Journal of Postgraduate Medicine Medknow Publications and Staff Society of Seth GS Medical College and KEM Hospital, Mumbai, India ISSN: 0022-3859 EISSN: 0972-2823 Vol. 48, Num. 1, 2002, pp. 34-36

Journal of Postgraduate Medicine, Vol. 48, Issue 1, 2002 pp. 34-36

Pressure Controlled Inverse Ratio Ventilation in Acute Respiratory Distress Syndrome Patients

Tripathi M, Pandey RK, Dwivedi S^*

Departments of Anaesthesiology and Critical Care and Medicine*, BP Koirala Institute of Health Sciences, Dharan, Nepal.
Address for Correspondence: Mukesh Tripathi, MD, Department of Anaesthesiology and Critical Care Medicine, SGPGI of Medical Sciences, Lucknow-226014, India. E-mail: mukesh_tripathi@hotmail.com

Code Number: jp02010

Abstract:

Appropriate ventilatory intervention is life saving in acute respiratory distress syndrome (ARDS). Pressure controlled inverse ratio ventilation (PC-IRV) is the likely mode of ventilation benefiting in extreme conditions of ARDS. However, guidelines when to start PC-IRV are not yet well defined. The ventilation-related dilemma, which we faced in two illustrative cases of ARDS are presented. The first patient presenting clinically with ARDS but with high peak airway pressure (PIP) and low dynamic lung compliance, PC-IRV helped in reducing PIP, improved haemodynamics and the oxygenation of blood. In second patient with similar clinical presentation of ARDS, where although PIP was high but the dynamic compliance was better, the PC-IRV caused deterioration in PaO₂. Here, patient rather did better with high PEEP (15cmH₂O) and usual I:E ratio (1:2). It is probable that the dynamic lung compliance (<20ml/cmH₂O), PIP (>50cmH₂O) at conventional I:E ratio (1:2) ventilation (10ml/kg) with hypotension might form the basis to develop a scoring system for guidance to switch over to PC-IRV ventilation. Further randomised prospective controlled clinical trials will then be required to establish indication to start PC-IRV in ARDS.

Key Words: ARDS, Pressure controlled ventilation, Inverse ratio ventilation, PC-IRV, Positive pressure ventilation.

Mechanical ventilation still remains the main stay in the management of the acute respiratory distress syndrome (ARDS). High tidal volume and high airway pressure lead to lung barotrauma and the morbidity hence, there is need to use alternative modes of ventilation. Pressure control with inverse ratio ventilation (PC-IRV) has been extensively studied in recent past.^1-4 Contradictory claims regarding its efficiency have been reported in different studies on animal^5,6 and in human.^7-9 The indications to start PC-IRV and to terminate it are still not very clear. We wish to share our experience of two patients, who required completely different modes of ventilation inspite of similar clinical presentation.

Case History

Case No 1

A 28-year female patient weighing 50 kg, and height 163 cm was admitted with fever, altered sensorium, irrational behaviour and convulsions. She had high-grade fever, generalised bodyache and weakness since 20 days with a history of foul smelling discharge per vaginam.

At the time of admission heart rate was 108/min and blood pressure of 120/60 mmHg. She was semiconscious; responding to painful stimuli, Glasgow coma scale (GCS) of 6, and normal reacting pupils. Deep tendon reflexes were depressed and the plantar reflex was flexor. While the patient was breathing room air, the arterial oxygen saturation (SaO₂) was 86%. The radiograph of chest showed diffuse bilateral infiltrates in all the quadrants. ECG revealed a normal sinus rhythm. A probable diagnosis of cerebral malaria/viral encephalitis was made and patient was given quinine, mannitol, dexamethasone and phenytoin. In addition, she received ceftriaxone, metronidazole, ranitidine, aminophylline infusion and oxygen (5l/min) by facemask.

Second day, she developed hypotension, respiratory distress, and fall in SaO₂ from 96% to 78%. Coarse crepitations and scattered ronchi were heard bilaterally. At this time, dopamine infusion was started, tracheal intubation was done and ventilatory assistance was provided (SIMV-10, tidal volume (V_T)-500ml, I:E-1:2, FIO₂-0.8, CPAP-7cmH_2O and ASB 20cmH₂O). With this mode, her SaO₂ deteriorated to 70%, after 4 hours and the FiO₂ was increased to 1.00. The chest radiograph revealed ill defined blotchy opacities in the middle and lower zones with air-bronchogram corresponding to extensive intra-alveolar pulmonary oedema and lung injury score (LIS) 2.7 and APACHI-II 26 (Table). Arterial blood gases analysis showed pH 7.32, PaO₂ 56mmHg and PaCO₂ 28mmHg. Haematological and other laboratory values were suggestive of septic shock. The patient was pharmacologically paralysed and volume controlled IPPV was instituted. This restored the SaO₂ to 90%.

Third day morning, the SaO₂ again dropped to 74% at FiO₂ 1.00 and arterial blood gases showed further deterioration pH 7.16; PaO₂ 64mmHg; PaCO₂ 23mmHg. At this time peak inspiratory pressure (PIP) was as high as to 62 cmH₂O for the ventilator setting (V_T-500ml, RR-15 I:E-1:2) and monitored total dynamic compliance 14ml/cmH₂O and resistance-21cmH₂O/ml. Tachycardia (heart rate-178bpm), hypotension (ABP-79/41mmHg) was noted. It was decided to change the ventilator mode to PC-IRV. The ventilator settings were P_max 30cmH₂0, RR 15/min, V_T 500ml, PEEP 10cmH₂0, FiO₂ 1.00, I:E-3:1. This mode of ventilation improved oxygenation at FiO₂ 1.00; SaO₂ 97%; PaO₂ 74mmHg and pH 7.14 and PaCO₂ 32mmHg. Haemodynamically, she improved as well (ABP-106/67mmHg and heart rate-137bpm). Dobutamine and adrenaline infusions were also started along with the renal dose of dopamine. Suitable antibiotics were started on the basis of blood culture report.

On day four, her condition facilitated reduction of FiO₂ to 0.5 in next 20 hours of starting of PC-IRV to attain SaO₂ 97%; PaO₂ 94mmHg; pH 7.3, and it was decided to discontinue muscle relaxant, change the ventilator mode to SIMV (RR 10/min; V_T 500ml) and ASB 20cmH₂O with normal I:E ratio (1:2). Within two hours of it, the oxygenation of the patient deteriorated (PaO₂ 58mmHg; SaO₂ 74%) and the FiO₂ was increased back to 0.9. Patient developed frank pulmonary oedema and coarse crepitations. At this time, we noted that the compliance was still very low (10ml/cmH₂O) with high resistance (21cmH₂O/ml). The ventilator mode was reverted back to PC-IRV, and continued for the next 36 hours. After this time, when compliance improved to 28ml/cmH₂O and resistance to 14cmH₂O/ml to get V_T 500ml, the ventilation could be changed to SIMV with ASB successfully. Over next four days the patient could be successfully weaned from the ventilator and was extubated.

Case 2

A 50-year female, weighing 70 kg, height 158 cm was brought with respiratory distress, and irrational violent behaviour developing after seven days of fever, dysentery and weakness. Oxygen by mask and prophylactic antibiotic therapy (ceftrioxone, amikacin, metronidazole) and oral paracetamol initially helped.

Second day, the patient became extremely restless and violent with irrelevant talking. Clinically, patient was showing cyanosis and tachypnoea (RR>50/min). It was decided to start elective positive pressure ventilation and accordingly patient was sedated with midazolam (8mg IV) and muscle relaxation with vecuronium (8mg). Nasotracheal intubation was done. IPPV was started at FiO₂ 0.60 (tidal volume 650ml; RR 12/min). Arterial blood gas analysis 20min later revealed (PaO₂ 48mmHg; pH 7.02; and PaCO₂ 18mmHg). FiO₂ was increased to 1.00 and sodium bicarbonate (7.5%) 100ml was given in slow infusion. Immediate X-ray chest showed diffuse bilateral infiltrates in all the quadrants, LIS 2.7 and APACHI II 32 (Table). ECG revealed a normal sinus rhythm.

Third day, during conventional mode of ventilation (tidal volume 10ml/kg; respiratory rate 12/min), PIP was 42cmH₂O, dynamic lung compliance 34ml/cmH₂O, airway resistance 13cmH₂O/ml. Her ABP ranged (70-75mmHg) and heart rate (136-149bpm) with inotropic support (Dopamine-10mcg/kg/min) but to maintain SaO₂ 88%, PaO₂ (64mmHg), we had to keep FiO₂ 1.00. To improve oxygen saturation at safer FiO₂ (<0.6) limits, we tried to increase PEEP 15cmH₂O but this did not improve the oxygenation and it was decided to try PC-IRV (PC 30cmH₂O; V_T 500ml, I:E 3:1; and RR-12/min). At this ventilator setting, patient desaturated (SaO₂ 74%) in place of expected improvement. ABP showed marginal improvement (mean 80mmHg) but we had to abandon the PC-IRV setting and started again the conventional ventilation with PEEP 15cmH₂O and tidal volume (10ml/kg). Arterial blood gas improved gradually to pH-7.32, PaO₂ 102mmHg at FiO₂ 0.9 and PaCO₂ 36mmHg. Patient continued to improve with conventional mode of ventilation gradually.

Discussion

In early course of ARDS, physicians are more commonly using volume targeted ventilation.⁹ Recently, significant emphasis has been given to some unconventional modes of ventilation strategies in ARDS e.g. open lung ventilation, low tidal volume with permissible hypercapnia, and pressure controlled inverse ratio ventilation (PC-IRV). All these strategies are aimed to achieve better gas exchange without lung barotrauma. Since the inception of prolonged inspiration time in place of high PEEP by Reynolds, PC-IRV has remained controversial mode of ventilation, having almost even reports in favour and against its use.^1-8 No definite guidelines are available as to when to execute PC-IRV leaving us to try PC-IRV in extremes of respiratory failure and then probably by the time it is too late to achieve positive results.

In the presented two patients, we observed that in spite of very similar clinical and respiratory failure presentation, patients required different mode of ventilation strategy. In first patient, PC-IRV worked well to improve oxygenation and haemodynamics. Here, the patient had significantly low dynamic lung compliance (9ml/cmH₂O), high PIP (>60cmH₂O) during I:E/1:2 ventilation at V_T (10ml/kg) and the hypotension even on inotropic support. After starting PC-IRV, her mean ABP improved to 68mmHg. Adrenaline infusion helped to maintain mean ABP above 75mmHg. PC-IRV in this patient improved ventilation as well.

In second patient, where during conventional mode of ventilation (V_T 10ml/kg; RR 12/min), although PIP (42cmH₂O) was high, but the dynamic compliance (34ml/cmH₂O) was better. Patient was maintaining mean ABP at 70-75mmHg and heart rate (136-149bpm) with inotropic support, and oxygenation (SaO₂ 88%; PaO₂ 64mmHg at FiO₂ 1.0), and PaCO₂ (36mmHg). To get oxygenation at safe FiO₂ limits (<0.60), we tried to increase PEEP from 7 to 15cmH₂O, but when this failed, we decided to try PC-IRV (PC 30cmH₂O; V_T 500ml; I:E 3:1; and RR 12/min). On PC-IRV, patient developed desaturation to (SaO₂ 74%) in place of expected improvement. ABP improved marginally (mean ABP 80mmHg) and we abandoned the PC-IRV setting after a trial for 30min and started again the conventional ventilation with PEEP 15cmH₂O, V_T 10ml/kg, and I:E 1:2 with improvement.

Although PC-IRV has been reported with increased PaCO_2,^10,11 and low cardiac output¹¹ but we found a definite improvement in first patient in terms of decreased PIP,¹¹ and better oxygenation,¹² while did not show any change in second patient. These two patients point out that probably the low dynamic lung compliance (<20ml/cmH₂O), high PIP (>50cmH₂O) at conventional I:E ratio (1:2), tidal volume setting (10ml/kg), hypotension and low acute lung injury score (<2.7) should form a basis to develop a scoring system to help guide us as to when to switch over PC-IRV ventilation.

In summary, with this experience, we wish to highlight that it is not uncommon to find two patients of similar clinical picture requiring different ventilation strategy in ARDS. We need to tailor various modes of ventilation depending upon individual patient's response and his clinical condition, to achieve goals of ventilation. PC-IRV is still a mode of ventilation benefiting in extreme conditions of ARDS and rigorous clinical trials are required to establish the appropriate time of its initiation or termination, and the correct patient selection to get the benefit of PC-IRV in ARDS subjects.

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