Systolic time interval changes after aorto-coronary bypass

How to perform indexing of extravascular lung water: a validation study

BACKGROUND

Extravascular lung water is a quantitative marker of the amount of fluid in the thoracic cavity besides the vasculature. Indexing to both predicted and actual body weight have been proposed to compare different individuals and provide a uniform range of normal.

OBJECTIVE

We explored extravascular lung water measured by single-indicator transpulmonary thermodilution in a large cohort of patients without cardiopulmonary instability, in order to evaluate current and alternative indexing methods.

DESIGN

Prospective, observational.

SETTING

Neurosurgical ICU in a tertiary referral academic teaching hospital.

PATIENTS

One hundred and one consecutive patients requiring elective brain tumor surgery and postoperative ICU surveillance.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

Indexed to predicted body weight, females had a mean extravascular lung water of 9.1 (SD=3.1, range: 5-23) mL/kg and males of 8.0 (SD=2.0, range: 4-19) mL/kg (p<0.001). Values indexed to predicted body weight were inversely correlated with the patient's height (p<0.001). Indexed to the traditionally used actual body weight, data showed a significant relationship to weight (p<0.001) and gender (p<0.05). In contrast, indexing to body height presented a method without dependencies on height, weight, or gender, yielding a uniform 95% confidence interval of 218-430 mL/m. Extravascular lung water increased with positive perioperative fluid balance (p=0.04).

CONCLUSIONS

Using either predicted or actual body weight for indexing extravascular lung water does not lead to independence of height, weight, and gender of the patient. Specifying a fixed range of normal or a uniform upper threshold for all patients is misleading for either method, despite widespread use. Our data suggest that indexing extravascular lung water to height is superior to weight-based methods. As we are not aware of any abnormal hemodynamic profile for brain tumor patients, we propose our findings to be a close approximation to normal values.

Pulmonary abnormalities after cardiac surgery are better explained by atelectasis than by increased permeability oedema

BACKGROUND

Cardiac surgery can be complicated by pulmonary abnormalities, but it is unclear how various manifestations interrelate.

METHODS

A prospective study in the intensive care unit was performed on 26 mechanically ventilated patients without cardiac failure within 3 h after elective cardiac surgery involving cardiopulmonary bypass. Oedema (extravascular lung water, EVLW) was measured by the thermal-dye technique and permeability by a dual radionuclide technique, yielding a pulmonary leak index (PLI). Radiographic, mechanical and gas exchange features were used to calculate the lung injury score (LIS), ranging between 0 and 4. Evidence for left lower lobe atelectasis was obtained from plain radiographs. The plasma colloid osmotic pressure (COP) was measured by an oncometer.

RESULTS

The EVLW (normal, <7 ml/kg) was elevated in 36% of patients and the PLI (normal, <14.1 x 10(-3)/min) in 44%, but the variables did not interrelate directly. Patients with a supranormal EVLW had a lower COP than patients with normal EVLW. The duration of mechanical ventilation was prolonged in patients (20%) with EVLW > 10 ml/kg. There was no difference in EVLW and PLI in patients with LIS < 1 and LIS > 1 (31% of patients). In patients with radiographic evidence for atelectasis (46%), the positive end-expiratory pressure and inspiratory O2 fraction to maintain oxygenation were higher than in those without.

CONCLUSIONS

After cardiac surgery, mild pulmonary oedema is relatively common, even in the absence of high filling pressures, and is mainly attributable to a low COP, irrespective of increased permeability in about one-half of patients. It may prolong mechanical ventilation at EVLW > 10 ml/kg. However, pulmonary radiographic and ventilatory abnormalities may result, at least in part, from atelectasis rather than increased permeability oedema.

Breathing pattern and gas exchange in emergency and elective abdominal surgical patients

OBJECTIVE

To evaluate the effects of intra-abdominal surgical emergency on breathing pattern and gas exchange and compare it with the changes induced by elective abdominal surgery.

DESIGN

Prospective clinical study.

SETTING

Abdominal surgical departments in a university hospital.

PATIENTS

Patients operated for intra-abdominal emergency (n = 10, EAS), elective upper abdominal (n = 19, UAS).

MEASUREMENTS AND RESULTS

Breathing pattern and gas exchange were measured with a respiratory inductive plethysmograph and a gas exchange monitor. EAS patients had pre-operatively a classical rapid shallow breathing pattern and increased ventilatory demand due to increased energy expenditure. The operation improved the breathing to normal pattern (frequency, 26 +/- 5/min and 17 +/- 3/min, p < 0.01; tidal volume, 439 +/- 128 ml and 541 +/- 165 ml, NS., before and after surgery, respectively). Sighing was absent before and after EAS and strictly reduced after elective surgery (p < 0.01 for UAS). The operation restricted the abdominal-diaphragmatic breathing movement which was reflected as increased contribution of the rib cage to VT (%RC: from 37% +/- 15 to 57% +/- 15 for UAS p < 0.001; from 47% +/- 16 to 61% +/- 14 for EAS NS.). After EAS and UAS hypoxemia was common (p < 0.001) with frequent radiological pathology. We conclude that intra-abdominal surgical emergencies increase the ventilatory demand and challenge the respiratory system to marked adaptive changes both pre- and post-operatively.

Preoperative estimation of pulmonary extravascular thermal volume in patients undergoing pneumonectomy

Pulmonary extravascular thermal volume (PETV) was measured during pulmonary artery occlusion in 18 patients preoperatively and 7 patients postoperatively who were undergoing pneumonectomy. We found that the PETV decreased from 6.6±2.3 ml·kg^-1 before occlusion to 4.1±1.6 ml·kg^-1 during occlusion. There was a significant correlation between the PETVs before and during occlusion multiplied by the fraction of pulmonary perfusion (r=0.77,P<0.001). Although the PETV increased in two patients and decreased in four within 48 h after pneumonectomy, it returned to the value during occlusion at 3 weeks after pneumonectomy in seven patients. There was a significant correlation between the PETV during occlusion and that at 3 weeks after pneumonectomy (r=0.66,P<0.05). In conclusion, PETV during pulmonary artery occlusion is a reliable baseline value in the assessment of postoperative pneumonectomy values.

Respiratory changes after open-heart surgery

Breathing pattern was studied non-invasively in 20 coronary artery bypass surgery patients before the operation and post-operatively after weaning from mechanical ventilation. Post-operatively minute ventilation (VE), breathing frequency (Fr) and mean inspiratory flow (VT/TI) increased (28%, 42%, 27%; p less than 0.01, p less than 0.001, p less than 0.01, respectively), while tidal volume (VT) decreased (15%, p less than 0.025). CO2 production (VCO2) and oxygen consumption (VO2) increased postoperatively (p less than 0.001 for both), contributing to the increase in ventilatory demand. Reduced variation of VT and Fr (p less than 0.001, p less than 0.01, respectively) and number of sighs (p less than 0.001) were characteristic of the post-operative breathing pattern. Post-operatively an increase in the contribution of rib cage (%RC) to tidal volume in the supine position was observed suggesting reduced motion of the diaphragm. All patients had atelectasis, 17 had pleural fluid and only 6 normal vascularity post-operatively. The shallow breathing in combination with increased ventilatory demand, impaired gas exchange and the surgical trauma of the thorax predispose to postoperative respiratory complications.

Measurement of extravascular lung water during abdominal aortic surgery

Cross-clamping of the abdominal aorta can be associated with significant changes in haemodynamic variables. However, intraoperative changes in extravascular lung water (EVLW) have not been studied. Nine patients undergoing elective surgery, either aortic aneurysm repair or aorto-bifemoral grafting, were monitored invasively with arterial lines, pulmonary artery catheters and Edwards lung water catheters inserted in either the brachial or axillary artery. Determinations of EVLW were made prior to and five minutes after application of the aortic cross-clamp and at 30-minute intervals during the course of the operation. Baseline EVLW was found to be 7-9 ml.kg-1. There were no significant changes in haemodynamic variables and no changes in EVLW with cross-clamping of the aorta. The EVLW did not change during the course of surgery. The EVLW did not increase in the absence of sustained elevation of pulmonary capillary wedge pressure. One patient developed an axillary artery thrombosis which required thrombectomy at the site of lung water catheter insertion. Two other patients lost their distal pulses without overt ischaemic changes. It was felt that such relatively high incidence of complications precluded further use of the lung water catheter in the axillary or brachial artery.

End-tidal carbon dioxide tension and temperature changes after coronary artery bypass surgery

Variations in end-tidal carbon dioxide partial pressure (PETCO2) and temperature were measured for six hours following coronary artery bypass surgery in twenty patients. In the recovery room, the patients were mechanically ventilated with a tidal volume of 12 ml X kg-1. Arterial blood gases were drawn every two hours, and the respiratory frequency was adjusted to maintain arterial carbon dioxide pressure (PaCO2) in the range of 30-45 mmHg. Naso-pharyngeal temperature was recorded every 30 minutes, and PETCO2 was measured continuously. The mean difference between temperature-corrected arterial and end-tidal CO2 pressure measurements was 3.2 mmHg (SD = 2.8; r = 0.963). This difference did not vary with time, temperature or PCO2. The largest temperature increases (mean 1.7 degree C/hour) occurred at a mean of 253 minutes after the end of surgery. End-tidal PCO2 increased markedly as temperature rose, in spite of a coincident increase in ventilation and then decreased as temperature stabilized. Large increases in CO2 production, caused by the metabolic demands during rewarming, most likely account for these changes. It is concluded that end-tidal CO2 recordings are reliable, and can help in maintaining normocarbia during the short but unstable period associated with rewarming following cardiac surgery.

Increased pulmonary artery diastolic-pulmonary wedge pressure gradient after cardiopulmonary bypass

In 29 patients undergoing elective coronary artery bypass grafting, the diastolic pulmonary arterial pressure-pulmonary capillary wedge pressure gradient (DPAP-PCWP) and related haemodynamic parameters were determined before and after induction of anaesthesia, immediately after cardiopulmonary bypass (CPB) and one and three hours after CPB. The DPAP-PCWP gradient remained unchanged after induction of anaesthesia but was significantly increased after CPB. A gradient of 5 mmHg or greater was observed in 16 patients after CPB, whereas none of the patients showed such a gradient before CPB. A significant correlation was found between the change in DPAP-PCWP and the change in pulmonary vascular resistance (PVR). It is concluded that DPAP should not be used as a substitute of PCWP in the early postbypass period without frequent confirmation of the presence of the normal small DPAP-PCWP gradient. Since an increase of PVR may impair right ventricular ejection, we recommend the routine measurement of DPAP-PCWP gradient in the postbypass period.

Continuous airway pressure with oxygen minimizes the metabolic lesion of 'pump lung'

Water distribution and energy status of the lung were measured in ten rabbits at two hours of hypothermic cardiopulmonary bypass (CPB) with left heart venting and incision of the parietal pleurae. During CPB, half the animals had their airways open to room air at ambient atmospheric pressure (ZEEP), and the remainder had their lungs inflated (CPAP) at a pressure of 5 cms H2O with the oxygen-enriched (70-75 per cent) gas mixture exiting from the disc oxygenator. In both the ZEEP and CPAP groups, there was more than doubling of the pulmonary extravascular sodium-free water (intracellular) space and reciprocal reduction of the pulmonary extravascular sodium (extracellular) space, compared with 12 control animals not undergoing CPB. In the ZEEP group, there was an 18-fold increase in the pulmonary lactate/pyruvate (L/P) ratio compared with controls, and the pulmonary energy charge (E.C.) was significantly less than in controls (0.74 +/- 0.02 vs. 0.89 +/- 0.01). In the CPAP group the pulmonary L/P ratio was 2 1/2 times control values and the pulmonary E.C. was virtually identical with that of the control group. These data suggest that CPB results in a shift of fluid from the pulmonary interstitium into the pulmonary intracellular compartment with no net increase in total pulmonary extravascular water. The data also suggest that pulmonary deflation during CPB results in a significant pulmonary energy deficit which can be prevented by keeping the lung inflated with an oxygen-enriched gas mixture during CPB.

Flow-dependence of extravascular thermal volume as an index of pulmonary edema

Using a double indicator (dye and heat) dilution technique of extravascular lung water measurement, we examined the effect of a reduction in cardiac output and positive pressures on the extravascular thermal volume (EVTV) in dogs. Following baseline EVTV measurements, cardiac output was lowered by inflation of balloons in the superior and inferior vena cavas, as well as by bleeding, and positive pressures were applied to the airways. There was good agreement between the baseline EVTV and post-mortem lung water; however, as the cardiac output was lowered there was a reduction in the measured EVTV. In other animals following application of positive airway pressure there was a decrease in the EVTV which appeared to be related to the reduction in cardiac output caused by positive airway pressure. At least in part, loss of thermal indicator appeared to explain the reduction in EVTV. Measurement of EVTV as an index of pulmonary edema may not be accurate in the face of a changing cardiac output.

Intubation and other experiences in cardiac surgery: the consumer's views

Recently publications have advocated earlier weaning and early extubation of the trachea in patients after cardiac surgery. Greater comfort of the patients is one of the advantages claimed for this policy. One hundred consecutive adult patients were questioned on the fifth postoperative day to assess the relative comfort or discomfort of the patients during tracheal intubation and ventilation. Overnight tracheal intubation and ventilation were easily tolerated by over 90 per cent of patients who had received morphine and diazepam. There was a high incidence of amnesia. Some of the other claimed advantages appear inconclusive and further study is desirable to elucidate the effect of early removal of the tracheal tube upon the indicence of pulmonary complications.

Pulmonary atelectasis and other respiratory complications after cardiopulmonary bypass and investigation of aetiological factors

Radiological evidence of pulmonary complications and possible aetiological factors were investigated in 50 consecutive patients after heart operations with cardiopulmonary bypass. Atelectasis was the most frequent pulmonary complication except for small pleural effusions, with an incidence of 64 per cent. Several types of atelectasis frequently co-existed, with a predominance of the less extensive plate and subsegmental forms. The incidence of atelectasis was the same on each side and the site of atelectasis was basal in three quarters of the patients. Preoperative clinical and catheter data were unrelated to the incidence of atelectasis. There was a significant positive correlation between a short cardiopulmonary bypass time and plate atelectasis, between a large fluid load after bypass and segmental atelectasis, between re-operation for bleeding and subsegmental atelectasis and between post-operative gastric dilation and atelectasis. The type of operation, the use of the intra-aortic balloon and the length of postoperative respiratory ventilation were unrelated to the incidence of atelectasis. The mechanism of development of atelectasis is discussed.

[Critical study of the hemodynamic status during anesthesia]

Physicians must choose the anaesthetics for their patients and select the methods to cheek their haemodynamic status. Experimental works do not always bring sufficient information to help them in their daily practice. Circulatory reaction to a pharmacodynamic agent is diffuse and non-specific. Some examples drawn from theoretical considerations and practical situations support this view. The difficulty of assessing the actual damage caused by one apparent variation still remains. On the other hand, haemodynamic indices such as PA, CVP, dP/dt are composed of elementary data that are interdependent; so indices are interdependent, too. A puzzling fact is that depressing anaesthetics can initiate severe haemodynamic crisis, generally badly tolerated by tissues, but better tolerated by myocardium which is not definietely altered if the anoxia inflicted is accompanied by a real decrease in cardiac work. This explains the often slight effect of short anesthetic overdosage. Previous haemodynamic variations to be countered relative to anaesthesia are not really known. Biochemistry of the venous coronary blood does not yet bring striking features in this field.

Influence of elevated pulmonary vascular resistance on the relationship between central venous pressure and pulmonary artery occluded pressure following cardiopulmonary bypass

This study has demonstrated that CVP measurement is an unreliable index of left ventricular filling pressure in unselected patients undergoing A-C bypass procedures. The influence of altered PVR on the disparity between right and left ventricular filling pressures is not of prime importance. The inconsistent relationship between CVP and PAo is primarily a result of ventricular dysfunction and not altered PVR. This suggests that following A-C bypass surgery patients cannot be grouped with respect to altered PVR as the variable which determines whether CVP will be a reliable guide to changes in left atrial pressure. Routine use of Swan-Ganz catheters would improve the precision of monitoring and fluid administration in all patients following CPB. However, it would also increase the cost and complexity of routine care and add a very low incidence of complications to all patients having cardiac surgery. At present, we monitor high risk patients from the time of surgery and advocate early insertion of Swan-Ganz catheters in low risk patients if they are not responding to appropriate management.