CHANGES IN FUNCTIONAL RESIDUAL CAPACITY DURING CARDIAC SURGERY

A gas washout technique was used to measure the functional residualcapacity (FRC) in eight patients during anaesthesia for cardiacsurgery. The patients were anaesthetized with droperidol, fentanyland nitrous oxide, alcuronium was given and the lungs were ventilatedwith a volume controlled ventilator. FRC was measured at threestages: before skin incision, after sternotomy but before cardiopulmonarybypass, and after closure of the sternum. The pleural cavitieswere intact in all patients during the operation. FRC beforeskin incision was 1.7±0.5 litre (mean±1 SD). A55% mean increase in volume was noted after sternotomy and placementof the sternal retractor (P<0.001). Mean FRC after sternalclosure was 16% lower than the preincision value (P<0.05).Arterial Po₂ was measured in 22 other patients who underwentcoronary artery bypass surgery and in whom F1_o2 was 0.5. Pa_o2increased significantly when the sternum was opened, but decreasedafter cardiopulmonary bypass. There was a further significantdecrease on closure of the sternum.     

Effect of apparatus on functional residual capacity

As the route of breathing and use of airway apparatus such as mask, mouthpiece and noseclip can alter breathing pattern, this study has used the helium dilution method to estimate the effects of mouthpiece and mask breathing on functional residual capacity (FRC) in the supine position, and the change in FRC that occurs between the sitting and supine positions while breathing by mouthpiece. In 13 normal subjects, breathing by mouthpiece, FRC was smaller, by a median of 1.07 litre (interquartile values 0.73-1.43 litre) in the supine compared with the sitting position (P < 0.01), but residual volume (RV) did not change significantly. FRC measured in the supine position was significantly greater when breathing by mask than by mouthpiece (0.25, 0.04-0.38 litre) and RV was greater by similar amounts (0.20, -0.02 to 0.49 litre). This difference may result from increased inspiratory activity while breathing via the mask.      

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.     

Applied respiratory physiology

Anaesthesia has many effects on respiratory physiology, the knowledge of which is relevant to clinical practice. Anaesthesia causes decreased muscle tone in the upper airway, which can lead to airway obstruction. Pulmonary hypoventilation occurs in the spontaneously breathing patient. There is a progressive decrease in the ventilatory response to CO₂ with increasing concentration of volatile agents, and even low doses of volatile agents have a profound effect on the ventilatory response to hypoxia. Functional residual capacity (FRC) is significantly reduced in the anaesthetized patient. Airway closure occurs when closing capacity exceeds FRC; with a reduced FRC this is more likely to happen especially in older patients or patients with coexisting lung pathology when closing capacity may be increased. The resulting atelectasis will affect oxygenation. Respiratory system compliance reduces very early during anaesthesia and there is little difference between the paralysed and spontaneously breathing patient. Alveolar dead space is decreased due to impairment of V/Q matching. During anaesthesia, venous admixture accounts for 10% of cardiac output due to increased shunt and changes in V/Q scatter. During anaesthesia and surgery, patient position, type of surgery, smoking and obesity all have specific effects on respiratory physiology. Exercise physiology parameters such as anaerobic threshold have a role as a measure of cardiorespiratory fitness such as in cardiopulmonary exercise testing (CPX). CPX is increasingly used in risk stratification in patients undergoing major surgery. Anaerobic threshold is the point at which oxygen delivery mechanisms can no longer match the oxygen demand required in exercise.     

Applied respiratory physiology

Anaesthesia has many effects on respiratory physiology, the knowledge of which is relevant to clinical practice. Anaesthesia causes decreased muscle tone in the upper airway, which can lead to airway obstruction. Pulmonary hypoventilation occurs in the spontaneously breathing patient. There is a progressive decrease in the ventilatory response to CO₂ with increasing concentration of volatile agents, and even low doses of volatile have a profound effect on the ventilatory response to hypoxia. Functional residual capacity (FRC) is significantly reduced in the anaesthetized patient. Airway closure occurs when closing capacity exceeds FRC, with a reduced FRC this is more likely to happen especially in older patients or patients with coexisting lung pathology when closing capacity may be increased. The resulting atelectasis will affect oxygenation. Respiratory system compliance reduces very early during anaesthesia and there is little difference between the paralysed and spontaneously breathing patient. Alveolar dead space is decreased due to impairment of V/Q matching. During anaesthesia venous admixture accounts for 10% of cardiac output due to increased shunt and changes in V/Q scatter. During anaesthesia and surgery patient position, type of surgery, smoking and obesity all have specific effects on respiratory physiology. Exercise physiology parameters such as anaerobic threshold have a role as a measure of cardiorespiratory fitness such as in cardiopulmonary exercise testing (CPX). CPX is increasingly used in risk stratification in patients undergoing major surgery. Anaerobic threshold is the point at which oxygen delivery mechanisms can no longer match the oxygen demand required in exercise     

Applied respiratory physiology

Anaesthesia has many effects on respiratory physiology, the knowledge of which is relevant to clinical practice. Anaesthesia causes decreased muscle tone in the upper airway, which can lead to airway obstruction. Pulmonary hypoventilation occurs in the spontaneously breathing patient. There is a progressive decrease in the ventilatory response to CO₂ with increasing concentration of volatile agents, and even low doses of volatile have a profound effect on the ventilatory response to hypoxia. Functional residual capacity (FRC) is significantly reduced in the anaesthetized patient. Airway closure occurs when closing capacity exceeds FRC, with a reduced FRC this is more likely to happen especially in older patients or patients with coexisting lung pathology when closing capacity may be increased. The resulting atelectasis will affect oxygenation. Respiratory system compliance reduces very early during anaesthesia and there is little difference between the paralysed and spontaneously breathing patient. Alveolar dead space is decreased due to impairment of V/Q matching. During anaesthesia, venous admixture accounts for 10% of cardiac output due to increased shunt and changes in V/Q scatter. During anaesthesia and surgery patient position, type of surgery, smoking and obesity all have specific effects on respiratory physiology. Exercise physiology parameters such as anaerobic threshold have a role as a measure of cardiorespiratory fitness such as in cardiopulmonary exercise testing (CPX). CPX is increasingly used in risk stratification in patients undergoing major surgery. Anaerobic threshold is the point at which oxygen delivery mechanisms can no longer match the oxygen demand required in exercise.     

Airway Closure and Distribution of Inspired Gas in the Extremely Obese, Breathing Spontaneously and During Anaesthesia with Intermittent Positive Pressure Ventilation

Airway closure (closing capacity, CC), FRC, total efficiency of ventilation (lung clearance index, LCI) and distribution of inspired gas (nitrogen washout delay percentage, NWOD) were determined by nitrogen washout techniques and arterial Po₂ and Pco₂ measured by standard electrodes in 10 extremely obese subjects, prior to and during anaesthesia and artificial ventilation. CC was normal, but because of small FRC, airway closure occurred within a tidal breath in 9 out of 10 subjects during spontaneous breathing, when awake. Po₂ was reduced, the hypoxaemia correlating to the magnitude of airway closure. LCI was normal, but NWOD was borderline. During anaesthesia, CC was unaltered but FRC was further reduced, so that in nine subjects airway closure occurred above FRC and tidal volume together. A marked increase in relative hypoxaemia was recorded. LCI and NWOD rose, indicating less efficient and less even ventilation. It is concluded that airway closure reasonably explains the marked hypoxaemia in obese subjects during anaesthesia, and that it may also be the reason for the uneven distribution of inspired gas.     

Lung mechanics during upper abdominal surgery

Functional residual capacity (FRC) and breath-by-breath compliance of the respiratory system (Crs) were studied after induction of anaesthesia, after insertion of retractors and after wound closure in patients undergoing upper abdominal surgery via a subcostal (n = 8) or a midline (n = 8) incision. After anaesthesia induction the mean FRC was 1.6 +/- 0.3 l. In the subcostal incision group FRC did not change between the studied stages, but Crs fell after retractor placement from 51 +/- 3 to 43 +/- 5 ml/cmH2O (p less than 0.01). In the midline incision group FRC rose by 21% (p less than 0.01) when the retractors were inserted, but regained outset level after wound closure. Crs in this group did not change significantly after retraction, but after closure of the wound it fell to 44 +/- 6 ml/cmH2O, i.e. less (p less than 0.05) than the outset value (52.6 ml/cmH2O). FRC thus did not decrease in either group, but Crs fell by about 15%. The authors conclude that the known difference in postoperative pulmonary complications between midline vs. subcostal incisions is not caused by the studied intraoperative events.     

VARIATIONS IN LUNG VOLUME AND COMPLIANCE DURING PULMONARY SURGERY

Functional residual capacity (FRC) and breath-by-breath complianceof the ventilatory system (C₁₅) were measured in 10 mechanicallyventilated patients during anaesthesia for lung surgery (pneumonectomy,lobectomy, lung or pleural resections or exploratory thoracotomy).In eight patients not requiring pneumonectomy, FRC of the lowerlung decreased by 8±9% (mean± 1 SD) (P < 0.05)while that of the upper lung increased by 75±24% (P <0.001) when the patient was turned to the lateral position.When the pleura was opened, FRC of the lower lung decreasedby a further 10±10% (P <0.01). One-lung ventilation(OLV), however, increased FRC of the lower lung back to thevalue found in the supine position before surgery. When two-lungventilation was re-established, FRC of the lower lung was aboutthe same as during corresponding stages before OLV. In the twopatients who underwent pneumonectomy, FRC of the remaining lungwas about 30% greater after OLV than at corresponding stagesbefore surgery. In the patients not requiring pneumonectomy,C₁₅ decreased from 29±6ml/cm H₂O to 23±6ml/cmH₂O (P < 0.05) on the lower side when the patient was turnedon his side. The corresponding figures on the upper side were24±8 ml/cm H₂O and 30±5ml/cm H₂O respectively(P < 0.05). There was no further significant change whenthe pleura was opened. After surgery when the patient was turnedto the supine position, C₁₅ of the lung not operated on wasalmost the same as before surgery.     

Lung mechanics after cardiac valve replacement.

Fourteen patients undergoing single aortic or mitral valve replacement had measurements made of lung volumes, static pressure-volume (P-V) relationships, and conductance-pressure relationships during deflation before operation and again between one and two years later. At follow-up, total lung capacity (TLC), functional residual capacity (FRC), residual volume (RV), and static tidal compliance (slope of static P-V deflation line for one litre above FRC) had increased significantly, in association with a decrease in heart size. There was a change in the shape and position of some P-V curves both in the aortic and mitral patients. In the patients with aortic disease P-V deflation curves shifted to the left after operation. In the patients with mitral disease the P-V deflation curves before operation crossed those measured after operation, so that at high lung volumes recoil became less after operation, but at low lung volumes recoil increased. Conductance had increased at high lung volumes. The data suggest that in longstanding pulmonary congestion, airways are more rigid making them less distensible at high and less compressible at low transpulmonary pressures than after operation when congestion has been at least partly relieved.     

PaO2 during anaesthesia and years of smoking predict late postoperative hypoxaemia and complications after upper abdominal surgery in patients without preoperative cardiopulmonary dysfunction

Background: The incidence of late postoperative hypoxaemia and complications after upper abdominal surgery is 20–50% among cardiopulmonary healthy patients. Atelectasis development during anaesthesia and surgery is the main hypothesis to explain postoperative hypoxaemia. This study tested the predictive value of PaO₂<19 kPa during combined general and thoracic epidural anaesthesia and the preoperative functional residual capacity (FRC) reduction in the 30° head tilt‐down position for the development of late prolonged postoperative hypoxaemia, PaO₂<8.5 kPa for a minimum of 3 out of 4 days, and other complications. Methods: Forty patients without cardiopulmonary morbidity, assessed by ECG, spirometry, FRC and diffusion capacity preoperatively, underwent upper abdominal surgery. PaO₂ during anaesthesia and preoperative FRC reduction were compared to known risk factors for the development of hypoxaemia and complications: age, pack‐years of smoking and duration of operation. The effect of optimizing pulmonary compliance with peroperative positive end‐expiratory pressure (PEEP) on postoperative hypoxaemia and complications was evaluated in a blinded and randomized manner. Results: Late prolonged postoperative hypoxaemia and other complications were found in 37% and 38% of the patients, respectively. Patients with PaO₂>19 kPa during anaesthesia with F_IO₂=0.33 exhibited a risk, irrespective of PEEP status, of suffering late prolonged hypoxaemia of 0% (0;23) and patients with PaO₂<19 kPa a risk of 52% (32;71), P<0.005. Having smoked more than 20 pack‐years was associated with a 47% (19;75) higher incidence of postoperative complications than having smoked less than 20 pack‐years, P<0.006. Conclusions: PaO₂ during anaesthesia and smoked pack‐years provide new tools evaluating patients undergoing upper abdominal surgery in order to predict the patients who develop late postoperative hypoxaemia and complications.     

Serum fluoride levels in morbidly obese patients: enflurane compared with isoflurane anaesthesia

Obese patients are known to metabolise anaesthetic agents more than patients of normal weight. The extent of this was investigated by the measurement of serum fluoride concentrations in 10 morbidly obese patients undergoing gastroplasty. Five were allocated to receive enflurane and five to receive isoflurane supplemented anaesthesia. The mean peak serum fluoride concentrations after enflurane anaesthesia were greater (22.7 mumol/litre, SE 2.9) than after isoflurane anaesthesia (6.5 mumol/litre, SE 0.6). The mechanisms and implications of this finding are discussed.     

PRESSURE-VOLUME RELATIONSHIPS IN THE LUNG DURING LAPAROSCOPY

Pressure-volume relationships for the total respiratory systemand for the lung were recorded in anaesthetized and paralysedpatients, during deflation from an airway pressure of 3 kPato FRC at a rate of 2 litre min^-1. Pleural pressure was estimatedby means of an oesophageal balloon. A group of nine female patients(mean age 32.7 yr) about to undergo laparoscopy were each studiedin four successive states: supine, 15° head down tilt, tiltand lithotomy position, and again in this position after abdominalinflation with nitrous oxide to a pressure of 0.8–1 kPa.Compliance values were calculated from the curves. Mean totalcompliance was increased significantly by moving to the lithotomyposition, and reduced markedly after inflation of the abdomen,because of a large reduction in thoracic compliance. Mean lungcompliance was unaltered, except for a slight but statisticallysignificant increase on moving from the supine to the Trendelenburgposition. Measurement of FRC by helium dilution in a group ofseven patients showed that abdominal inflation caused a meandecrease of 19%. Airway closure manoeuvres were carried outusing a helium bolus technique from FRC in five patients, butclosing volume could be measured in only one patient, in thesupine position. The absence of an inflexion in the slope ofthe pressure-volume curves for the other patients supportedthis negative finding. ^* Present address: Department of Anaesthesia, Royal VictoriaHospital, Montreal, Quebec, Canada.     

CHANGES IN INTRAGASTRIC PRESSURE ON INDUCTION OF ANAESTHESIA

Intragastric pressure was measured in 20 patients before, andimmediately after, the induction of anaesthesia with thiopentone.Intragastric pressure decreased in patients whose weights werethe same as or less than expected, and increased in fire ofseven patients whose weights were greater than expected (P=0.0013). These findings suggest that the decrease in FRC knownto occur on the induction of anaesthesia is caused by a decreasein inspiratory muscle tone in the diapbragrn and other muscles Footnotes ^*Present address: Department of Anaesthesia, Adden-brookes Hospital,Hills Road, Cambridge CB2 2QQ     

MEASUREMENT OF THE RELATIVE CONTRIBUTIONS OF RIB CAGE AND ABDOMEN/DIAPHRAGM TO TIDAL BREATHING IN MAN

A simple mathematical model of the chest wall was constructedso that during tidal breathing the relative volume contributionsof the rib cage and abdomen/diaphragm could be measured in man,using four mercury-in-rubber strain gauges around the trunk.From the dimensions of the trunk and the change in circumferencedetermined by the four gauges, the separate contributions ofrib cage and abdomen/diaphragm could be determined using a purpose-builtanalog computer. The system was evaluated in 13 laboratory personnel,and in 13 other subjects before and after anaesthesia. Therewas a linear relationship between tidal volumes computed andmeasured at the mouth, over the residual volume to (FRC+1 litre)range, with an error of +8%. The relative contribution of ribcage to tidal breathing showed a large scatter from 5 to 42%with a non-significant tendency to decrease with age. ^*Present address: R.A.F. Institute of Aviation Medicine, Farnborough,Hants     

DETECTION AND REVERSAL OF PULMONARY ABSORPTION COLLAPSE

Six healthy seated male subjects (aged 36–54 yr) inhaled100% oxygen for 10 min at normal lung volume and then for afurther 5 min with forced maximal expirations. Following thismanoeuvre, changes in arterial Po₂ ranged from a decrease of16.7 kPa to an increase of 2 kPa (—125 to +15 mm Hg) whilebreathing 100% oxygen, and a decrease of 1.8 kPa to an increaseof 1.1 kPa (—13.5 to + 8.6 mm Hg) while breathing air.Functional residual capacity (FRC) was unchanged in one subject,but decreased by 0.04–1.15 litre in the others. The chestradiograph was unaltered in the subjects with zero and 0.04litre decreases in FRC. Three of the other subjects showed lineshadows at the bases, while the subject with the largest decreasein FRC showed extensive areas of collapse. There was a goodcorrelation between changes in arterial Po₂, FRC and the chestradiograph. Substernal discomfort was felt when those subjectswith positive radiographic changes attempted to make a maximalinspiration. All changes were reversed by taking five maximalforced inspirations. Arterial Po₂ changes while breathing 100%oxygen provided the most sensitive indication of collapse. Changesin FRC and arterial Po2 while breathing air were generally withinthe normal reference range of these variables, and thereforeof little diagnostic value in the absence of control measurements.Under such circumstances, chest radiography seems to be themost sensitive indication of pulmonary collapse occurring inthe upright position. ^* Present address: Great Yarmouth General Hospital, Dene Side,Great Yarmouth NR30 2LX.     

Ventilatory consequences of the lateral position and thoracotomy in children

Functional residual capacity (FRC), breath-by-breath compliance of the respiratory system (Crs) and arterial oxygen tension (PaO2) were measured in ten children, two months to nine years of age, during anaesthesia for surgical correction of patent ductus arteriosus or coarctation of the aorta. The children were mechanically ventilated with halothane, nitrous oxide and oxygen. FIO2 was kept constant in each child. After induction of anaesthesia, FRC was 17 +/- 7 ml X kg-1 (mean +/- 1 SD), corresponding to 60 +/- 22 per cent of a predicted awake value. FRC increased to 21 +/- 8 ml X kg-1 (p = 0.0005) when the child was turned to its right side and decreased to 13 +/- 5 ml X kg-1 (p = 0.0003) when the pleura was opened. No significant change in Crs or PaO2 occurred during these manoeuvres. Retraction of the upper lung to visualize the great vessels caused a significant decrease in FRC, Crs, and PaO2. The lowest PaO2 observed during this stage was 70.0 mmHg. After surgery FRC and PaO2 were about the same as before surgery while Crs had decreased from 0.87 +/- 0.18 preoperatively to 0.64 +/- 0.15 ml X cmH2O-1 X kg-1 (p = 0.0069). This study shows that FRC increases when mechanically ventilated children are placed in the lateral position, and that thoracotomy is associated with marked changes in FRC, Crs and PaO2.     

Influence of Lung Volume History on Closing Volume Measurement During Anaesthesia

Airway closure measurements were made with the bolus technique on eight healthy subjects, who were in a supine position prior to and during anaesthesia. Measurements were made on an expiration following vital capacity (VC) and 30% VC. Closing volume (CV) was calculated prior to anaesthesia, and closing capacity (CC)—functional residual capacity (FRC) was estimated during anaesthesia. When measured from VC, CV was 703±20 ml (s.e. mean) and from 30% VC it was 440±51 ml (s.e. mean) ( P< 0.005) prior to anaesthesia. When measured from VC, CC-FRC was 370±34 ml (s.e. mean), and from 30% VCit was 343±37 (s.e. mean) ( P >0.05) during anaesthesia. It is concluded that volume history has little effect on CC measurement during anaesthesia and artificial ventilation, but a major influence on CC measurement in the conscious patient. Hence, it is suggested that CC, within the tidal range, is increased during anaesthesia.     

Effect of positive end-expiratory pressure on lung mechanics during anaesthesia in dogs.

We studied supine anaesthetized dogs to determine whether the use of positive end-expiratory pressure to increase functional residual capacity (FRC) could thereby prevent the small lung compliance values frequently reported during anaesthesia. We first made control measurements of FRC and of both static (CSTAT) and dynamic (CDYN) lung compliance. Continuous positive pressure ventilation (CPPV) with 5 cm H2O positive end-expiratory pressure was then commenced, and FRC and compliance measurements were repeated at 15-minute intervals during one hour of CPPV. We found that the positive end-expiratory pressure served to increase FRC to a level expected for unanaesthetized normally breathing dogs in the same posture (J. Appl. Physiol. 36: 93, 1974), and this FRC was well maintained during the one-hour observation period. Despite the volume preservation, we found a marked decline in lung compliance, equal in both CSTAT and CDYN; however, these changes were completely reversed to control values following hyperinflation of lung (artificial sighting). We conclude that diminution in lung compliance in our anaesthetized dogs was not dependent on low lung volume breathing but must have been due to another mechanism such as suppression of sighing or possibly regional hypoventilation. It might well be important to recognize that reversible reduction of compliance may occur despite increase of FRC in patients managed with positive end-expiratory pressure.     

DISPOSITION OF PROPOFOL IN PATIENTS UNDERGOING SURGERY: A Comparison in Men and Women

The disposition kinetics of propofoi have been determined in12 patients (six female) receiving propof of 2.5 mg kg^–1for induction of anaesthesia, which was maintained with 67%nitrous oxide in oxygen and 1–1.5% halothane. Peripheralblood samples were collected at selected times up to 8 h afterthe injection of the drug, and whole blood propofoi concentrationsdetermined by HPL C with fluorescence detection. Drug concentration–timedata were analysed by the non-linear regression program EL SFIT.This showed the data to be describable by a tri-exponentialequation, corresponding to a three-compartment model. Therewere no differences in the derived kinetic indices for the maleand female patients, with the exception of a greater Vd^ss: Vratio in the males. The terminal half-life in the male patientswas 262 min (SEM 44), and in the female patients 309min(60).Vd^ss was 329 litre (67) and 313 litre (69) in male and femalepatients, respectively. The clearance in both groups was 1.8litre min^–1. Seven out of 12 patients showed significantsecondary peaks in blood propofoi concentration associated withrecovery from anaesthesia.