Mechanical Ventilation

Indications

• Improve gas exchange

* oxygenation

T alveolar ventilation and/or reverse acute respiratory acidosis

• Relieve respiratory distress

I work of breathing (can account for up to 50% of total oxygen consumption) I respiratory muscle fatigue

• Airway protection

• Pulmonary toilet

Choosing settings {NEfM 2001 ;344 1986)

1) Pick ventilator mode

2) Choose volume-targeted or pressure-targeted

3) Set or ✓ remaining variables

Step 1: Pick Ventilator Mode

Mode

Description

Assist control (AC)

Vent delivers a minimum number of supported breaths Additional Pt-initiated breaths trigger fully-assisted vent breaths

Vent-triggered breaths identical to Pt-triggered breaths Tachypnea • ? respiratory alkalosis, breath-stacking. & auto-PEEP May be pressure-targeted or volume-targeted

Synchronized intermittent mandatory vent (SIMV)

Vent delivers min. no. of supported breaths (synch, to Pis efforts) Additional Pt-initiated breaths -.Vt determined by Pt's efforts . Vent-assisted breaths * spontaneous breaths Must overcome resp. circuit during spont. breaths — T fatigue SIMV AC in patients who are not spontaneously breathing

Pressure support vent (PSV)

Vent supports Pt-initiated breaths with a set inspiratory pressure

A mode of partial vent support because no set rate

Can combine with SIMV to partially assist spontaneous breaths

Continuous positive airway pressure (CPAP)

Pt breathes spont at their own rate while vent maintains constant positive airway pressure throughout respiratory cycle (7 cm H20 overcomes 7 Fr ETT)

T-piece

No airway pressure, no rate set; patient breathes through ETT

Other

High-frequency vent (a/rccm 2002.166:801; cow 20035-31:s-317) ECMO and ECCOjR {Am Sorg 2004:240:595)

Volume-targeted

Step 2: Choose Volume-Targeted or Pressure-Targeted

Vent delivers a setVr

Airway pressures depend on airway resist & lung/chest wall compliance Benefit T control over ventilation (ideal initial ventilator setting); evidence-based benefit in ALI/ARDS; easy to measure mechanical respiratory properties (PIR P,*,. airway resistance, compliance) Risk: Patient at risk for t pressures -» barotrauma (and volutrauma if set volume to high!)

Pressure-targeted

Vent delivers a fixed inspiratory pressure regardless of Vt Vt depends on airway resistance and lung/chest wall compliance Benefit: May t patient comfort (PSV) requiring less sedation Risk: Pt at risk for A volumes — inadequate Vf

General principles

Institutional/practitioner preference and patient comfort usually dictate ventilator strategy; no strategy has proven superior Common reasons for changing ventilator strategies include: dysynchrony. poor gas exchange. A mechanical respiratory properties. A goals of care (eg. sedation, weaning, lung protection) Alarms can be set for T volumes and T airway pressures in pressure-targeted and volume-targeted strategies, respectively

Step 3: Set or ✓ Remaining Variables

FiOj Fraction of inspired air that is oxygen

Positive Positive pressure applied during exhalation end- Generated by a resistor in exhalation port expiratory Benefits: prevents alveoli collapse. 1 intrapulmonary shunt. T Oj pressure Cardiac effects:

(PEEP) I preload by t intrathoracic pressure and impeding venous return i afterload by i cardiac transmural pressure may t or i CO and may ! or ; oxygen delivery based on the above "Auto-PEEP" or "intrinsic PEEP": inadequate exhalation time - lungs unable to completely empty before the next breath (ie."breath stacking"); if flow at end-expiration must be pressure auto-PEEP Will i preload and may i CO. especially if Pt. hypovolemic Will T work of breathing as must be overcome by Pt to trigger breaths; can prevent Pt from successfully triggering ventilator Can be detected if end-expiratory flow * 0 before next breath Can measure by occluding expiratory port of vent at end-expiration Inspiratory Normally l:E ratio is 1:2; however, can alter I time (and consequently time flow rate, see below): use in pressure control mode

Inspiratory ? flow rate -♦ 1 I time * E time -» .. improved ventilation in flow rates obstructive disease; use in volume control mode Peak Dynamic measurement during inspiration inspiratory Determined by airway resistance and lung/chest wall compliance pressure Set in pressure-targeted ventilation

(PIP) t PIP w/o T Ppu — T airway resist (eg. bronchospasm. plugging)

; PIP ; airway resistance or air leak in the system Plateau Static measurement at the end of inspiration when there is no flow pressure Determined by resp system compliance (resist, not a factor since 0 flow) (Pp.«) t Pput i lung or chest wall compliance (eg. PTX. pulmonary edema, pneumonia, atelectasis), t PEEP, or auto-PEEP

Initial Settings

Mode

Tidal volume

Respiratory rate

F.O,

PEEP

Assist control

8 mL/kg

12-14

1.0 (ie. 100%)

! 5 cm H,0

Volume-targeted

IBW

breaths/min

Initial VT -9 ml/kg is risk factor for subsequent development ofVILI in non-ALI acutc respiratory failure (ccm 2004:311817);. . start with 8 mUkg and subsequently tailor.

Initial VT -9 ml/kg is risk factor for subsequent development ofVILI in non-ALI acutc respiratory failure (ccm 2004:311817);. . start with 8 mUkg and subsequently tailor.

Noninvasive Ventilation

Continuous positive PEEP

airway pressure No limit on Oj delivered (ie. can give hi-flow F.02 1.0) (CPAP) Used in Pts whose primary problem is hypoxemia (eg. CHF)

CHF: i intub. & mortality ¡jama 2005:294:3124: Lancet 2006:367 1155)

PSV - PEEP

Able to set both inspiratory (usually 8-10 cm HjO) and expiratory pressures (usually ■ 5 cm HjO) Used when primary problem hypoventilation: Oj delivery limited COPD:. intub. & mortality (Loncet 2000.355:1931) PN A- controversial - T secretions. I mort. I intubations in subgroup w/ acute hypoxemic resp. failure (AJRCCM 2003:168 1438) Immunocompromised i mort.. A intub..» infections Postextubation: if high risk (age 65 y. CHF.APACHE-II >12) routine NIV ■ 24 h i reintub. and. if P,C02 -45 during SBT. i mortality WRCCM 2006:173:164) Rescue strategy after failed extubation: I mort. (nejm 2004:350.2452) Mask ventilation Tight-fitting mask connecting patient to a standard ventilator Can receive pressure support of up to 20-30 cm H?0. PEEP of up to 10 cm H2O. F.Oj of up to 1.0 Used for short-term support (<24 h) for a reversible process (asthma. CHF. COPD) Contraindications: altered MS. vomiting, unable to protect airway, extrapulmonary organ failure, hemodynamic instability, severe UGIB. inability to fit mask (jama 2002.288:932)

NPPV/Bilevel positive airway pressure (BiPAP)

(nb. recent trials w/ ICU-style NIV w/ alarms, new modes. ' O2 delivery, etc.; ? equivalent to basic BiPAP device)

Tailoring the ventilator settings

• To improve oxygenation: f F.Oj. t PEEP (optimize based on lung mechanics)

• To improve ventilation: T Vt or inspiratory pressures, t RR (may need to i I time to accomplish As)

• Permissive hypercapnia: tolerating T P4COj to avoid excessive barotrauma or volutrauma may have protective effect in ALI/ARDS (ccm 2006:34 1) ^^ VT 4-6 mL/kg IBW (as long as P,COj < 80 and pH 7.15)

rel. contraindic.: cerebrovascular disease, hemodynamic instability, renal failure. PHT Acute ventilatory deterioration (usually T PIP)

• Response to T PIP: disconnect Pt from vent. bag. auscultate, suction. ✓ CXR & ABG

Figure 2-8 Approach to acute ventilatory deterioration

PIP

/ 1

\

decreased increased normal

^ 1

air leak Ppiat

PE

1 airway resistance

extrathoracic process

normal increased

S

Ni

airwav obstruction

decreased compliance

bronchospasm

asynchronous breathing

secretions

auto-PEEP

aspiration

pneumothorax

ventilator tubing problem

atelectasis

pneumonia

pulmonary edema

PE

(Adapted from Marino. P.L The icu Book. 2nd ed.. Baltimore: William* & Wilkin*. 1990:430) Weaning from the ventilator

• Weaning strategy (no single proven approach): spontaneous breathing trial (SBT) ? better than PSV (i 2-4 cm HjO q12h) and SIMV (i RR 2-4 breaths/min q12h - backup 5 cm H20 PSV); IMV not effective (NE/M 1995:331345)

• Identify Pts who can breathe spontaneously (NEJM 1991:324:1445 & 1996:335:1864)

screening criteria: sedation reversed.VS stable, minimal secretions, adequate cough, cause of respiratory failure or previously failed SBT reversed vent parameters: P,02/Fi02 200. PEEP ^S.f/VT <• 105,VE <12 L/min.VC >10 mL/kg SBT (CPAP orT-piece x 30-120 min) (Ajrccm 1999;1S9.512) failure if: deteriorating ABGs, T RR. i or I HR. T or i BP. diaphoresis, anxiety rapid shallow breathing index (f/VT) -105 predicts failure; NPV 0.95 [nejm 1991;3241445)

• If tolerate SBT consider extubation

• If fail SBT ? cause work to correct on comfortable vent settings — retry SBT qd Complications

• Barotrauma and volutrauma (eg, pneumothorax, pneumomediastinum)

high PIPs are usually not harmful unless ? Pph, (? >33 cm HjO. but the lower the better) alveolar damage

• Oxygen toxicity (theoretical); proportional to duration - degree of T oxygen (F,02 >0.6)

• Alterations in cardiac output

• Ventilator associated pneumonia (1%/day. mortality rate 30%)

typical pathogens: MRSA. Pseudomonos, Acinetobacter and Enterobocter species preventive strategies (ajrccm 2005:171:388) nonpharm: wash hands, semirecumbent position, non-nasal intubation, enteral nutrition rather than TPN, routine suction of subglottic secretions pharm: avoid unnecessary abx and transfusions, routine oral antiseptic, daily sedation withdrawal, stress-ulcer prophylaxis w/ ? sucralfate (i VAP.T GIB) vs. Hj-ant or PPI

• Laryngeal edema: for Pts ventilated >36h. ? consider methyfprednisohne 20 mg IV 12 h before extubation and q4h until tube removed -» 86% I in clinically evident laryngeal edema and 50% A in reintubation (tone« 2007:369:1003)

ulceration: consider tracheostomy for patients in whom expect >14 d of mech. vent I duration mech. vent. 1 # ICU days (8mj 2005:330:1243)

ACUTE RESPIRATORY DISTRESS SYNDROME

Definition (nejm 2000:342:133-4)

• Acute respiratory distress syndrome (ARDS) / Acute lung injury (AU)

clinical -- acute onset, severe hypoxemia, refractory to O2 w/ bilateral infiltrates pathophys. = noncardiogenic pulmonary edema path. = diffuse alveolar damage (DAD)

• American-European Consensus Conference (1994): 4 criteria to define ARDS w/o bx

1) acute onset

2) bilateral patchy air-space disease (need not be diffuse)

3) PCWP <18 mmHg or no clinical evidence of t LA pressure

Etiologies

Direct Injury

Indirect Injury

• Near drowning

• Shock • Trauma/multiple fractures

• DIC • Hypertransfusion (TRAU)

Pathophysiology

• T intrapulmonary shunt (. . refractory to T FiOi);can develop 2° PHT

Diagnostic studies

• CXR: bilateral infiltrates developing w/in 24 h of appearance of air-space disease

• Chest CT: patchy infiltrate mixed w/ normal lung, densities greater in dependent areas

• Lung biopsy: not required, but often provides useful dx information (Chest 2004:125:197)

Treatment (primarily supportive) (lone« 2007:369.1553)

• Mech. vent.: maintain systemic O2 deliv. while min. ventilator-induced lung injury (VILI)

Mechanisms of VILI

Barotrauma/volutrauma:

alveolar overdistention mech. damage; biotrauma — SIRS

Ventilator Strategies

VT s6 mL/kg, Ppu, s30 cm H20. tolerate 1 P,CO; (but keep pH >7.15). sedation/paralysis as needed (but try to minimize); i mortality (ne/m 2000.342:1301)

Atelectrauma: repetitive alveoli recruitment & decruitment

Titrate PEEP to prevent tidal alveolar collapse may 1 mortality (ne/m 1997338:347). However, no mortality diff. in high vs. low PEEP strategy at a given vt (alveou. n£j/m 2004:351 327).

Hyperoxia: Oj rad. ? injury

T PEEP rather than FOj (keep 0.60)

• Fluid balance: target CVP 4-6 cm H2O (if nonoliguric & normotensive) — t ventilator/ICU

free days, but no mortality difference (factt. nejm 2006:354:2564)

• Routine PA catheter use to guide fluid management in ARDS offers no advantage over central venous catheter (factt. nejm 2006:354:2213)

• Steroids: early ARDS • no benefit (nejm 1987:317.-1 SOS)

ARDS onset 7-13 d: ? benefit i ventilator/ICU free days.no mort. difference ARDS onset ^14 d: clear harm — t mortality (L»srs. nejm 2006:354 1671)

• Experimental:

Inhaled nitric oxide: transient I PAP. t PtOilhOj, no i mort. (jama 2004.291:1603) Prone ventilation: t PjOj. no i mortality (/ama 2004:292:2379)

? 1 mortality if initiated early and for longer (17 h/d) (ajrccm 2006:173:1233) IV B? agonist 1 lung H2O. I Ppa, (ajrccm 2005:173 281): await results of large RCT Lung recruitment: insp. hold to recruit lung and then t PEEP to maintain (nejm 2006:354:1775 & 1839)

ECMO: selected Pts. (young, immunocompetent w/o MODS) (Ant1 Surj 1999:134:375) Prognosis

• Mortality: 40% overall: 9-15% resp. causes. 85-91% extrapulmonary (MODS)

• Higher dead-space fraction [(P,C02-PeC02)/P.C02] predicts t mort. (nejm 2002:346:1281)

• Sequelae: PFTs normal. ¿ DLCO. muscle wasting, weakness persists (nejm 2003:348:683)

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