[Perioperative management with short-acting intravenous anesthetics]

Total intravenous anaesthesia (TIVA) with short-acting drugs is a standard procedure for day case surgery and is increasingly used for neurosurgical, cardiac surgical and paediatric surgical operations. The combination of propofol with alfentanil or remifentanil is frequently applied due to its favourable pharmacological properties. Propofol is characterized by a large volume of distribution at steady state and a relatively long elimination half time (t1/2 beta). Because of a high metabolic clearance, the clinical effects of propofol decline rapidly even after prolonged intravenous drug infusion. In patients with increased age, obesity or liver or renal failure, decreased doses of propofol for induction of anaesthesia are recommended. The short-acting opioids alfentanil and remifentanil provide small volumes of distribution at steady state, a short blood-brain equilibration time and decreased t1/2 beta. Remifentanil has unique pharmacological properties due to an ester binding and its elimination via extrahepatic hydrolysis by non-specific blood and tissue esterases. The context sensitive half time of remifentanil is significantly shorter than that of other opioids. Its analgetic potency is equal to fentanyl and 20 to 30 times higher than alfentanil. The advantages of total intravenous anaesthesia include fewer haemodynamic side-effects, a decreased incidence of postoperative nausea and vomiting and less neurohumoral stress response to surgery. Adequate pain therapy is mandatory after total intravenous anaesthesia with short-acting drugs. Continuous infusion of remifentanil for postoperative analgesia or supplementation of regional anaesthesia requires careful monitoring of vital functions. The economic aspects of TIVA remain to be determined.

Induction of P-glycoprotein by rifampin increases intestinal secretion of talinolol in human beings: a new type of drug/drug interaction

BACKGROUND

P-Glycoprotein is an efflux pump in many epithelial cells with excretory function. It has been demonstrated that rifampin (INN, rifampicin) induces P-glycoprotein, particularly in the gut wall. We therefore hypothesized that rifampin affects pharmacokinetics of the P-glycoprotein substrate talinolol, a beta1-blocker without appreciable metabolic disposition but intense intestinal secretion in human beings.

METHODS

Pharmacokinetics of talinolol (a single dose of 30 mg administered intravenously or 100 mg administered orally for 7 days) and duodenal expression of the MDR1 gene product P-glycoprotein as assessed by reverse transcriptase-polymerase chain reaction of the MDR1-messenger ribonucleic acid, by immunohistochemistry and Western blot analysis were analyzed before and after coadministration of rifampin (600 mg per day for 9 days) in 8 male healthy volunteers (age 22 to 26 years).

RESULTS

During rifampin treatment, the areas under the curve of intravenous and oral talinolol were significantly lower (21% and 35%; P < .05). Treatment with rifampin resulted in a significantly increased expression of duodenal P-glycoprotein content 4.2-fold (2.9, 6.51) (Western blot) and messenger RNA was increased in six of the eight volunteers. P-Glycoprotein expression in biopsy specimens of gut mucosa correlated significantly with the systemic clearance of intravenous talinolol (rs = 0.74; P < .001).

CONCLUSIONS

Rifampin induces P-glycoprotein-mediated excretion of talinolol predominantly in the gut wall. Moreover, clearance of talinolol from the blood into the lumen of the gastrointestinal tract may be predicted by the individual intestinal P-glycoprotein expression. Thus we describe a new type of steady-state drug interaction affecting compounds that are subject to transport rather than metabolism.

[Acute traumatic myocardial infarction with cardiogenic shock in severe polytrauma--a case report]

A 41-year-old man suffered severe polytrauma and developed a traumatic myocardial infarction with cardiogenic shock. Thrombolysis as well as coronary bypass grafting was contraindicated due to accompanying injuries. An attempted early coronary revascularization by percutaneous transluminal coronary angioplasty (PTCA) failed due to dissection of the left interventricular coronary artery. Treatment of cardiac insufficiency was complicated by intraabdominal haemorrhage enforcing emergency laparotomy. Intraaortic balloon counterpulsation proved to be efficient in supporting circulation in these circumstances. The case report documents the practicability and importance of treating both myocardial ischaemia and attending injuries in an equivalent and coordinated manner in traumatic myocardial infarction.

[Acute failure of the intestinal barrier--pathophysiology, diagnosis, prophylaxis and therapy]

The gut not only serves as a main target for the detrimental effects of stress during and after surgery, but may also promote the development of multiple organ failure after different types of severe shock. According to a current hypothesis, an impaired intestinal barrier function is associated with a decreased separation of intraluminal bacteria and toxins and systemic circulation, which may induce sepsis and multiple organ failure. Hypoperfusion during shock, reperfusion injury of the splanchnic mucosa, alterations of the micro-ecology of the gut and immunologic and hormonal disturbances are important underlying pathophysiological mechanisms. Various therapeutic concepts have been proposed such as improvement of splanchnic perfusion, nutritive and metabolic treatment by means of immunomodulating nutrients, parenteral substitution of glutamine, early onset of enteral nutrition, normalization of gut motility and selective decontamination of the gut. However, no clinical study to date could clearly demonstrate a key role of the gut in the pathogenesis of sepsis and multiple organ failure. Likewise, the efficacy of different prophylactic and therapeutic procedures remain to be studied. An aggressive treatment of shock and avoidance of microcirculatory disturbances are of principal importance for prophylaxis of multiple organ failure.

Blood separation with two different autotransfusion devices: effects on blood cell quality and coagulation variables

The quality of blood products obtained from two different autotransfusion devices (CATS- Fresenius and Sequestra 1000 - Medtronic) was tested in 27 patients undergoing elective orthopaedic surgery. Blood products provided from our institutional blood bank (n = 16) served as controls. Hemodiluted blood was separated into platelet poor plasma (PPP), platelet rich plasma (PRP), and packed red cells (PRC) and analysed for blood cell count, fibrinogen concentration, thromboplastin time, partial thromboplastin time, platelet aggregation and platelet recovery rate. Coagulation variables showed no differences between the CATS-group (n = 14) and the Sequestra 1000-group (n = 13). The volume of PRP was lower in the Sequestra 1000-group (45+/-3 ml vs. 89+/-1 ml, p<0.05), but hematocrit was higher (14.4+/-7.8% vs. 8.5+/-2.8%, p<0.05). PPP produced with CATS contained a higher concentration of white blood cells (0.6+/-0.2 Gpt/l vs. 0.1+/-0.01 Gpt/l, p<0.05) and thrombocytes (163+/-74 Gpt/l vs. 11+/-12 Gpt/l, p<0.05). Hematocrit of PRC was significantly higher in the CATS-group (73.8+/-2.0% vs. 69.0+/-6.5%, p<0.05). Blood products were of high quality in both groups and comparable to or superior than blood products provided from our institutional blood bank.

Thoracoscopic interruption of patent ductus arteriosus compromises cardiopulmonary function in newborn pigs

Interruption of patent ductus arteriosus (PDA) using video-assisted thoracoscopic surgery (VATS) has recently been introduced into clinical practice. To study cardiovascular and pulmonary function during VATS, we treated 16 newborn pigs (weight 1421+/-44 g) with PDA with conventional surgical interruption (CSI; n = 7) or interruption via VATS (n = 9). Measurements of hemodynamics and gas exchange were performed before, during, and after surgery. Systemic perfusion was calculated using Fick's equation. Stress hormones (ACTH, epinephrine, and norepinephrine) were determined before and after surgery. The duration of the surgical procedure was 41+/-8 min for CSI and 49+/-9 min for VATS (mean+/-SEM). With VATS, PaO2 decreased during and after surgery (P < 0.05), whereas alveolar-arterial PO2 difference (PA-aO2) and PaCO2 were increased (P < 0.05). Compared with CSI after surgery, PaO2 with VATS was decreased (130+/-10 vs 171+/-12 mm Hg; P < 0.05). Systemic perfusion was lower during VATS (76.7% of baseline) than during CSI (107% of baseline; P < 0.05). Heart rate, mean arterial pressure, and right ventricular end-diastolic pressure remained unchanged with both treatments. Stress hormones were comparable between groups. We conclude that systemic perfusion and oxygenation were impaired during VATS compared with CSI. Therefore, VATS may be contraindicated in pediatric patients with minor cardiopulmonary reserve.

IMPLICATIONS

We studied the cardiopulmonary effects of endoscopic interruption of the patent ductus arteriosus in an animal model of newborn pigs. Gas exchange and systemic perfusion were impaired compared with conventional interruption of the patent ductus arteriosus after thoracotomy.

[Dissociative stupor--differential diagnosis of coma following injury]

Two cases are described in which a dissociative stupor originating from conversion neurosis simulated a coma following a sustained trauma. At first both patients showed no response to being addressed or to pain stimuli. They presented an upward eye gaze deviation, cardiorespiratory functions were stable. Following extensive diagnostic procedures revealing no organic cause for the clinical symptoms, the diagnosis of a hysterical consciousness disorder was stated. Symptoms of conversion neuroses include lacking call response, gait disorder, seizure-like conditions and strength diminution in one or more extremities. In these cases suspicious facts are the absence of injuries (for example by falling down or tongue bite during a dissociative attack), eye gaze deviation and the phenomenon that, when the patient's arm is raised above the head and let fall, it never hits the face but glides down beside the body.

Intrathoracic and pulmonary blood volume during CO2-pneumoperitoneum in humans

BACKGROUND

Induction of CO2-pneumoperitoneum may have significant effects on systemic and pulmonary haemodynamics. We hypothesized that intrathoracic (ITBV) and pulmonary blood volume (PBV) are affected during intra-abdominal CO2-insufflation, which may be pronounced by positional changes of the patient.

METHODS

Sixteen anaesthetized patients were studied before, during and after CO2-pneumoperitoneum for laparoscopic cholecystectomy. A dye indicator technique was used to assess ITBV and PBV. In addition, gas exchange and haemodynamics were recorded.

RESULTS

In the supine position, induction of CO2-pneumoperitoneum had no effects on ITBV, PBV and cardiac output. Mean systemic arterial pressure increased from 10.9 +/- 1.5 kPa (82 +/- 11 mmHg) to 12.7 +/- 1.5 kPa (95 +/- 11 mmHg, P < 0.01). In the reverse Trendelenburg position ITBV decreased from 19.8 +/- 5.1 ml.kg-1 to 16.7 +/- 3.7 ml.kg-1 (P < 0.05) during CO2-insufflation, but increased to control values after 20 min. PBV decreased from 4.2 +/- 1.2 ml.kg-1 to 3.4 +/- 1.1 ml.kg (P < 0.05) and remained decreased during CO2-pneumoperitoneum. Calculated venous admixture was unchanged throughout the study. Deflation of CO2-pneumoperitoneum increased ITBV (22.4 +/- 5.2 ml.kg-1, P < 0.05) and cardiac output above control values.

CONCLUSIONS

In anaesthetized-paralyzed patients in the reverse Trendelenburg position intra-abdominal CO2-insufflation is associated with significant alterations of ITBV and PBV. The release of CO2-pneumoperitoneum is associated with a re-distribution of blood into the thorax.

Atelectasis and gas exchange after cardiac surgery

BACKGROUND

Sometimes a high intrapulmonary shunt occurs after cardiac surgery, and impairment of lung function and oxygenation can persist for 1 week after operation. Animal studies have shown that postoperative shunt can be explained by atelectasis. In this study the authors tried to determine if atelectasis can explain shunt in patients who have had cardiac surgery.

METHODS

Nine patients having coronary artery bypass graft surgery and nine patients having mitral valve surgery were examined using the multiple inert gas elimination technique before and after operation. On the first postoperative day, computed tomography scans were made at three levels of the thorax.

RESULTS

Before anesthesia, the average shunt was low (2+/-3%; range, 0-13%), but on the first postoperative day shunt had increased to 12+/-60% (range, 3-28%). The computed tomography scans showed bilateral dependent densities in all patients but one. The mean area of the densities was 8+/-8% (range, 0-37%) of total lung area, corresponding to a calculated fraction of collapsed lung tissue of 20+/-14% (range, 0-59%). In the basal region, the calculated amount of collapsed tissue was 28+/-19% (range, 0-73%). One mitral valve patient was an outlier and had a large shunt both before and after the operation.

CONCLUSIONS

Large atelectasis in the dorsal part of the lungs was found on the first postoperative day after cardiac surgery. However, there was no clear correlation between atelectasis and measured shunt fraction.

[Ventilation-perfusion distribution with volume-reduced, pressure-limited ventilation with permissive hypercapnia]

PURPOSE

Low volume pressure-limited ventilation with permissive hypercapnia (PH) may decrease the mechanical stress of the lung in acute respiratory insufficiency. Alveolar PCO2 is a determinant of regional ventilation, whereas increased mixed-venous and arterial PCO2 may affect systemic and pulmonary haemodynamics. The aim of this study was to analyse the ventilation-perfusion (VA/Q) distribution during controlled ventilation with permissive hypercapnia.

METHODS

The study was approved by the ethical committee of the Ernst-Moritz-Arndt University of Greifswald. Eleven patients with severe ARDS (lung injury severity score 2.77 +/- 0.47) were studied. Intrapulmonary shunt (QS/QT, % of QT), lung areas with 0.005 < or = VA/Q < or = 0.1 ("low" VA/Q, % of QT), lung areas with 10 < or = VA/Q < or = 100 ("high" VA/Q, % of VE), dead space ventilation (VD/VT = VA/Q > 100, % of VE) and the mean distribution of ventilation (Vmean VA/Q) and perfusion (Qmean VA/Q) were determined by the multiple inert gas elimination technique during normocapnic (NC) and hypercapnic (HC) mechanical ventilation. In addition, systemic mean arterial and pulmonary arterial pressure, cardiac output (CO) and arterial and mixed venous partial pressures for oxygen (PaO2, PvO2) and carbondioxide (PaCO2, PvCO2) were assessed.

RESULTS

Low-volume pressure-limited ventilation was associated with moderate hypercapnia (PaCO2 = 61 +/- 12 mmHg vs. 39 +/- 6 mmHg, p < 0.01). QS/QT increased (28 +/- 16% [NC] vs. 36 +/- 17% [HC], p < 0.05), whereas Qmean VA/Q decreased from 1.01 +/- 0.37 (NC) to 0.65 +/- 0.49 (HC), (p < 0.01) and Vmean VA/Q decreased from 1.54 +/- 0.58 (NC) to 1.12 +/- 0.93 (HC) (p < 0.05). Hypercapnia induced mild systemic hypotension and pulmonary hypertension. CO increased from 10.8 +/- 2.3 l/min to 11.6 +/- 2.6 l/min (p < 0.05). PaO2 was almost unchanged, but PvO2 increased significantly from 40 +/- 4 mmHg (NC) to 49 +/- 7 mmHg (HC) (p < 0.01).

CONCLUSION

The mechanical ventilation with permissive hypercapnia may increase shunt due to alveolar derecruitement and an impaired hypoxic pulmonary vasoconstriction. PaO2 was unchanged due to an increased CO, PvO2 and--to a lesser extent--shift of the oxyhaemoglobin dissociation curve.

Stress failure of the blood-gas barrier

The blood-gas barrier must be extremely thin because oxygen and carbon dioxide cross the alveolar-capillary membrane by passive diffusion, and the diffusion resistance is proportional to thickness. Despite its remarkable size (harmonic mean thickness approximately 0.6 microm) the membrane must be immensely strong, because maintenance of its integrity is fundamental for pulmonary gas exchange. The basement membrane is probably the principal anatomical structure providing the strength of the blood-gas barrier. Experimental studies have demonstrated that wall stress of the capillaries can become very high when perfusion pressure is increased to 5.2 kPa (39 mmHg) or more, which was associated with breaks of the capillary endothelium, the alveolar epithelium, or both. These values are potentially reached or exceeded in different cardiac or pulmonary diseases, or in healthy humans subjected to heavy exercise. Stress failure of pulmonary capillaries may play a role in neurogenic pulmonary oedema, high-altitude pulmonary oedema, re-expansion pulmonary oedema, and some forms of the adult respiratory distress syndrome. Increased alveolar pressure due to lung inflation potentiates damage of the blood-gas barrier, suggesting that increases in capillary transmural pressure and transpulmonary pressure are equivalent in terms of their effects on capillary wall stress. These data may have importance for the management of patients with acute respiratory failure requiring mechanical ventilation.

The effect of dopexamine on ventilation perfusion distribution and pulmonary gas exchange in anesthetized, paralyzed patients

We studied the effects of the beta2-adrenoceptor and DA1-receptor agonist dopexamine on ventilation perfusion (V(A)/Q) distribution in anesthetized, paralyzed patients (n = 17) undergoing major abdominal surgery. Intrapulmonary shunt (Q(S)/Q(T)) (percentage of cardiac output [CO]), perfusion of low V(A)/Q areas (percentage of CO), ventilation of high V(A)/Q areas (percentage of total ventilation [V(E)]), and dead space ventilation [percentage of V(E)]) were calculated from the retention/excretion data of six inert gases. In the control state, Q(S)/Q(T) was 11% +/- 9% (mean +/- SD) and little perfusion of low V(A)/Q areas (3% +/- 4%) was observed. Infusion of 1.0 microg kg(-1) x min(-1) dopexamine had no effect on Q(S)/Q(T) and low V(A)/Q areas despite an increased CO (7.7 +/- 2.2 L/min versus 6.2 +/- 1.2 L/min; P < 0.01). Pao2 increased from 15.5 +/- 5.6 kPa (116 +/- 42 mm Hg) to 17.3 +/- 6.3 kPa (130 +/- 47 mm Hg) (P < 0.05). Infusion of 2.0 microg x kg(-1) x min(-1) dopexamine further increased CO to 8.4 +/- 2.7 L/min (P < 0.01) without alterations of Q(S)/Q(T), perfusion of low V(A)/Q areas, and Pao2. We concluded that dopexamine (1.0 microg x kg(-1) x min(-1) and 2.0 microg x kg(-1) x min(-1)) has no adverse effects on V(A)/Q relationships and Q(S)/Q(T) in anesthetized, paralyzed patients.

IMPLICATIONS

The I.V. administration of vasoactive drugs can improve oxygen delivery to different organ systems but may impair pulmonary gas exchange. In anesthetized, paralyzed patients, we studied the effects of beta2-adrenoceptor and DA1-receptor agonist dopexamine on ventilation perfusion distribution. Dopexamine (1.0 microg x kg(-1) x min(-1) and 2.0 microg x kg(-1) min(-1)) improved cardiac output and oxygenation without alterations of intrapulmonary shunt.

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.

Cardiopulmonary effects of enoximone or dobutamine and nitroglycerin on mitral valve regurgitation and pulmonary venous hypertension

OBJECTIVE

To compare the cardiovascular and pulmonary effects of the phosphodiesterase III inhibitor enoximone (EN) or a combination of dobutamine (DOB) and nitroglycerin (NTG) before and after mitral valve repair or replacement.

DESIGN

Prospective, randomized, controlled clinical study.

SETTING

University hospital.

PARTICIPANTS

Twenty patients with mitral regurgitation and pulmonary venous hypertension scheduled for elective mitral valve surgery.

INTERVENTIONS

Patients fulfilling the inclusion criteria of the study were randomly allocated into a group treated with EN (group 1, n = 10) or DOB and NTG (group 2, n = 10). A cardiopulmonary status was obtained after induction of anesthesia and mechanical ventilation during stable hemodynamic conditions (control). Then the patients received either EN (bolus dose 1.0 mg/kg followed by a continuous infusion of 10 micrograms/kg/min) or DOB (8.0 micrograms/kg/min) and NTG (1.0 microgram/kg/min) according to the randomization. After a period of 20 minutes, all parameters were measured again. The study drugs were stopped, and cardiac surgery was performed. Infusions of EN (without additional loading dose) or DOB and NTG were started again in the above-described doses 10 minutes before separation from cardiopulmonary bypass (CPB). Respiratory and hemodynamic measurements were made 20 minutes after weaning from CPB and 60 minutes after admission of the patient to the intensive care unit.

MEASUREMENTS AND MAIN RESULTS

Both groups were comparable regarding preoperative and control data. Before mitral valve surgery, cardiac output (CO) and heart rate (HR) increased by 46% (p < 0.05) and 31% (p < 0.01) during infusion of EN with minor changes of mean systemic arterial pressure (PSA) and gas exchange. Mean pulmonary arterial pressure (PPA) decreased from 32 +/- 11 mmHg to 23 +/- 11 mmHg (p < 0.05). Similar alterations were observed in group 2 (delta CO + 26%, p < 0.05, delta HR + 39%, p < 0.01); however, PPA and calculated pulmonary vascular resistance remained unchanged. After separation from CPB, EN and DOB-NTG achieved comparable effects on CO, HR, and PSA, but PPA was significantly lower in group 1. In addition, venous admixture and alveolo-arterial oxygen tension gradient were lower in EN-treated patients.

CONCLUSION

Enoximone or DOB and NTG have comparable effects on CO, PSA, and HR in mitral regurgitation and pulmonary hypertension, but EN is more effective in reducing PPA without deterioration of gas exchange.

The ventilation-perfusion relation and gas exchange in mitral valve disease and coronary artery disease. Implications for anesthesia, extracorporeal circulation, and cardiac surgery

BACKGROUND

Patients with mitral valve disease (MVD) are at greater risk for respiratory complications after cardiac surgery compared with patients with coronary artery disease (CAD). The authors hypothesized that ventilation-perfusion (VA/Q) inequality is more pronounced in patients with MVD before and after induction of anesthesia and during and after surgery when extracorporeal circulation (ECC) is used.

METHODS

In patients with MVD (n = 12) or with CAD (n = 12), VA/Q distribution was determined using the multiple inert gas elimination technique. Intrapulmonary shunt (Qs/Qr) defined as regions with VA/Q < 0.005 [% of total perfusion (Qr)], perfusion of "low" VA/Q areas (0.005 < or = VA/Q < 0.1, [% of Qr]), ventilation of "high" VA/Q regions (10 < or = VA/Q < or = 100 [% of total ventilation VE]), and dead space (VA/Q > 100 [% of VE]) were calculated from the retention/excretion data of the inert gases. Recordings were obtained while patients spontaneously breathed air in the awake state, during mechanical ventilation after induction of anesthesia, after separation of patients from ECC, and 4 h after operation.

RESULTS

Qs/Qr was low in the awake state (MVD group, 3% +/- 3%; CAD group, 3% +/- 4%) and increased after induction of anesthesia to 10% +/- 8% (MVD group, P < 0.05) and 11% +/- 7% (CAD group, P < 0.01). Qs/Qr increased further after separation from ECC (MVD group, 24% +/- 9%, P < 0.01; CAD group, 23% +/- 7%, P < 0.01). Similarly, alveolar-arterial oxygen tension difference (PA-aO2) increased from 168 +/- 54 mmHg (anesthetized state) to 427 +/- 138 mmHg after ECC (MVD group, P < 0.01) and from 153 +/- 65 mmHg to 377 +/- 101 mmHg (CAD group, P < 0.01). In both groups, PA-aO2 was correlated with Qs/Qr. Four hours after operation, Qs/Qr had decreased significantly to 8% +/- 6% (CAD group) and 10% +/- 6% (MVD group). PA-aO2 and Qs/Qr showed no significant differences between the CAD and MVD groups.

CONCLUSIONS

Qs/Qr is the main pathophysiologic mechanism of gas exchange impairment during cardiac surgery for MVD or CAD. Impairment of pulmonary gas exchange secondary to general anesthesia, cardiac surgery, and ECC are comparable for patients undergoing myocardial revascularization or mitral valve surgery.

[Augmented spontaneous breathing]

Impaired pulmonary gas exchange can result from lung parenchymal failure inducing oxygenation deficiency and fatigue of the respiratory muscles, which is characterized by hypercapnia or a combination of both mechanisms. Contractility of and coordination between the diaphragm and the thoracoabdominal respiratory muscles predominantly determine the efficiency of spontaneous breathing. Sepsis, cardiac failure, malnutrition or acute changes of the load conditions may induce fatigue of the respiratory muscles. Augmentation of spontaneous breathing is not only achieved by the application of different technical principles or devices; it also has to improve perfusion, metabolism, load conditions and contractility of the respiratory muscles. Intermittent mandatory ventilation (IMV) allows spontaneous breathing of the patient and augments alveolar ventilation by periodically applying positive airway pressure tidal volumes, which are generated by the respirator. Potential advantages include lower mean airway pressure (PAW), as compared with controlled mechanical ventilation, and improved haemodynamics. Suboptimal IMV systems may impose increased work and oxygen cost of breathing, fatigue of the respiratory muscles and CO2 retention. During pressure support ventilation (PSV), inspiratory alterations of PAW or gas flow (trigger) are detected by the respirator, which delivers a gas flow to maintain PAW at a fixed value (usually 5-20 cm H2O) during inspiration. PSV may be combined with other modalities of respiratory therapy such as IMV or CPAP. Claimed advantages of PSV include decreased effort of breathing, reduced systemic and respiratory muscle consumption of oxygen, prophylaxis of diaphragmatic fatigue and an improved extubation rate after prolonged periods of mechanical ventilation. Minimum alveolar ventilation is not guaranteed during PSV; thus, close observation of the patient is mandatory to avoid serious respiratory complications. Continuous positive airway pressure breathing (CPAP) maintains PAW above atmospheric pressure throughout the respiratory cycle, which may increase functional residual capacity and decrease the effort of breathing. CPAP has been conceptually designed for the augmentation of spontaneous breathing and requires the intact central and peripheral regulation of the respiratory system. Airway pressure release ventilation (APRV) improves alveolar ventilation by intermittent release of PAW, which is kept above atmospheric pressure by means of a high-flow CPAP system. The opening of an expiratory valve for 1-2 s induces a decreased PAW and lung volume, which increases rapidly to pre-exhalation values after closure of the valve due to the high gas flow within the circuit (90-100 1/min). APRV may improve haemodynamics and VA/Q distribution as compared with conventional mechanical ventilation. Biphasic positive airway pressure (BIPAP) is characterized by the combination of spontaneous breathing and time-regulated, pressure-controlled mechanical ventilation. During the respiratory cycle the ventilator generates two alternating CPAP levels, which can be modified with regard to time and pressure. As with APRV, alveolar ventilation is maintained even if the spontaneous breathing efforts of the patient cease, which improves the safety of both modes of respiratory therapy. The contribution of spontaneous breathing to total minute ventilation may be important, since a decreased shunt and improved VA/Q relationship have been observed in experimental non-cardiogenic lung oedema. These data give support to the concept that spontaneous breathing should be maintained and augmented in the setting of acute respiratory failure.

Ventilation-perfusion inequality in patients undergoing cardiac surgery

BACKGROUND

Impaired gas exchange is a major complication after cardiac surgery with the use of extracorporeal circulation. Blood gas analysis gives little information on underlying mechanisms, in particular if the impairment is multifactorial. In the current study we used the multiple inert gas technique with recordings of hemodynamics to analyze the separate effects of intrapulmonary shunt (QS/QT), ventilation-perfusion (VA/Q) mismatch, and low mixed venous oxygen tension on arterial oxygenation during cardiac surgery.

METHODS

VA/Q distribution was studied in nine patients undergoing coronary artery revascularization surgery. The obtained data related to VA/Q distribution were perfusion of lung regions with VA/Q < 0.005 (QS/QT), perfusion of lung regions with 0.005 < VA/Q < 0.1 ("low"-VA/Q regions), ventilation of lung regions with 10 < VA/Q < 100 ("high"-VA/Q regions), and ventilation of lung regions with VA/Q > 100 (dead space [VD/VT]). In addition, arterial and mixed venous oxygen and carbon dioxide tensions and systemic and pulmonary hemodynamics were analyzed. Recordings were made before and after induction of anesthesia, after sternotomy, 45 min after separation from extracorporeal circulation, 4 h postoperatively during mechanical ventilation, and on the 1st postoperative day during spontaneous breathing.

RESULTS

In the awake state, QS/QT was 4 +/- 4%, and perfusion of low-VA/Q regions was 3 +/- 5%. The sum of QS/QT and low-VA/Q units correlated with the alveolar-arterial oxygen tension gradient (PA-aO2) (r = 0.63, P < 0.05). After induction of anesthesia, QS/QT increased to 10 +/- 9% (P = 0.069). Sternotomy had little effect on shunt, but QS/QT increased to 22 +/- 8% (P < 0.01) after separation from extracorporeal circulation, which was correlated with a significantly higher PA-aO2 (r = 0.77, P < 0.05). Postoperatively, gas exchange improved rapidly, as assessed by a decrease of PA-aO2 from 341 +/- 77 to 97 +/- 36 mmHg (P < 0.01) and a reduced QS/QT (5 +/- 4%, P < 0.05). On the 1st postoperative day, arterial oxygen tension was significantly lower than preanesthesia values (58 +/- 6 vs. 68 +/- 8 mmHg, P < 0.05). QS/QT had increased to 11 +/- 6% (P < 0.05), but little perfusion of low-VA/Q units was observed. A correlation was found between PA-aO2 and QS/QT (r = 0.82, P < 0.03).

CONCLUSIONS

QS/QT is a major component of impaired gas exchange before, during, and after cardiac surgery. QS/QT increases after induction of general anesthesia, probably because of development of atelectasis. After separation from extracorporeal circulation, accumulation of extravascular lung water or further collapse of lung tissue may aggravate QS/QT. Postoperatively, oxygenation improves, possibly because of recruitment of previously nonventilated alveoli or resolution of extravascular lung water. During spontaneous breathing, additional mechanisms such as altered mechanics of the chest, perfusion of low-VA/Q regions, and decreased mixed venous oxygen tension may contribute to impaired gas exchange.

Thoracic intravascular and extravascular fluid volumes in cardiac surgical patients

BACKGROUND

One possible mechanism of impaired oxygenation in cardiac surgery with extracorporeal circulation (ECC) is the accumulation of extravascular lung water (EVLW). Intrathoracic blood volume (ITBV) and pulmonary blood volume (PBV) also may increase after separation from ECC, which can influence both cardiac performance and pulmonary capillary fluid filtration. This study tested whether there were any relationships between lung fluid accumulation and pulmonary gas exchange during the perioperative period of cardiac surgery and ECC.

METHODS

Ten patients undergoing myocardial revascularization were studied. ITBV, PBV, and EVLW were determined from the mean transit times and decay times of the dye and thermal indicator curves obtained simultaneously in the descending aorta. Gas exchange was assessed by arterial and mixed venous partial pressure of oxygen (PO2) and carbon dioxide (PCO2), and calculation of alveolo-arterial PO2 gradient (PA-aO2) and venous admixture (QVA/QT). Recordings were made after induction of anesthesia, after sternotomy, 15 min after separation from ECC, and 4 and 20 h postoperatively.

RESULTS

After induction of anesthesia, EVLW (6.0 +/- 1.0 ml/kg, mean +/- SD), PBV (3.6 +/- 1.3 ml/kg), and ITBV (18.4 +/- 2.7 ml/kg) were within normal ranges. Oxygenation was moderately impaired, as indicated by an increased PA-aO2 (144 +/- 46 mmHg) and QVA/QT (11 +/- 4%). After separation from ECC, EVLW had increased to 9.1 +/- 2.6 ml/kg, which was accompanied by an increase of ITBV (26.0 +/- 4.4 ml/kg) and PBV (5.6 +/- 1.9 ml/kg). PAa-O2 (396 +/- 116 mmHg) and QVA/QT (29 +/- 7%) also were increased. ITBV and PBV remained increased 4 and 20 h postoperatively, but EVLW decreased to presurgery values. No correlations were found between thoracic intravascular and extravascular fluid volumes and gas exchange.

CONCLUSIONS

Cardiac surgery with the use of ECC induces alterations of thoracic intravascular and extravascular fluid volumes. Postoperatively, increased ITBV and PBV need not be associated with higher EVLW. Thus, sufficient mechanisms protecting against lung edema formation or providing resolution of EVLW probably are maintained after ECC. Since oxygenation is impaired during and after cardiac surgery, it is concluded that mechanisms other than or in addition to changes of ITBV, PBV, and EVLW predominantly influence gas exchange.

Mechanical ventilation in the prone position for acute respiratory failure after cardiac surgery

Ten patients with acute respiratory failure (ARF) after coronary artery bypass grafting were studied during conventional mechanical ventilation in the supine and in the prone position. Impaired gas exchange was defined as an inspired oxygen fraction (FIO2) greater than 0.5 to maintain an arterial oxygen tension (PaO2) > or = 70 mmHg, an alveolar-arterial PaO2 gradient (PA-aO2) > 200 mmHg and a venous admixture (QVA/QT) > 15% during mechanical ventilation with a tidal volume (VT) = 10 to 12 mL/kg, frequency (f) = 10 to 15 VT/min, inspiratory-expiratory (I:E) ratio = 0.5, and positive end-expiratory pressure (PEEP) of 5 to 7.5 cm H2O. In the supine position, systemic and pulmonary hemodynamics were in the normal range, but oxygenation was severely impaired. In all patients thoracic computed tomography scans were obtained and revealed crest-shaped bilateral densities in the dependent lung regions. FIO2 of 0.67 +/- 0.22 was required to maintain a PaO2 greater than 70 mmHg during mechanical ventilation in the supine position. Under these conditions PA-aO2 and QVA/QT were 362 +/- 153 mmHg and 32.5 +/- 8.3%, respectively. CO2 elimination was not severely affected. The patients were turned into the prone position after an average of 30.6 +/- 5.4 hours postoperatively and ventilated with unchanged VT, f, PEEP, and inspiratory-expiratory ratio for 26.7 +/- 11.7 hours (range, 10 to 42 hours). A second cardiopulmonary status was obtained within 2 to 5 hours of ventilation in the prone position.(ABSTRACT TRUNCATED AT 250 WORDS)

Analysis of lung density by computed tomography before and during general anaesthesia

Pulmonary structure was analysed by means of computed tomography (CT) in 20 lung-healthy patients, relating tissue density to the attenuation value (AV) of a picture element. Regional density of pulmonary tissue (rlung) was determined using mean lung density in five regions of interest (ROI1-5) (sector method). Vertical and horizontal distributions of x-ray attenuation were analysed by density profiles, relating AV values to evenly distributed and normalised length scales. In group I (n = 12), CT-densitometry was obtained in awake, supine patients and after induction of general anaesthesia. In group II (n = 8), the effect of mechanical ventilation with positive end-expiratory pressure (PEEP, 1.0 kPa [10 cmH2O]) was studied. In the awake state, a vertical tissue density difference between the top and bottom of the lung was found in all patients, accounting for a mean of 0.235 g.cm-3 (right lung) and 0.199 g.cm-3 (left lung). Only minor changes were seen in the horizontal lung density profiles. After induction of anaesthesia, x-ray attenuation of ROI1-4 showed no significant differences when compared with the awake state. The basal lung areas (ROI5) revealed a significantly increased tissue density (P < or = 0.01), reaching mean values of 0.94 g.cm-3 (right lung) and 0.814 g.cm-3 (left lung). Similarly, vertical density profiles showed a markedly enhanced rlung of the bottom of the lung in all patients, interpreted as atelectasis. The amount of atelectasis accounted for 4.8 +/- 2.6% (right lung) and 4.7 +/- 2.1% (left lung) of the intrapulmonary area.(ABSTRACT TRUNCATED AT 250 WORDS)

Effective pulmonary capillary pressure in experimental myocardial ischaemia

Effective pulmonary capillary pressure and extravascular lung water were investigated in dogs (n = 9) with normal heart function and after development of acute myocardial ischaemia. During control, no impairment of cardiopulmonary performance was observed. Extravascular lung water was in the normal range (8.1 +/- 2.8 ml.kg-1) and the effective pulmonary capillary pressure accounted for 1.36 +/- 0.53 kPa (10.2 +/- 4 mmHg). No correlation between extravascular lung water and effective pulmonary capillary pressure was observed (r2 = 0.347, P = 0.06). Arterial (RPA) and venous pulmonary resistance (RPV) were 70 +/- 15% and 30 +/- 6%, respectively. Acute myocardial ischaemia was induced by one stage occlusion of the left anterior descending (LAD) coronary artery; measurements during the ischaemia phase were performed 60 min following LAD occlusion. Myocardial ischaemia resulted in moderate changes of cardiac output, heart rate and left ventricular end-diastolic pressure. Oxygenation deteriorated, but no hypoxaemia occurred in any animal and CO2 elimination remained unchanged. Extravascular lung water was elevated (16.5 +/- 7.9 ml.kg-1, P < or = 0.01), and effective pulmonary capillary pressure was higher when compared with the control state (2.32 +/- 1.05 kPa (17.4 +/- 7.9 mmHg), P < or = 0.01). There was a significant correlation between both parameters (r2 = 0.528, P < or = 0.05). Longitudinal distribution of pulmonary vascular resistance was altered, and RPA decreased to 60 +/- 13% (P < or = 0.05), while RPV increased to 40 +/- 8% (P < or = 0.05). It is concluded that development of lung oedema is related to elevated effective pulmonary capillary pressure in dogs with acute myocardial ischaemia.(ABSTRACT TRUNCATED AT 250 WORDS)