), operation duration, forced expiratory volume in 1 second (FEV₁), forced vital capacity (FVC), FEV₁/FVC ratio, visual analog scale (VAS), atelectasis scores, and lengths of intensive care unit (ICU) and hospital stays were recorded.

RESULTS

In the TIVA+EA group, IPD, MPD, delta-FEV₁ delta-FVC, VAS for all time, atelectasis score and length of hospital stay were significantly lower than in the TIVA group (P < .001 for all). Whereas VAS at the end of the operation was negatively correlated with delta-FEV₁ and delta-FVC (r² = 0.26 P < .001; r² = 0.41 P < .001; respectively), it was positively correlated with atelectasis score and length of ICU stay (r² = 0.49, P < .001; and r² = 0.41, P < .001; respectively). Atelectasis score was positively correlated with length of ICU stay (r² = 0.86, P < .001).

CONCLUSIONS

Reduced anesthetic requirement, better postoperative analgesia, reduced atelectasis score, and preserved PFTs can be provided with the use of EA in LTDs. Positive effects of EA on anesthesia requirement, pain management and pulmonary function are associated with outcomes.

Caudal anesthesia in a patient with peritonitis: Is it safe??

Neuraxial anesthesia combined with general anesthesia has become a widely accepted method of providing effective postoperative analgesia and decreasing intraoperative anesthetic needs in the pediatric population. In clinical practice, there still appears to be hesitancy for the use of a neuraxial technique (spinal or epidural) in patients at risk for bacteremia or with an on-going systemic infection. However, evidence-based medicine lacks any data to support an increase in the risk of infectious complications following neuraxial anesthesia. We present two pediatric patients with intra-abdominal infectious processes who received caudal epidural blockade for postoperative operative analgesia. The use of neuraxial techniques in patients at risk for bacteremia is reviewed, evidence-based medicine regarding the risks of infection discussed, and the potential favorable effects of neuraxial blockade on the neurohumoral response to sepsis and the systemic inflammatory responses presented.

Respiratory function during anesthesia: effects on gas exchange

Anaesthesia causes a respiratory impairment, whether the patient is breathing spontaneously or is ventilated mechanically. This impairment impedes the matching of alveolar ventilation and perfusion and thus the oxygenation of arterial blood. A triggering factor is loss of muscle tone that causes a fall in the resting lung volume, functional residual capacity. This fall promotes airway closure and gas adsorption, leading eventually to alveolar collapse, that is, atelectasis. The higher the oxygen concentration, the faster will the gas be adsorbed and the aleveoli collapse. Preoxygenation is a major cause of atelectasis and continuing use of high oxygen concentration maintains or increases the lung collapse, that typically is 10% or more of the lung tissue. It can exceed 25% to 40%. Perfusion of the atelectasis causes shunt and cyclic airway closure causes regions with low ventilation/perfusion ratios, that add to impaired oxygenation. Ventilation with positive end-expiratory pressure reduces the atelectasis but oxygenation need not improve, because of shift of blood flow down the lung to any remaining atelectatic tissue. Inflation of the lung to an airway pressure of 40 cmH2O recruits almost all collapsed lung and the lung remains open if ventilation is with moderate oxygen concentration (< 40%) but recollapses within a few minutes if ventilation is with 100% oxygen. Severe obesity increases the lung collapse and obstructive lung disease and one-lung anesthesia increase the mismatch of ventilation and perfusion. CO2 pneumoperitoneum increases atelectasis formation but not shunt, likely explained by enhanced hypoxic pulmonary vasoconstriction by CO2. Atelectasis may persist in the postoperative period and contribute to pneumonia.

Dependence of the gradient between arterial and end-tidal P(CO(2)) on the fraction of inspired oxygen

BACKGROUND

End-tidal P(CO(2)) (Pe'(CO(2))) is routinely used in the clinical assessment of the adequacy of ventilation because it provides an estimate of Pa(CO(2)). How well Pe'(CO(2)) reflects Pa(CO(2)) depends on the gradient between them, expressed as ΔPa-e'(CO(2)). The major determinant of ΔPa-e'(CO(2)) is alveolar dead space (Vd(alv)). The fraction of inspired O(2) (Fi(O(2))) is not thought to substantially affect ΔPa-e'(CO(2)) in anaesthetized patients. We hypothesized that a high Fi(O(2)) may indeed increase ΔPa-e'(CO(2)) by preferentially vasodilating well-perfused alveoli, resulting in the redistribution of blood flow to these alveoli from poorly perfused alveoli and an increase in Vd(alv). We therefore investigated the effects of changes in Fi(O(2)) on ΔPa-e'(CO(2)) and Vd(alv).

METHODS

With Institutional Review Board approval and informed consent, we studied 20 ASA I-II supine patients undergoing elective lower abdominal surgery under combined general and epidural anaesthesia. At constant levels of ventilation, Fi(O(2)) levels of 0.21, 0.33, 0.5, 0.75, and 0.97 were applied in a random order and ΔPa-e'(CO(2)) and Vd(alv) were calculated.

RESULTS

The ΔPa-e'(CO(2)) values were, in order of ascending Fi(O(2)), {mean [standard error of the mean (SEM)]} 0.13 (0.04), 0.28 (0.08), 0.29 (0.09), 0.44 (0.11), and 0.53 (0.09) kPa. The corresponding values of Vd(alv) were 25.5, 33.8, 35.8, 48.9, and 47.4 ml. Each successive hyperoxic level showed a significant increase in ΔPa-e'(CO(2)) except between the 0.33-0.5 and 0.75-0.97 Fi(O(2)) levels.

CONCLUSIONS

These data demonstrate that ΔPa-e'(CO(2)), in anaesthetized patients depends on Fi(O(2)).

The effect of caudal block on functional residual capacity and ventilation homogeneity in healthy children*

Caudal block results in a motor blockade that can reduce abdominal wall tension. This could interact with the balance between chest wall and lung recoil pressure and tension of the diaphragm, which determines the static resting volume of the lung. On this rationale, we hypothesised that caudal block causes an increase in functional residual capacity and ventilation distribution in anaesthetised children. Fifty-two healthy children (15-30 kg, 3-8 years of age) undergoing elective surgery with general anaesthesia and caudal block were studied and randomly allocated to two groups: caudal block or control. Following induction of anaesthesia, the first measurement was obtained in the supine position (baseline). All children were then turned to the left lateral position and patients in the caudal block group received a caudal block with bupivacaine. No intervention took place in the control group. After 15 min in the supine position, the second assessment was performed. Functional residual capacity and parameters of ventilation distribution were calculated by a blinded reviewer. Functional residual capacity was similar at baseline in both groups. In the caudal block group, the capacity increased significantly (p < 0.0001) following caudal block, while in the control group, it remained unchanged. In both groups, parameters of ventilation distribution were consistent with the changes in functional residual capacity. Caudal block resulted in a significant increase in functional residual capacity and improvement in ventilation homogeneity in comparison with the control group. This indicates that caudal block might have a beneficial effect on gas exchange in anaesthetised, spontaneously breathing preschool-aged children with healthy lungs.

Thoracic epidural anesthesia as an adjunct to general anesthesia for cardiac surgery: effects on ventilation-perfusion relationships

OBJECTIVE

To determine the effects of thoracic epidural anesthesia (TEA) on ventilation-perfusion (VA/Q) relationships, atelectasis, and oxygenation before and after coronary artery bypass graft surgery (CABG).

DESIGN

Prospective, controlled, unblinded, randomized trial.

SETTING

Cardiothoracic clinic at a major university referral center.

PARTICIPANTS

Twenty-eight patients undergoing elective CABG.

INTERVENTIONS

Perioperative and postoperative TEA was added to general anesthesia (GA) in 14 patients, and 14 patients receiving GA alone served as controls.

MEASUREMENTS AND MAIN RESULTS

VA/Q relationships were measured by the multiple inert gas elimination technique, and, 20 hours postoperatively, atelectasis was assessed by computerized tomographic scans. Arterial and mixed venous blood gases and hemodynamic variables were measured by standard techniques. TEA per se caused no change in shunt, VA/Q matching, or oxygenation. Induction of GA in the control group and induction of TEA caused similar reductions in mean arterial pressure. The TEA patients needed less morphine analgesia postoperatively and were extubated earlier. Extubation caused significant improvement in VA/Q matching. On the first postoperative day, a slight reduction in PaCO2 was seen in the TEA group, but no differences in shunt, VA/Q matching, or oxygenation compared with the GA group. Both groups showed extensive bilateral atelectasis.

CONCLUSION

TEA can reduce respirator time and the need for morphine analgesics after CABG without negative effects on VA/Q matching, oxygenation, or atelectasis formation.

Lung aeration and pulmonary gas exchange during lumbar epidural anaesthesia and in the lithotomy position in elderly patients

We investigated a total of 36 subjects with a mean (SD) age of 65 (13) years, during baseline conditions (supine, before any anaesthesia), and then during one of the following protocols: (1) lithotomy positioning (n = 12), (2) epidural anaesthesia (n = 12), (3) general anaesthesia in the supine position (n = 12). Lung aeration, ventilation/perfusion matching, gas exchange and functional residual capacity were measured. Lung aeration was normal during baseline assessment with almost no regions with poor aeration and no substantial dependent densities. Shunt and perfusion of poorly ventilated regions were minor. Lithotomy positioning did not reduce functional residual capacity and did not affect aeration of the lung or ventilation/perfusion matching. Epidural anaesthesia, in general, had no effect on aeration, ventilation/perfusion matching or gas exchange, regardless of whether the patient was in the supine or lithotomy position. General anaesthesia, however, caused significant increases in poorly aerated lung regions and in dependent densities (interpreted as atelectasis). In conclusion, no or little impairment of lung aeration and ventilation/perfusion matching was caused by the lithotomy position and/or epidural anaesthesia, contrary to the effects seen during general anaesthesia. However, our findings also suggest that being overweight is a factor that may cause impairment of lung aeration.

Effect of thoracic epidural anaesthesia on ventilation-perfusion distribution and intrathoracic blood volume before and after induction of general anaesthesia

BACKGROUND

Gas exchange is impaired during general anaesthesia due to development of shunt and ventilation-perfusion mismatching. Thoracic epidural anaesthesia (TEA) may affect the mechanics of the respiratory system, intrathoracic blood volume and possibly ventilation-perfusion (VA/Q) distribution during general anaesthesia.

METHODS

VA/Q relationships were analyzed in 24 patients undergoing major abdominal surgery. Intrapulmonary shunt (Qs/QT), perfusion of "low" VA/Q areas, ventilation of "high" VA/Q regions, dead space ventilation and mean distribution of ventilation and perfusion were calculated from the retention/excretion data of six inert gases. Intrathoracic blood volume (ITBV) and pulmonary blood volume (PBV) were determined with a double indicator technique. Recordings were made before and after administration of 8.5 +/- 1.5 ml bupivacaine 0.5% (n = 12) or 8.3 +/- 1.8 ml placebo (n = 12) into a thoracic epidural catheter and after induction of general anaesthesia.

RESULTS

Before TEA, Qs/QT was normal in the bupivacaine group (2 +/- 2%) and the placebo group (2 +/- 3%). TEA covering the dermatomal segments T 12 to T 4 had no effect on VA/Q relationships, ITBV and PBV. After induction of general anaesthesia Qs/QT increased to 8 +/- 4% (bupivacaine group, P < 0.05 and to 7 +/- 2% (placebo group, P < 0.05). ITBV and PBV decreased significantly to the same extent in the bupivacaine group and the placebo group.

CONCLUSIONS

TEA has no effect on VA/Q distribution, gas exchange and intrathoracic blood volume in the awake state and does not influence development of Qs/QT and VA/Q inequality after induction of general anaesthesia.

Arterial to end-tidal carbon dioxide tension difference during laparoscopy. Magnitude and effect of anaesthetic technique

The relationship between arterial carbon dioxide tension and end-tidal carbon dioxide tension was studied in 25 patients during laparoscopy. Thirteen patients received general anaesthesia and 12 epidural anaesthesia. The overall mean difference between arterial and end-tidal carbon dioxide tensions was 0.44 kPa (95% confidence intervals 0.28-0.60 kPa) which was significantly less than that reported in studies during other procedures. The reasons for this difference are probably associated with the physiological changes induced by CO2 pneumoperitoneum and steep Trendelenburg positioning. The choice of anaesthetic technique did not affect the arterial to end-tidal carbon dioxide tension difference significantly (p greater than 0.9).

[Total pulmonary atelectasis after epidural anesthesia]

A case is reported of a 69-year-old man who sustained a total atelectasis of his right lung while undergoing ilio-tibial bypass surgery. This patient had a history of chronic obstructive pulmonary disease. The plain chest film showed a possible right-sided segmental atelectasis as well as pleural thickening on the same side. Lung function tests showed up a small obstructive syndrome, with mild hypoxaemia (Pao2 60.5 mmHg). The procedure was performed under epidural anaesthesia, carried out with 20 ml of 0.5% bupivacaine and 0.1 mg fentanyl. The upper level of anaesthesia was T4. The patient was sedated with flunitrazepam. He was given oxygen because arterial blood appeared to be somewhat dark. Despite this, the patient became grey, agitated, and had tachypnoea, together with absent breath sounds on the right side. A chest X-ray confirmed the diagnosis of total atelectasis of the right lung. Fiberoptic bronchoscopy was carried out, and a purulent mucous plug was removed. The patient improved and recovered from this episode totally. Blood gases were measured 1 month later; there was no difference with the preoperative values, except for correction of the hypoxaemia (Pao2 76 mmHg). The different factors possibly involved in the pathogenesis of the plug are discussed. It is likely that general anaesthesia would not have avoided this complication.

Effect of anaesthesia on respiratory function after major lower extremity surgery. A comparison between bupivacaine spinal analgesia with low-dose morphine and general anaesthesia

Postoperative pulmonary function was studied in 16 patients undergoing total hip or knee arthroplasty. Their mean age was 65 years. Half of them received spinal analgesia (22.5 mg bupivacaine + 0.3 mg morphine) and the other half underwent general anaesthesia with halothane-nitrous oxide. Four hours postoperatively, the forced expirogram was maintained in the spinal analgesia group, compared with preanaesthesia measurements. Functional residual capacity (FRC) measured by multiple breath nitrogen washout was reduced by 0.51, as was closing capacity (CC) measured by the bolus technique. The gas distribution index (nitrogen washout delay) was unaltered. The alveolar-arterial oxygen tension difference (PA-ao2) was not significantly altered, but arterial oxygen tension (Pao2) was reduced by 1.7 kPa, and arterial carbon dioxide tension (PaCo2) was increased. No respiratory measurements could be made in the general anaesthesia group 4 h postoperatively, but arterial blood gases were unaltered compared with preanaesthesia values. Eighteen hours postoperatively, forced vital capacity (FVC) was reduced in the spinal analgesia group, FRC and CC remained diminished and the gas distribution index was increased, indicating less efficient gas mixing. Simultaneously, PA-ao2 was increased, and Pao2 remained reduced despite increased alveolar ventilation (lowered PaCo2). In the general anaesthesia group FVC, FRC and CC were also reduced, but the gas distribution index remained at the awake level and blood gases were unaltered. It is suggested that the slight hypoventilation in the spinal analgesia group early after surgery may have contributed to impaired gas distribution and ventilation-perfusion matching later postoperatively.