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Effects of manual versus ventilator hyperinflation on respiratory compliance and arterial blood gases in mitral valve replacement patients
Objective: To compare the effects of manual hyperinflation (MHI) and ventilator hyperinflation (VHI) delivered to completely sedated and paralyzed mitral valve replacement (MVR) patients while maintaining minute ventilation.
Design: This was a randomized study with two group pre-test post-test experimental design. Effects of hyperinflation were studied on static compliance (Cstat), dynamic compliance (Cdyn), oxygenation (PaO2:FiO2), partial pressure of carbon-dioxide in arterial blood (PaCO2) and cologarithmÂ ofÂ activityÂ of dissolved hydrogen ions in arterial blood (pH).
Sample: A sample of 30 immediate post-operative MVR surgery patients was taken for the study.
Results: No significant differences were found between the groups. Significant improvements were found in oxygenation at both, 1 and 20 minutes after MHI, but only at 1 minute after VHI (p<0.05). VHI led to improved Cdyn (p<0.05).
Conclusions: In the immediate post-operative MVR patients, both techniques produce similar effects on respiratory compliance and oxygenation. MHI produced longer lasting improvements in oxygenation than VHI, whereas VHI produced better improvements in dynamic compliance. PaCO2 and pH were maintained by both.
Patients who have undergone mitral valve replacement (MVR) are said to be at a greater risk of developing serious respiratory complications than those who have undergone cardiac surgery for coronary artery disease.1 MVR surgery patients who have a history of Rheumatic heart disease (RHD) usually have long standing pulmonary hypertension. During this surgery, the patients are subjected to general anesthesia, median sternotomy, mechanical ventilation and cardiopulmonary bypass. All of these are known to considerably affect lung function1 and respiratory compliance.2 Induction of anesthesia is closely followed by formation of atelectasis and it may take weeks for the lung to recover to the preoperative status.2
Manual hyperinflation (MHI) is known to improve pulmonary compliance and oxygenation (PaO2:FiO2).3,4 Ventilator hyperinflation (VHI), on the other hand, is a newer technique and comparatively less popular than MHI. Yet, few studies comparing manual and ventilator hyperinflation have found VHI to be an equally beneficial5 alternative of MHI, or even a better alternative than MHI6. It needs to be investigated whether any of these techniques can be effectively employed to improve respiratory compliance and oxygenation in the immediate post-operative phase in patients who have undergone MVR surgery. Also, literature related to the isolated effects of any of these techniques, or the effects of hyperinflation without hyperventilation, or the effects of hyperinflation at constant FiO2 of 1.0, is scarce. Compliance is said to be best measured at the ideal state of complete paralysis, but literature on effects of hyperinflation at complete paralysis could not be found by the authors. Therefore, this study was conducted to investigate and compare the efficacy of manual and ventilator hyperinflation techniques in improving compliance and oxygenation when these techniques were performed in isolation and at the state of complete paralysis in immediate post-operative mitral valve replacement surgery patients.
MATERIALS AND METHODS
This was a randomized, two group interventional study using manual or ventilation hyperinflation as the intervention. A sample of 30 patients who underwent MVR because of Rheumatic heart disease was enrolled from the Cardiothoracic and Vascular Surgery (CTVS) department. Institutional Board of Studies approval and written informed consent from each patient were obtained. The sample consisted of patients who were between 18 and 40 years old, had Cardiopulmonary bypass (CPB) time between 50 and 120 minutes, who were completely sedated and paralyzed under the influence of anaesthesia, were between 3 to 4 hours after surgery, were intubated and mechanically ventilated (at volume control mode) and were cardiovascularly stable. Cardiovascular stability was based on haemodynamics as well as medications. A patient was considered cardiovascularly unstable when the patient needed ionotropic support of more than 10 micro gram (Î¼g)/kilogram body weight/minute of dopamine and/or 0.1 Î¼g/Kg body weight/minute of adrenaline or the patient was on intra aortic balloon pump (IABP) or there was continuous mediastinal drainage of more than 6 ml/Kg body weight/min or the systolic blood pressure was less than 80 mmHg or the heart rate was more than 140 beats/min. Only patients with severe mitral stenosis and moderate to severe pulmonary hypertension were included in the study. The patients had the same type of lesion of mitral valve (stenotic lesions were included and regurgitant lesions were excluded). The diagnoses of RHD were based on echocardiography by 2-D echocardiography and Doppler studies. All patients were administered the same anaesthesia during the surgery and were on similar medications after surgery i.e. Dopamine , Adrenaline and Nitroglycerine (NTG) were administered to all the patients but the dosage of each was variable as per the hemodynamic status of the patient.
Obstructive lung disease patients, patients with positive end expiratory pressure (PEEP) 10 cmH2O or more, and patients with any of the complications due to which the intervention (hyperinflation) was contraindicated were excluded.
The withdrawal criteria included unstable cardiovascular status of the patient including arrhythmias compromising the cardiovascular status of the patient and any other serious complication arising during hyperinflation.
Post-surgically, between 3-4 hours after surgery, all patients were positioned into supine position. The breath frequency was set as 12 breaths per minute on the ventilator at volume control mode. Care was taken to ensure that no patient was triggering a spontaneous breath and the measured breath frequency equalled the preset breath frequency on volume control mode of mechanical ventilation. Therefore, any patient capable of triggering a spontaneous breath was suspected not to be completely sedated and paralysed, and was not included in the study. Thus, the effects were studied only for hyperinflation under the state of pure positive pressure ventilation. Tidal volume delivered was 10 ml per Kg body weight or 550 ml, whichever was lesser. If any patient was suspected of having secretions in the central airways, endotracheal suctioning was done about half an hour prior to intervention, so that excess secretions did not influence the dependent variables. Fifteen minutes prior to intervention, the FiO2 of the ventilator was set as 1.0. After fifteen minutes of being ventilated at FiO2 1.0, a pre-intervention (i.e. pre-hyperinflation) measure of the dependent variables, which are, static and dynamic compliance, PaO2:FiO2, PaCO2 and pH was done. After recording the pre-intervention (baseline) measurements of the dependent variables, the intervention (i.e. VHI or MHI) was given. The preset breath frequency was decreased from 12 breaths per minute to 8 breaths per minute during hyperinflation and the tidal volume was increased to 150% of the tidal volume the patient had been breathing upon at the baseline, in order to maintain the minute ventilation. During the entire procedure all precautions were undertaken to prevent infection to the patient. MHI was performed by the same person for all the patients. Heart rate, electrocardiogram (ECG), arterial blood pressure (ABP) (invasive), arterial blood oxygen saturation (SpO2) and temperature were monitored using bedside monitor (Philips Intelli Vue MP 40).
Manual Hyperinflation (MHI). Manual hyperinflation was given by "bag squeezing" using the self-inflating Intersurgical 1.5 litre manual resuscitation bag with oxygen reservoir (Intersurgical Ltd., Berkshire, United Kingdom) by two handed technique. The bag was connected to oxygen supply at a flow rate of 15 L/min. A disposable positive end expiratory pressure (PEEP) valve (Vital Signs Inc, Totowa, NJ) was connected to the resuscitation bag. Hyperinflation breathes with a two second inspiration, two second inspiratory pause, and one second expiration were given for three minutes, at a rate of 8 breaths per minute and FiO2 1.0.
Ventilator Hyperinflation (VHI). Ventilator hyperinflation was given by Siemens Servo - 300 (Siemens-Elema AB, Sweden) mechanical ventilator by increasing the inspiratory tidal volume to 150% of the tidal volume the subject had been breathing upon at baseline. Breathing rate was brought down to 8 breaths per minute, and FiO2 was maintained at 1.0 for the duration of the technique (3 minutes). Upper pressure limit was set at 35 mmHg.
Post intervention measures of the dependent variables were taken at 1-minute after the intervention and again 20 minutes post-intervention. PaO2, pH and PaCO2 were measured by the Arterial Blood Gas analyzer (Radiometer ABL 800 Basic, Copenhagen, Denmark). Fresh arterial blood samples were taken from the arterial line in situ, at all the three times of data collection. A discard sample was removed each time to clear the tubing. Peak inspiratory pressure, end inspiratory plateau pressure, positive end expiratory pressure (PEEP) and exhaled tidal volume were read from the Siemens Servo ventilator for calculating static and dynamic compliance. A two second 'inspiratory hold' was incorporated with the use of the inspiratory hold knob on the ventilator, after complete inspiration, for the measurement of exact plateau pressure. The formula used for static compliance (Cstat) was corrected eVt / Plateau pressure - PEEP and for dynamic compliance (Cdyn) the formula used was corrected eVt / Peak Inspiratory pressure - PEEP, where corrected eVt is exhaled tidal volume with compensation for tubing compression.
Measured values were compared for the two techniques by the use of T-test. Changes within the groups 1 minute and 20 minutes after hyperinflation were analysed using repeated measures ANOVA. Probability values of p<0.05 were deemed to be significant for PaO2:FiO2, PaCO2, Cstat and Cdyn. For pH, significance was set at p<0.01.
The demographic data of the selected patients is presented in Table 1.
Results are shown in Table 2 and 3. There was no significant difference between groups in terms of age, height, weight, Cardiopulmonary bypass (CPB) time and body-mass index (BMI). Baseline values were not found to be significantly different between the groups in case of any dependent variable. Also, there were no significant differences between the groups in terms of changes in any of the dependent variables (figures 1-3). There was a significant improvement (p = 0.0001) in PaO2:FiO2 ratio in the group MHI 1 minute and 20 minutes after MHI (Table 2, figure 3), whereas in the group VHI there was a significant improvement (p = 0.0001) in PaO2:FiO2 only at 1 minute after VHI (Table 3, figure 3). Cdyn showed a significant improvement at 1 minute after MHI (p = 0.001) (Table 2, figure 2). The remaining dependent variables (except PaCO2), in both the groups, showed improvements which were statistically non-significant. pH and PaCO2 showed non-significant and negligible change after both techniques (Table 2 & 3).
There were no adverse changes in blood pressure, heart rate or rhythm during the intervention.
This study compared the effects of manual hyperinflation (MHI) and ventilator hyperinflation (VHI) on arterial blood gas (ABG) values which included PaO2, PaCO2 and pH and the respiratory mechanics which included static and dynamic compliance, in sedated, paralyzed, volume control mode mechanically ventilated, hemodynamically stable, immediate post-operative mitral valve replacement patients. This is perhaps the first study to investigate the effects of hyperinflation while maintaining constant minute ventilation, first to examine the effects of VHI on dynamic compliance and also the first to compare the effects of hyperinflation techniques at the ideal state of complete paralysis.
Although MHI has been shown to be an effective technique in the management of intubated patients, it has several known limitations like the need of disconnection of the patient from the ventilator resulting in loss of PEEP, poor control of airway pressure & flow, and lack of accurate control of FiO2 delivered.5 These limitations are eliminated with the use of ventilator hyperinflation as it can give accurate tidal volume, easily limit airway pressure at desired limit of 35 mmHg, maintain constant flow and deliver an accurate FiO2. The present study found that VHI is as effective as MHI in improving pulmonary compliance, although the improvements found were statistically not significant (p<0.05). For the purpose of convenience, the effects of MHI or VHI which were observed immediately 1-minute after the hyperinflation are referred here to as 'immediate effects' and the effects that were observed 20-minutes after the hyperinflation are referred to as 'delayed effects'.
Since, the improvements in Cstat after VHI were better maintained at 20 minutes post-hyperinflation compared to MHI, if VHI is given with intention of improving compliance, maintenance of Cstat above the baseline for a longer duration may be expected. The maintenance of the improved compliance in the VHI group may be due to recruitment of the collapsed alveoli by VHI initially, and later, maintenance of re-inflation and resumption of almost normal functioning of the alveoli in the 20 minutes post VHI. Although a few alveoli might have re-collapsed after resumption of normal tidal volume breaths, majority of the alveoli may have remained inflated leading to maintenance of Cstat. Constant maintenance of PEEP may be responsible for this maintenance of improvement as ventilator hyperinflation does not require disconnection from the ventilator. There may have been a better and greater recruitment of the collapsed alveoli immediately after MHI than by VHI as is shown by the better Cstat immediately post MHI than post VHI, but the decline in Cstat in the delayed effects of MHI signifies that there may have been a cascade fall in the number of re-inflated alveoli due to re-collapse, thus leading to a decline in compliance in the delayed effects phase. This may mean that VHI has better late effects on pulmonary mechanics than MHI, whereas, MHI has better immediate effects. Although improvements in static and dynamic compliance, were not found to be statistically significant (except in the immediate effects after VHI), a further study needs to be conducted to determine whether there is any statistically significant change in compliance or not if the procedure, as done in this study, is repeated 3-4 times within a period of two hours. If such a procedure results in a summation of improvements achieved after each set of treatment, it may dramatically improve the compliance. Such an improvement would be of clinical value as it may help reduce atelectasis and improve oxygenation considerably.
The elastic properties of respiratory system determine the static compliance (Cstat), whereas, dynamic compliance (Cdyn), in addition to dependence on elastic properties of respiratory system, also reflects flow and resistance of the airways.7 Cdyn improved more after VHI than MHI. The reason may be that MHI requires disconnection from the ventilator and connection to the bag and then disconnection from the bag and reconnection to the ventilator. Such connections and disconnections usually cause disturbance of the ET tube, which may in turn irritate the trachea. Tracheal irritation may cause bronchospasm or the disturbance of ET tube may disturb the mucus which is usually collected at the opening of ET in-situ near carina. This disturbance of mucus may cause narrowing of the ET tube, in turn leading to increase in airway resistance. According to Poiseuille equation, the airway resistance offered to the flow of air in the airways is inversely proportional to the fourth power of the effective radius of the airway. This means that if the effective radius of the airway is reduced to half, the airway resistance will increase by 16 times. The effective radius denotes the radius of the airway which is actually available for the flow of air after the radius of the airway has been compromised by the collected secretions in that part of the airway8. However small the obstruction in the airway may be, the effects it has on the airway resistance are exaggerated. This was not the case with VHI, in which endotracheal tube is not disturbed and considerably smoother flow may be expected which keeps airway resistance low. Paratz et al4 also found improvements in Cdyn after MHI. The effects were maintained at above baseline at 30 minutes after MHI in their study also. Suter et al7 also found improvements in Cstat and Cdyn on increasing tidal volume from 10 L/min. To 15 L/min. on the mechanical ventilator in mechanically ventilated patients, but the measurements were taken by them during ventilation at the raised tidal volume of 15 L/min. and not after returning back to 10 L/min.
Absence of significant change in PaCO2 in any group post-hyperinflation is in agreement with the attempts made to maintain alveolar ventilation by decreasing the respiratory rate and increasing the tidal volume for hyperinflation. A maintained ventilation of the alveoli during hyperinflation (MHI or VHI) may have prevented a marked washout of CO2 from the blood. The non-significant difference in change of PaCO2 (immediate effects) between the groups indicates that both the techniques were equally effective in maintaining the minute ventilation. Non-significant changes (significance accepted at p<0.01 for pH) observed in pH may be due to the fact that the value of PaCO2 was maintained due to constant minute ventilation and, therefore, no corresponding changes occurred in pH as well. Hyperinflation achieved without hyperventilation by maintaining minute ventilation has clinical importance in those intensive care unit (ICU) patients who require hyperinflation for secretion mobilization, but have already decreased levels of PaCO2. If hyperventilation is produced by hyperinflation in such patients, it may lead to further respiratory alkalosis.
It is important to note that the rise in PaO2:FiO2 was not due to increased FiO2 during MHI as FiO2 was 1.0 throughout the duration of the study. Also PaO2 cannot be expected to increase with increased ventilation of the lungs during hyperinflation, since minute ventilation was kept constant by decreasing respiratory rate from 12 to 8 breaths per minute and increasing the tidal volume to about 150% of baseline tidal volume. The probable reason is that with the increase in the recruitment of functional alveolar units after hyperinflation, there may have been an improvement in the ventilation of that area of the lung, resulting in improved ventilation-perfusion ratio, decreased shunting of blood in the lungs and improved oxygen transport in the blood, i.e. increased PaO2 of the pulmonary venous blood. Most clinical interventions apply endotracheal suctioning after MHI. Since endotracheal (ET) suction may have hypoxic effects9, the effects of improvement in PaO2:FiO2 by MHI done prior to ET suction may be contaminated by ET suctioning. Since no ET suction or any other intervention was done in the present study before or after hyperinflation, the results are not expected to be influenced by any intervention other than MHI or VHI.
In a similar study, Patman et al3 investigated effects of MHI on Cstat and PaO2:FiO2 in patients within 4 hours of coronary artery bypass graft surgery (CABG). They found about four times greater improvement in PaO2:FiO2 and about double the improvement in Cstat. The great difference in the improvement in PaO2:FiO2 between the present study and the study by Patman et al may be due to the difference in the average age of the patients and that minute ventilation was maintained in the present study.
In a previous study, Denehy et al5 compared MHI and VHI in intubated and ventilated patients. Their results support the results of the present study, except in the case of 30 minutes post-VHI measurements of Cstat, where Cstat continued to increase till 30 minutes after VHI, making the measured static compliance better at 30 minutes post-VHI compared to the measurements recorded immediately after VHI. This was not observed in the present study. Also, the other results which are supported by the study of Denehy et al are not found to be statistically significant. The reasons may be different MHI bag type, different patient population and pressure dependent hyperinflation protocol in their study.
In another study, Savian et al6 found no significant difference between effects of MHI and VHI in general intensive care patients. They used a self-inflating Laerdal resuscitation bag and found no significant improvements in oxygenation. The studies3,4 in which oxygenation showed improvement used a Mapleson circuit. In the present study, although a self-inflating Intersurgical resuscitation bag was used, significant improvements in oxygenation were found after MHI, which persisted significantly above baseline at 20 minutes after hyperinflation.
The tidal volume delivered to the lungs of the patients may be related to the changes in PaO2:FiO2 ratio, but the authors were unable to measure the exact tidal volume delivered during MHI. Oesophageal balloon was not used in the measurement of compliance. Also, both the hyperinflation procedures were volume dependent and, therefore, the results found may not be applicable to pressure dependent hyperinflation procedures. The findings of the present study where MHI was delivered by Intersurgical resuscitation bag may not be generalized to all the bags as variability exists in the effects of different bags especially when Intersurgical or Laerdal bag are compared to Mapleson-C circuit.
Both, manual and ventilator hyperinflation, produce similar effects on respiratory compliance and oxygenation. Both techniques improve static and dynamic compliance and oxygenation. But VHI seems to better improve dynamic compliance. Both techniques are equally effective in producing hyperinflation without causing hyperventilation. Thus, any of the techniques may be used interchangeably without compromising on major benefits. Longer lasting improvements in oxygenation may be obtained by MHI, whereas better dynamic compliance may be achieved using VHI in immediate post-operative MVR patients. The improved oxygenation may be of great advantage in intubated patients who have atelectasis and pulmonary shunting which compromise the oxygenation, whereas improved dynamic compliance holds value in patients who have stiffer lungs resulting out of atelectasis or segmental collapse. Therefore, based on the patient's condition and the need of a specifically targeting method, a choice among the two techniques can be made by the clinician. However, the clinical significance and benefits of these improvements on outcomes is unclear. Therefore, further studies are required to investigate the effects of improved oxygenation and dynamic compliance on clinical outcomes.