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4 Basic Mechanical Ventilation

37

 

 

Suggested Reading

1.MacIntyre NR. Is there a best way to set positive expiratoryend pressure for mechanical ventilatory support in acute lung injury? Clin Chest Med. 2008;29:233–9.

How, when and how much PEEP is indicated in ARDS

2.MacIntyre NR. Is there a best way to set tidal volume for mechanical ventilatory support? Clin Chest Med. 2008;29:225–31.

Basis of deciding the correct tidal volume in ventilating critically ill patients.

3.Dhand R, Guntur VP. How best to deliver aerosol medications to mechanically ventilated patients. Clin Chest Med. 2008;29:277–96.

This article discusses the role of aerosolized medications in the management of mechanically ventilated patients.

4.Garpestad E, Brennam J, Hill NS. Noninvasive ventilation for critical care. Chest. 2007; 132:711–20.

This article discusses role of noninvasive ventilation in managing critically ill patients

5.Girard TD, Bernard GR. Mechanical Ventilation in ARDS: a state-of-the art review. Chest. 2007;131(3):421–29.

Review of Mechanical Ventilation including noninvasive ventilation.

6.Calverly PMA. Chronic obstructive airway disease. In: Fink M, Abraham E, Vincent JL,

Kochanek PM, editors. Text book of critical care. 5th ed. Philadelphia: SAUNDERS; 2005. p. 599–619.

Source material

Mechanical Ventilation in Acute

5

Respiratory Distress Syndrome

Farhad N. Kapadia and Umakant Bhutada

A 25-year-old female patient presented with fever with chills and vomiting. The following day, she noted increased difÞculty in breathing. The chest X-ray showed extensive bilateral inÞltrates, and she needed supplemental oxygen to keep the saturation at more than 90%. Echocardiography showed normal cardiac function. She was getting progressively fatigued and increasingly drowsy and was intubated.

Acute respiratory distress syndrome (ARDS) is characterized by a brief precipitating event followed by rapidly developing dyspnea. These patients have markedly impaired respiratory system compliance and reduced aerated lung volume. The hypoxemia is refractory to low fraction of oxygen concentration and low positive end expiratory pressure (PEEP). The mortality from ARDS is around 35Ð40%. Current therapy of ARDS revolves around treatment of underlying cause, lung protective ventilatory strategy, and appropriate ßuid management.

Step 1: Initiate resuscitation and identify the reason for deterioration.

¥Initial resuscitation should be done, as mentioned in Chap. 78.

¥Take history, perform quick physical examination, and initiate basic investigation such as arterial blood gas and the chest X-ray to arrive at a probable cause for deterioration in respiratory status.

F.N. Kapadia, M.D., F.R.C.P. (*)

Department of Medicine & Intensive Care, P.D. Hinduja National Hospital and Medical Research Centre, Mumbai, India

e-mail: fnkapadia@gmail.com

U. Bhutada, M.D.

Department of Respiratory Medicine, P.D Hinduja Hospital, Mumbai, India

R. Chawla and S. Todi (eds.), ICU Protocols: A stepwise approach,

39

DOI 10.1007/978-81-322-0535-7_5, © Springer India 2012

 

40

F.N. Kapadia and U. Bhutada

 

 

Table 5.1 Conditions

Pulmonary (primary)

associated with ARDS

Pneumonia

 

 

Aspiration

 

Smoke inhalation

 

Lung contusion

 

Near-drowning

 

Venous air embolism

 

Extrapulmonary (secondary)

 

Sepsis

 

Pancreatitis

 

Blood transfusion

 

Fat emboli

 

Major burn

 

Poly trauma

 

Amniotic ßuid embolism

 

Neurogenic pulmonary edema

 

Cardiopulmonary bypass

 

Drug reactions (aspirin, nitrofurantoin)

¥Acute respiratory distress syndrome (ARDS) used to be diagnosed when a patient fulÞlled the following criteria:

ÐAcute onset

ÐPresence of a predisposing condition (Table 5.1)

ÐBilateral inÞltrates on the chest skiagram

ÐPaO2/FiO2 less than 200 for ARDS

ÐPaO2/FiO2 less than 300 for ALI

ÐPulmonary arterial occlusion pressure less than 18 mmHg or no clinical evidence of the left-sided heart failure

The deÞnition of ARDS has been revised recently .The new deÞnition of ARDS is called ÒBerlin ARDS deÞnitionÓ (Table 5.2).

Step 2: Assess the need for continuous positive airway pressure (CPAP) or noninvasive ventilation (NIV)

¥NIV/CPAP has a very limited role in a patient developing ARDS.

¥It may be used in selected patients who are immunosuppressed, with close monitoring to help improve oxygenation and decrease the work of breathing.

¥A minority of patients show a marked improvement, and they may be taken off this cautiously after a few days of stabilization.

¥Most of the patients show very little or transient improvement, and it is important not to persist with NIV and instead proceed to tracheal intubation before a major deterioration occurs (see Chap. 3).

5 Mechanical Ventilation in Acute Respiratory Distress Syndrome

41

 

 

 

Table 5.2 The Berlin deÞnition of Acute Respiratory Distress Syndrome

 

 

Acute Respiratory Distress Syndrome

 

 

 

 

 

 

Timing

Within 1 week of a known clinical insult or new or worsening respiratory

 

symptoms

 

 

 

 

 

 

Chest imaginga

Bilateral opacitiesnot fully explained by effusion, lobar/lung/collapse, or

 

nodules

 

 

 

 

 

 

Origin of edema

Respiratory failure not fully explained by cardiac failure or ßuid overload

 

Need objective assessment (eg, echocardiography) to exclude hydrostatic

 

edema if no risk factor present

 

 

Oxygenationb

 

 

 

 

 

 

 

Mild

200 mg Hg <Pao

2

/FiO

2

<300 mm Hg with PEEP or CPAP > 5 cm H

Oc

 

 

 

2

 

 

Moderate

100 mg Hg <PaO2/FiO2 < 200 mm Hg with PEEP > 5 cm H2O

 

 

Severe

PaO2/FiO2 < 100 mg Hg with PEEP >5 cm H2O

 

 

Abbrevations: CPAP, continous positive airway pressure; FiO2, fraction of inspired oxygen; PaO2, partial pressure of arterial oxygen; PEEP, positive end-expiratory pressure.

aChest radiograph or computed tomograph scan.

bIf altitude is higher than 1000 m, the correction factor should be calculated as follows: [PaO2/ FiO2X (barometric pressure/760)].

cThis may be delivered noninvasively in the mild acute respiratory distress syndrome group.

Step 3: Assess the need for mechanical ventilation

¥In the following situations of ARDS, mechanical ventilation should be initiated electively. To avoid complications of emergent intubation, it is improper to wait till the patient deteriorates further.

ÐPersistent hypoxemia (SpO2 < 90%) on non-rebreathing facemask oxygen or NIV

ÐExcessive work of breathing and high minute ventilation, which is often a subjective assessment

ÐHemodynamic instability

Step 4: Understand principles of ventilation in ARDS

¥The major principle is to keep the patient stable and cause minimal iatrogenic damage till such time the underlying disease resolves.

¥In ARDS, mechanical ventilation is primarily used to reverse hypoxemia and decrease the work of breathing.

¥Positive-pressure ventilation is unphysiological, and adverse effects of this must be prevented or rapidly reversed.

¥Initially, there may be a signiÞcant hemodynamic deterioration, which mandates adequate monitoring and reversal with ßuids and appropriate inotrope and vasopressor agents.

¥High volumes, high airway pressures, and repeated opening and closing of collapsed alveoli may further damage the lung, worsen the ARDS, and contribute to systemic inßammation.

¥These patients are prone to ventilator-associated pneumonia due to prolonged ventilation required and occasionally due to use of corticosteroid.

42

F.N. Kapadia and U. Bhutada

 

 

¥The mechanical ventilation protocol for a patient with ARDS is based on the concept that the lung is largely consolidated, and may be viewed as a Òbaby lungÓ with only about a third of the alveoli remaining open. This ÒconsolidationÓ primarily results from the alveolar wall becoming stiff and shutting down, rather than the alveoli actually being ßuid-Þlled.

¥The concept of a Òsponge lungÓ implies the gravitational effect of lung injury and the appropriate ventilatory strategy can open up or recruit these shut alveoli. This is demonstrated in imaging studies that show that changes of position of the patient cause changes in the patterns of aeration of the lung, with the lowest or most dependent areas shutting down due to the weight of the lung above, the alveoli in the highest part remaining mostly open. On the basis of these concepts, one of the strategies of mechanical ventilation is to Òopen up the lung and keep it open.Ó

Step 5: Decide on the initial settings on the ventilator

¥A protocol should be followed for initiating ventilator setting, which may be customized according to the patientÕs need.

¥Initially the following setting needs to be decided:

ÐModeÑvolume control ventilation (ARDS network protocol) or pressure control as the starting mode

¥Tidal volume (in volume control mode)

ÐCalculate ideal body weight (IBW): Male IBW = 50 + 2.3 [height (inches) − 60]; female IBW = 45.5 + 2.3 [height (inches) − 60].

ÐSet initial tidal volume (TV) to 8 mL/kg IBW.

Ð Reduce TV by 1 mL/kg intervals every 2 h until TV = 6 mL/kg IBW.

¥Inspiratory pressure (pressure control)

ÐInspiratory airway pressure should be limited to less than 30 cm H2O.

¥FiO2 and positive end-expiratory pressure (PEEP)

ÐInitial FiO2 should be kept high and PEEP 5Ð10 cm H2O to keep oxygen saturation more than 90%.

ÐFiO2 should be titrated down subsequently if oxygen saturation is more than 90%. Titrate PEEP as per ARDSnet table.

¥Minute ventilation

ÐAdjust respiratory rate (maximum up to 35/min) to achieve a minute ventilation commensurate with patientsÕ demand.

¥Inspiratory ßow or inspiratory time or I:E ratio (depending on the ventilator type)

ÐSet the inspiratory ßow rate above patientsÕ demand (usually >80 L/ min); adjust ßow rate to achieve goal of inspiratoryÐexpiratory ratio of 1:1.0Ð1.3.

Step 6: Try to achieve goals of ventilation

¥After initial ventilator setup, the patient should be monitored for safety and efÞcacy of ventilator settings and an attempt should be made to ventilate within certain goals.

5 Mechanical Ventilation in Acute Respiratory Distress Syndrome

43

 

 

ÐOxygenation goal: PaO2 = 55Ð80 mmHg or SpO2 = 88Ð95%

¥Use these incremental FiO2ÐPEEP combinations to achieve oxygenation goal:

FiO2

0.3

0.4

0.4

0.5

0.5

0.6

0.7

0.7

PEEP

5

5

8

8

10

10

10

12

FiO2

0.7

0.8

0.9

0.9

0.9

1.0

1.0

1.0

PEEP

14

14

14

16

18

20

22

24

ÐPlateau pressure (Pplat) goal = 30 cm H2O

¥Keep inspiratory pressure in pressure control below 30 cm H2O.

¥In volume assist control, check Pplat (use 0.5-s inspiratory pause), SpO2, total RR, TV, and arterial blood gases.

¥If Pplat is more than 30 cm H2O, decrease TV by 1 mL/kg steps (minimum 4 mL/kg IBW).

¥If Pplat is less than 25 cm H2O and TV is less than 6 mL/kg, increase TV by 1 mL/kg until Pplat is more than 25 cm H2O or TV is 6 mL/kg.

¥If Pplat is less than 30 cm H2O and breath stacking occurs, one may increase TV in 1 mL/kg IBW increments (to a maximum of 8 mL/kg) as long as Pplat is less than 30 cm H2O.

¥In patients with obesity and stiff chest wall or high Intraabdominal pressure (IAP) a higher plateau pressure may be tolerated.

ÐpH goal: 7.30Ð7.45

Acidosis management: pH less than 7.30

¥If pH is 7.15Ð7.30, increase RR until pH is more than 7.30 or PaCO2 is less

than 25 mmHg (maximum RR = 35); if RR is 35 and PaCO2 is less than 25 mmHg, NaHCO3 may be given.

¥If pH is less than 7.15 and NaHCO3 is considered or infused, TV may be increased in 1 mL/kg steps until pH is more than 7.15 (Pplat goal may be exceeded).

Alkalosis management: if pH is more than7.45, decrease RR if possible.

Step 7: Management strategy for life-threatening hypoxemia

¥Rescue strategies need to be implemented in patients who remain persistently

hypoxemic in spite of maximum FiO2 and PEEP combination. The following strategies are used:

ÐRecruitment maneuver.

ÐAirway pressure release ventilation (APRV)/inverse ratio ventilation (IRV).

ÐProne positioning.

ÐHigh-frequency ventilation.

ÐExtracorporeal membrane oxygenation.

ÐInhaled nitric oxide.

1. Recruitment maneuver

44

F.N. Kapadia and U. Bhutada

 

 

Table 5.3

Recruitment maneuvers

Indications

1. As rescue measure to improve oxygenation

2. After disconnections in ventilator circuit (if responsive to recruitment maneuvers) 3. Postintubation to assess recruitability

Steps

1. The patient should be well sedated/paralyzed

2. The patient should be adequately hydrated

3. The patient should be hemodynamically stable and no arrhythmias

4. Avoid in patients with severe chronic respiratory disease, intracranial hypertension, morbid obesity, and pregnancy

Method 1:

Keep the patient in CPAP mode and deliver 40 cm H2O pressure for up to 30 s at FiO2 of 1.0

Method 2:

Put patient in pressure control mode FiO2 of 1.0

Inspiratory pressure 40Ð50 cm H2O PEEP 20Ð30 cm H2O

Rate 8Ð20/min Inspiratory time 1Ð3 s Duration 1Ð2 min

Start with lower inspiratory pressure (40) and PEEP (20) and if there is no response go to higher pressure

Complications

1. Hypotension (mean arterial pressure <60 mmHg)

2.Desaturation (SpO2 < 85%)

3.Cardiac arrhythmias

4. Barotrauma (pneumothorax, pneumomediastinum, new air leak)

Recruitment maneuver is to apply a high level of sustained airway pressure to open up the collapsed alveoli and then high PEEP to prevent recollapse (Table 5.3).

2.IRV/APRV (Inverse Ratio Ventilation/Airway Pressure Release Ventilation)

¥IRV and APRV may be considered in difÞcult-to-manage ARDS patients.

¥IRV, during which the ratio of inspiratory time to expiratory time exceeds one, can be achieved using either volume or pressure modes of ventilation. Prolongation of the inspiratory time results in increased mean airway pressures, often improving oxygenation.

¥APRV uses high continuous airway pressure to promote alveolar recruitment and to maintain adequate lung volume, and a time-cycled release phase to a lower pressure in supplementing spontaneous minute ventilation. By allowing unrestricted spontaneous breathing throughout the ventilator cycle, APRV allows for better ventilation of dependent lung regions; spontaneous breathing reduces atelectasis.

5 Mechanical Ventilation in Acute Respiratory Distress Syndrome

45

 

 

ÐThese modes of ventilation may lead to ventilation with a low tidal volume leading to hypercapnia, which is termed Òpermissive hypercapniaÓ as it is a necessity for safe ventilation.

ÐPermissive hypercapnia is usually safe but may have some harmful effects, which include pulmonary vasoconstriction and pulmonary hypertension, proarrhythmic effects due to increased discharge of the sympathetic nervous system, and cerebral vasodilation leading to increased intracranial pressure.

ÐPermissive hypercapnia should probably be used with caution in patients with heart disease and is relatively contraindicated in those with elevated intracranial pressure.

3.Prone position

¥Prone position can be considered if the patient has any of the following:

ÐSevere hypoxemia: PaO2/FiO2 < 140 mmHg.

ÐPaO2 < 55 with FiO2 ³ 0.7 or plateau pressure >30 cm H2O.

¥Contraindications:

ÐLife-threatening shock (mean arterial pressure <65 mmHg with or without vasopressors).

ÐRaised intracranial pressure more than 30 mmHg, or cerebral perfusion pressure less than 60 mmHg.

ÐSpinal instability or any unstable fracture.

ÐRecent thoracoabdominal surgery.

ÐOpen wound or burns on ventral body surface.

ÐMassive hemoptysis.

ÐArrhythmias.

¥Steps to proning:

ÐProning needs trained staff; it requires 4Ð6 persons.

ÐHave a central and arterial line in place.

ÐArrange for cushions.

ÐFix tube and lines well.

ÐEmpty the nasogastric tube.

ÐSedate well and paralyze if required.

ÐCover eyes.

ÐPlace electrocardiograph (ECG) electrodes on the back.

ÐOne person stands at the head end and holds the ETT with one hand and the head with the other.

ÐDisconnect monitoring.

ÐBring the patient on the edge of the bed.

ÐTurn the patient by three persons and place on supporting cushions under chest and lower pelvis.

ÐImmediately reconnect monitors and take BP, SpO2.

ÐAuscultate chest.

ÐMake sure abdomen is free (for respiration).One should be able to pass hands between abdomen and mattress.

46

F.N. Kapadia and U. Bhutada

 

 

ÐExtracushion pads for genitalia, axilla, ears, breasts, knees, foot.

ÐCheck ABG within 30 min.

ÐTurn the head alternately to right and left every 2 h.

¥DurationÑtwo methods used:

ÐIntermittent: 8Ð20 h/day

ÐContinuously for longer duration till patient shows improvement

¥Complications:

ÐPressure ulcers

ÐDisplacement of endotracheal tubes, thoracotomy tubes, and vascular catheters

4.High-frequency ventilation

¥Consider high-frequency ventilation, which implies application of mechanical ventilation with a respiratory rate that exceeds 100 breaths/min.

¥HFOV supports pulmonary gas exchange by entraining gas from a bias ßow circuit and delivering subnormal tidal volumes to the lungs at rates between 3 and 15 cycles per second (Hz).

¥Smaller tidal volume is delivered, which limits alveolar overdistension.

¥Higher mean airway pressure is attained, so there is more alveolar recruitment.

¥Constant airway pressure is applied during both inspiration and expiration, which prevents end-expiratory alveolar collapse.

¥Oxygenation can be adjusted independently of CO2 removal by adjusting the mean pressure and FiO2.

¥CO2 removal is increased by increasing the oscillation pressure amplitude, decreasing the frequency, increasing percent inspiratory time, and causing intentional cuff leak.

¥To maximize lung protection, emphasis is placed on achieving frequency as high as possible in combination with the lowest amplitude.

¥HFOV can be considered under the following circumstances:

ÐOxygenation failure: FiO2 equal to or more than 0.7 and PEEP more than 14 cm H2O

Ðor

ÐVentilation failure: pH less than 7.25 with tidal volume equal to or more than 6 L/kg predicted body weight and plateau airway pressure equal to or more than 30 cm H2O.

ÐIt may be considered for patients with ARDS who are failing conventional ventilation. In the absence of studies showing improved clinical outcomes, it remains an investigational tool for routine management of ARDS.

¥Contraindications:

ÐKnown severe airßow obstruction.

ÐIntracranial hypertension

¥Target

ÐOxygenation target: SpO2 88Ð95% or PaO2 55Ð80 mmHg

ÐVentilation target pH: 7.25Ð7.35

5 Mechanical Ventilation in Acute Respiratory Distress Syndrome

47

 

 

¥Initial HFOV settings

ÐBias ßow = 40 L/min

ÐInspiratory time = 33%

ÐmPaw = 34 cm H2O

ÐFiO2 = 1.0

ÐAmplitude (DP) = 90 cm H2O

ÐInitial frequency based on most recent arterial blood gas:

¥pH more than 7.10 = 4 Hz

¥pH 7.10Ð7.19 = 5 Hz

¥pH 7.20Ð7.35 = 6 Hz

¥pH > 7.35 = 7 Hz

¥Oxygenation can be improved by increasing the airway pressure and FiO2.

¥Recruitment maneuver, prone position, and nitric oxide can be used as adjuncts.

¥Ventilation goals are achieved using frequency as the primary adjustment, rather than the oscillation pressure amplitude.

¥Higher frequencies are emphasized, which will result in smaller TV.

¥Transition to conventional ventilation.

ÐWhen patients have reached at FiO2 of 0.4 and mPaw of 22 cm H2O and remained on those settings for at least 12 h.

ÐInitial settings:

¥VT = 6 mL/kg predicted body weight

¥FiO2 = 0.5

¥PEEP = 16 cm H2O

¥RR = 25

ÐCheck arterial blood gas in 30Ð60 min.

ÐRevert to HFOV if the patient again meets criteria for oxygenation or ventilation failure.

Step 8: Evaluate effects on oxygenation, static compliance, and dead-space ventilation

¥ Static lung compliance =

VT

 

Plateau pressure (PEEP + autoPEEP)

 

¥Normal value of static compliance is 100 mL/cm H2O.

¥If there is signiÞcant improvement, then continue with therapy. If there is no signiÞcant improvement, then proceed to the next intervention.

Step 9: Consider administration of glucocorticoids

¥Weigh the risks and beneÞts for individual patients. It should be avoided in patients with active infection.

¥It is used within 2 weeks of onset.

¥Dose should be methylprednisolone 1 mg/kg bolus followed by 1 mg/kg/day infusion.

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