Immediate changes in hemodynamics and gas exchange after initiation of noninvasive ventilation in cardiac surgical patients.

Introduction: Cardiac surgery is associated with pulmonary dysfunction and complications such as prolonged intubation and reintubation. Bilevel positive airway pressure (BiPAP) machine has been used in the clinical settings to improve oxygenation, reduce work of breathing, and avoid reintubation. The effect of BiPAP on cardiovascular parameters is not well established, and very few studies have targeted hemodynamic changes. The aim of the study was to assess the immediate effect of BiPAP on respiratory and hemodynamic parameters in post-cardiac surgery patients. Materials and
Methods: This quasi-experimental study was done on 33 adult cardiac surgery patients. Ethical review committee approval was sought and consent was taken. All patients who were in respiratory distress with respiratory rate of >30/min and/or PaO2:FiO2 ratio of <200 were included. Hemodynamic and respiratory parameters were recorded just before and 15 min after BiPAP application. Sample size was determined on the basis of BiPAP effect on one of the variables, PaO2:FiO2 ratio.
Results: A total of 33 patients were included in the study. The average age of the patients was 60.97 ± 10.8, of which 23 (69.7%) were males and 10 (30.7%) females. BiPAP application leads to statistically significant improvement in ventilator parameters including SaO2 29 (87.7%), PaO2 29 (87.8%), PaCO2 21 (63.6%), and PaO2:FiO2 ratio in 27 (81.8%).
Conclusion: Ventilatory parameters were significantly improved after BiPAP application in this study, but hemodynamic parameters showed no statistically significant change. BiPAP application was also able to decrease the need for reintubation in post-cardiac surgery patients.

Introduction

Cardiac surgery is associated with pulmonary dysfunction, which can lead to postoperative complications such as prolong intubation, respiratory distress, and reintubation. The reasons for pulmonary dysfunction after cardiac surgery includes general anesthesia, surgery time, mechanical ventilation, atelectasis, fluid overload, pleural opening, lung parenchymal injury due to cardiopulmonary bypass (CPB) and microembolization, pain due to incision, chest tubes' presence, and phrenic nerve injury. Preservation of pleural integrity is associated with better respiratory function and reduced length of stay.[1] Use of left internal mammary artery in cardiac surgery is also associated with more respiratory dysfunction than only saphenous vein grafts.[2] In addition, preoperative factors such as preexisting lung diseases, smoking, old age, and poor nutritional state among others are a predisposition to complications. Respiratory dysfunction in cardiac surgical patient appears early in the postoperative period, but these changes are usually transient and respond to interventions.

Bilevel positive airway pressure (BiPAP) machine has been used in the clinical settings for patients with pulmonary edema,[3] high-risk postoperative patients, and acute hypoxemic respiratory failure. When compared with endotracheal intubation,[4] the BiPAP is more comfortable, has a role in avoiding intubations, provides better outcome (mortality and nosocomial infection), and helps in avoiding ventilator-associated complications such as ventilator-associated pneumonia and need for deep sedation. Other advantages include improved oxygenation[56] and decreased work of breathing which, in turn, reduces myocardial oxygen demand. It recruits atelectatic alveoli and improves lung compliance. It may also have a role in reducing the risk of nosocomial infection. Studies have shown that intubation and mechanical ventilation in respiratory compromised patients is associated with higher mortality,[7] and BiPAP may help in reducing such complications.[8]

BiPAP supports respiration during inspiration by applying inspiratory positive airway pressure (IPAP; which is the sum of pressure support and PEEP), and during expiration it acts like positive end-expiratory pressure by application of expiratory positive airway pressure (EPAP). It also increases PaO2:FiO2 ratio. Low ratio in cardiac surgical patients is associated with higher intensive care unit (ICU) mortality and pulmonary complications such as atelectasis and pulmonary edema.[9] BiPAP application improves atelectasis, and BiPAP has been used in post-cardiac surgery patients to avoid intubation. Some patients may not tolerate the mask and become restless. BiPAP may not work well in extremely agitated, uncooperative, and claustrophobic patients. In addition, it may not be beneficial in hemodynamically unstable patients and those with excessive airway secretion.

The effect of BiPAP on cardiovascular system is not very well established, and very few studies have targeted hemodynamic changes. These changes are more important for post-cardiac surgery patients who are not only recovering from the effect of cardioplegia but also having variable volume status. This is probably the only study where all important invasive and noninvasive hemodynamic parameters are considered along with ventilator parameters.

The aim of the study was to assess the immediate effect of BiPAP on respiratory and hemodynamic parameters in those post-cardiac surgery patients who require noninvasive ventilation (NIV).

Materials and Methods

This quasi-experimental study was performed on 33 adult cardiac surgery patients between the ages of 35 and 70 years. Ethical review committee approval was sought. All patients on respiratory distress were initially managed by higher FiO2 by mask, respiratory therapy including nebulization, and prop up position. Those patients who remained in respiratory distress with respiratory rate of >30/min and PaO2:FiO2 ratio <200 were included in the study. BiPAP was applied as soon as the patient met these two criteria. Patient or family refusal and elective application of BiPAP were taken as exclusion criteria. Emergency surgery and preexisting pulmonary dysfunction patients were also excluded. Before initiating BiPAP, possible surgical complications such as anastomosis leakage, hemorrhage, pneumothorax, and cardiac tamponade were excluded. Decision about BiPAP application was taken by on-call resident after consultation with covering consultant of cardiac ICU (CICU). BiPAP was explained to these patients and consent was taken. For those patients who were unable to give written consent due to drowsiness or respiratory distress, it was taken from close family members (parents, spouse, and children). This decision was made after consultation with ethical review committee.

Consecutive cardiac surgical patients who fulfilled BiPAP application criteria were included in nonrandomized fashion. Sufficient communication was established with the patients, so that the procedure is well understood. BiPAP applied only to those patients who were present in CICU. Full face mask was used to cover the mouth and nose and then attached with portable BiPAP machine (VPAP III STA QuickNav; ResMed, Australia). It delivers a positive pressure through a single air circuit with the exhaled air exiting through a mask exhaust vent. Initial settings of IPAP 12 and EPAP of 6 were applied and gradually increased accordingly. No attempt was made to wean off during the study period. The patient was monitored for mask intolerance, gastric distension, and facial skin laceration.

All data were recorded on a proforma, which included demographics, reason for application, SaO2 at the time of first application, time it started, total duration of application, initial settings, range (minimum and maximum), and outcome of BiPAP application. These data were collected by residents and consultants who were involved in patient management.

Sample size was based on the study by Takami and Ina[10] and determined on the basis of the effect on one of the variables, PaO2:FiO2 ratio. Changes in the hemodynamic, BiPAP parameters, SaO2, PaO2, PaCO2, heart rate (HR), mean blood pressure (BP), central venous pressure (CVP), pulmonary artery (PA) pressure, systemic vascular resistance (SVR), and pulmonary vascular resistance (PVR) were calculated from relative changes from the baseline. A sample size of 33 achieved 90% power to detect a 20% and above mean paired difference with estimated standard deviation of the difference being 20, with a significance level (alpha) of 0.01 using a paired t-test.

All statistical analysis was performed using Statistical Package for the Social Sciences version 19 (SPSS Inc., Chicago, IL, USA). Frequency and percentage were computed for categorical observation, while mean and standard deviation were estimated for numeric variables. Pre- and post-BiPaP effects on dependent variables were analyzed by paired t-test. Repeated measure analysis of variance test was applied to observe the effect of other variables on dependent variables. A P value of less than 0.05 was considered as statistically significant.

Results

A total of 33 patients were included in the study. The average age of the patients was 60.97 ± 10.8, of which 23 (69.7%) were males and 10 (30.7%) females [Table 1]. The most common reason for BiPAP application was atelectasis (51.5%). Diagnosis was left at the discretion of CICU intensivist on the basis of X-ray findings' interpretation. Patients with pneumonia and obesity frequently required noninvasive ventilatory support. BiPAP application lead to statistically significant improvement in the following parameters: SaO2 29 (87.7%), PaO2 29 (87.8%), PaCO2 9 (27.3%), and PaO2:FiO2 ratio in 27 (81.8%). Only four patients in our study had PaO2:FiO2 ratio greater than 150, and of these only one patient did not show improvement in the ratio after BiPAP application [Table 2]. Only two patients required escalation of support during the study period.

Table 1

Demographic, Comorbid and Surgery status (n=33)

	Point estimation
Variables
Age (Years)	60.97±10.81
Weight (kg)	75.90±17.22
Height (cm)	163.07±6.74
Body mass index (kg/m)	28.36±5.56
Gender
Male	24 (72.7%)
Female	9 (27.3%)
Comorbid
Hypertension	27 (81.8%)
Diabetic Mellitus	18 (54.5%)
IHD	7 (21.2%)
Other*	4 (12.1%)
Surgery
CABG	28 (84.8%)
MVR	4 (12.1%)
Pericardiectomy	1 (3%)

Results are presented as mean±SD and n (%), Others*: CKD 2, obesity 1, AKI 1

Table 2

Reasons, Improvement and total duration of BiPAP in cardiac surgery patients (n=33)

Variables	n	%
Reason for BiPAP Application
Pneumonia	5	15.2%
Sepsis	2	6.1%
Muscle Weakness	4	12.1%
Obesity	4	12.1%
Low PaO2 ventilator	4	12.1%
COPD	1	3%
Pulmonary edema	1	3%
Respiratory distress due to unknown reason	2	6.1%
Respiratory distress due to atelectasis	17	51.5%
Single Reason	7	21.2%
Multiple Reasons	26	78.8%
Improvement seen in following parameters soon (15 min) afterBiPAP Application
SaO2	25	75.8%
PaO2	23	69.7%
PaCO2	9	27.3%
Clinical Mentation, Drowsiness	7	21.2%
Reduction in respiratory rate after first app BiPAP	12	36.4%
PaO2/FiO2 ratio	27	81.8%
Total Duration of BiPAP
<24 h	12	36.4%
2-3 days	19	57.6%
3-5 or above	2	6.1%

Results are presented as n (%)

Eight patients had body mass index (BMI) greater than 30. All (n = 3) patients with BMI >35 showed improvement in SaO2, PaO2, and PaO2:FiO2 ratio after BiPAP application. Nine of 17 patients who were in respiratory distress due to atelectasis needed BiPAP application for less than 24 h. BiPAP was applied preemptively on three patients soon after extubation due to low PaO2:FiO2 ratio on ventilator. HR reduced from mean 102 to 92 beats/min, while the mean BP was increased [Table 3]. Cardiac output which was measured after 15 min of BiPAP application also reduced from mean 4.45 ± 1.23 to 4.50 ± 1.34 L/min with a P value of 0.873.

Table 3

Effect of BIPAP application on hemodynamics and ventilator parameters

	n	Pre BiPAP	Post BiPAP	P
Gas exchange parameters
SaO2	33	90[88.4, 94]	95[91,97.7]	0.003
PaO2	33	60.2[55.7,70]	69[63.6,98.1]	0.0005
PaCO2	33	44[39.05,51]	40.5[37.1,45.6]	0.005
PaO2/FiO2 ratio	33	107[94,137]	115[103.5,173]	0.001
Base excess	21	0.20[-0.21, 1.85]	0.30[-2.3,2.05]	0.259
Hemodynamic parameters
HR	33	102[91,111]	92[89,108.5]	0.18
Systolic BP	33	112[104.5,128.5]	112[105,125.5]	0.443
Diastolic BP	33	63[59,83.5]	69[59,83]	0.702
Mean BP	33	79.5[70,89]	84.5[71,90.7]	0.142
CVP	32	8.5[7,10]	9[7,11]	0.062
PA pressure (Systolic)	20	32[25,37.5]	28[26,38]	0.887
PA pressure (Diastolic)	20	15[13,18]	16[14,18]	0.060
SVR	20	1259[1003,1712]	1264 [999.25,1553]	0.959
CO	14	4.45[2.60,6.25]	4.50[2.51,6.54]	0.873

Results are presented as median [25th, 75th percentile], Wilcoxon Signed Ranks Test

Thirteen (36.4%) patients required BiPAP support for less than 24 h. Only one patient required reintubation in the first 24 h, and overall 6 (18.2%) patients ultimately required reintubation after prolonged BiPAP application [Table 4]. Three patients required reapplication of BiPAP within 24 h after planned removal. Four of five patients who had low PaO2 on ventilator required BiPAP for more than 24 h.

Table 4

Outcome of BiPAP Application (n=33)

Outcome of BiPAP Application	Number of Patients	Percentage
Re-intubation	6	18.2%
Day 1	1
Day 2	2
Day 3	2
Day 4	1
Able to maintain SaO2 without BiPAP for 24 h	22	66.7%
BiPAP reapplication within 24 hrs. of removal	3	9.1%
Rate patient’s cooperation after BiPAP application
Fully Cooperative	30	90.90%
Reluctant to Cooperate	2	6.1%
Non Cooperate	1	3%

Discussion

Application of alternating IPAP and EPAP improves gas exchange by recruiting atelectatic alveoli. IPAP has an additional advantage of reducing the work of breathing. In other studies, respiratory parameters were improved in 30 min, while in this study NIV has shown improvement within 15 min of application. Zoremba et al.[11] in their study noticed that the short-term use of BiPAP can improve pulmonary function for 24 h. It decreases the need for sedation, and the patient can protect his own airway to protect against aspiration. It also decreases nosocomial infection rate when compared with reintubation. Diaphragmatic paralysis or dysfunction, which is not uncommon in cardiac surgical patients, can be an indication for BiPAP application.[12] The other advantage includes reduced inotrope requirements and respiratory infection.[13]

Early application may have a role in reducing mortality.[14] BiPAP also has a role in reducing the need for reintubation,[15] particularly in respiratory failure patients[3] and in cardiogenic pulmonary edema[6] but does not reduce the incidence of myocardial infarction and mortality.[16] A meta-analysis by Bajaj et al.[17] also concluded that NIV reduces reintubation (10.8% in NIV vs. 17.8% in conventional group) in chronic obstructive pulmonary disease and high risk for extubation failure patients.

It may also reduce intubation rate in acute respiratory failure.[18] It reduces the reintubation rate in patients who are at high risk for postoperative pulmonary complications or have ongoing acute respiratory failure.[19] Prophylactic application is ineffective in low-risk patients, but in high-risk patient even few hours of application[20] reduces reintubation rate. Preemptive BiPAP application was applied in three patients in our study, and not only it improved oxygenation but also it prevented reintubation. BiPAP also has a role in reducing intubation rate in acute and chronic congestive heart failure (CHF) patients.[21]

BiPAP has a failure rate of 10%–55%, and the main reason is pneumonia and older age group.[22] There are various definitions of failure. For study purpose, we considered reintubation within 4 days of application as BiPAP failure and 18% patients needed reintubation in this study. Five patients in our study also had pneumonia for which BiPAP was applied. Only two patients required reintubation on day 2 of application, which showed some success, but larger studies are needed to conclude in these patients. It may be suggested not to use BiPAP in this patient population due to higher failure rate, and conventional endotracheal intubation must be preferred.[23] Age of the patients seems to have no effect on failure rate in this study.

There was a statistically significant improvement in PaO2 within 15 min of application. It was more effective in 93% of patients in whom PaO2 was lower than 60 mmHg (14/15). BiPAP improves oxygenation more rapidly than continuous positive airway pressure in cardiogenic pulmonary edema patients.[3]

It usually increases SaO2 without significant changes in PaCO2[24] in normal patients. But in patients with hypercapnia, its role is different. In our study, BiPAP application significantly reduces PaCO2 in 21 (63%) patients. BiPAP seems to be more efficient in patients with hypercapnia as we have seen in our study that most of the patients (11 of 14) whose PaCO2 was higher than 45 mmHg showed reduction in PaCO2 within 15 min of BiPAP application. Our results are similar to other studies carried out earlier, which also showed a significant effect of BiPAP only in patients with hypercapnia.[625] Mehta et al. showed improvement in PaCO2 after BiPAP application, but it increases acute myocardial infarction rate (71%).[26] Tobias[5] also demonstrated reduction in respiratory rate and PaCO2 in postoperative patients. Those patients were in impending respiratory failure, and application of BiPAP improved oxygenation and CO2 levels and avoided reintubation.

We took PaO2:FiO2 ratio of <200 as an inclusion criteria, and BiPAP application did improve the ratio significantly. The average value of ratio before and after application was 107 versus 115. This increase in ratio reached more than 200 only in five (15.1%) patients. The reason may be that most of our patients (n = 29) had less than 150 ratio before BiPAP application. One of the study also used this ratio for indication[10] and the ratio increased significantly within 3 min. Park et al.[6] also showed improvement in the ratio within 10 min of application and required more than 30 min to reach 200 level but those were pulmonary edema patients.

Hemodynamic effects are varied according to the disease state and also on the IPAP and EPAP settings and the type of mask (nasal vs. face). In normal patients, it may decrease CO, while in chronic heart failure patients it may improve CO by reducing SVR[27] and preload. Cardiac function improved in distended heart but not in normal functioning heart. No significant hemodynamic changes were seen after BiPAP application during the initial 15 min. It may be inferred that it needed more time for hemodynamic improvement or it may be more effective in pulmonary edema cases as studies have shown a reduction in HR and SVR after BiPAP application in pulmonary edema patients. BiPAP reduces sympathetic activity,[28] preload, and afterload leading to enhanced ventricular function. Mean arterial pressure (MAP), CVP, and PA diastolic pressures slightly increase probably due to an increase in intrathoracic pressure exerted by BiPAP.[29] Insignificant reduction in HR and PA systolic pressure was seen in our patients. Decrease in HR occurs due to parasympathetic stimulation by stretched receptors in lungs. This is in contrast to a previous study in post-cardiac surgery patients by Kilic et al. which showed a slight increase in HR at 1 h along with a decrease in MAP. These values remain insignificant even after 12 h of BiPAP application. Cardiac surgery patients are usually in various stages of hemodynamic status; some may be hypovolemic or hypervolumic, while others may still be recovering from the effect of cardioplegia. In addition, hemodynamics are also affected by postoperative cardiac index, pain, BiPAP settings, and patient's cooperation. Significant HR reduction was seen in two previous studies, but those were with patients CHF and the improvement was seen after 30 min.[2630] This change in HR in patients with pulmonary edema was significant within 10 min of application in a study by Marcelo et al.

Although CO was increased in our patients, it was insignificant. The increase in CO and MAP was probably due to reduction in preload and improved cardiac contractility after BiPAP. Another study which is comparable to this study showed improvement in cardiac index(CI) without changes in systemic and PA pressures.[15] Atelectasis and overinflation of lung above functional residual capacity (FRC) can increase PVR. Takami and Ina showed higher systemic vascular resistance index and pulmonary arterial resistance index along with low CI in those patients who required BiPAP when compared with non-BiPAP patients. It will be interesting to see the changes in ionotropes and vasopressors' requirement in future studies. Another limitation relates to heterogeneity of the study subset unknown baseline respiratory function (we try to exclude these patients), different types of surgical procedures, CPB time, and length of postoperative ventilation. Although the most common mode for weaning at our institute is SIMV, it would have been ideal to standardize the weaning mode in all study patients.

Hypoxemia, atelectasis, and respiratory impairment are more common in obese cardiac surgery patients, and short-term use of BiPAP was able to improve pulmonary functions which lasted for about 24 h after discontinuation. It is recommended to commence BiPAP early to achieve maximum benefits. This study also showed improvement in respiratory parameters in obese patients,[22] particularly in patients with BMI greater than 35. There is a suggestion to apply BiPAP, soon after extubation in all patients with higher BMI. Prophylactic BiPAP also has a role in patients with low ejection fraction where it reduces the incidence of atelectasis and at the same time increase PaO2.[31] BiPAP can be used as a part of fast-track extubation. As a weaning mode, it reduces extubation time when compared with intermittent mandatory ventilation.[32]

Conclusion

Saturation, PaO2, PaCO2, and PaO2:FiO2 ratios were significantly improved soon after BiPAP application in this study, but no statistically significant changes were seen in hemodynamic parameters. BiPAP application was also able to reduce the need for reintubation in post-cardiac surgery patients.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.