Chapter 46 Respiratory Monitoring and Management of the Patient in the Intensive Care Unit

1. Chapter 46Respiratory Monitoring and Management of the Patient in the Intensive Care Unit

2. Objectives • Discuss the principles of monitoring the respiratory system • Discuss the risks and benefits of intensive care unit (ICU) monitoring techniques. • Discuss why the caregiver is the most important monitor in the ICU. • Describehow to evaluate measures of patient oxygenation in the ICU.

3. Objectives (cont.) • Define why Paco2 is the single best index of ventilation for critically ill patients. • Describe the approach used to evaluate changes in respiratory rate, tidal volume, minute ventilation, Paco2, and end-tidal Pco2 values for monitoring purposes. • Discuss monitoring techniques used in the ICU to evaluate lung and chest wall mechanics and work of breathing. • Discuss the importance of monitoring peak and plateau pressures in patients receiving mechanical ventilatory support.

4. Objectives (cont.) • Describe the approach used to interpret the results of ventilator graphics monitoring.

5. Introduction to Monitoring • Continuous monitoring or periodic checks • Gray area between diagnostic and monitoring procedures • Risk/benefit ratio

6. Monitored Values • All data must be evaluated in context of overall clinical presentation. • Instrument inaccuracyrecalibrate • Artifacts • Factitious results: true but temporary (cough) • Treat the pathology, not the errant number. • All values monitored must be considered in relation to what pathology has altered them and how best to treat the pathology.

7. Monitoring Oxygenation • Tissue oxygenation depends on CaO2 (PaO2 and SaO2), cardiac output, and oxygen uptake • Pulse oximetry (“fifth vital sign”) • Provides noninvasive measurement of SaO2, referred to as SpO2 • Monitors only oxygen, not ventilation • Significant limitations

8. Monitoring Oxygenation (cont.)

9. Other Oxygen Indices • Oxygen consumption • Difficult to measure, so seldom used • Normal 250 ml/min, 25% of oxygen delivery • P(A  a)O2 • Healthy patient • 21% O2, gradient is 5 to 15 mm Hg • 100% O2, gradient is 100 to 150 mm Hg • An abnormal increase is associated with gas exchange problems.

10. Other Oxygen Indices (cont.) • PaO2/FIO2 ratio (P/F ratio) • Normal P/F ratio is 400 to 500. • In ALI, this falls below 300. • In ARDS, it will be < 200. • Most reliable index of gas exchange if FIO2 > 0.50 and PaO2 < 100 mm Hg • QS/QT (physiologic shunt) • Increased if pulmonary venous admixture occurs (mixed venous blood exits A/C membrane unchanged)

11. Monitoring Ventilation • Routine monitoring includes • PaCO2, which defines adequacy of ventilation • T, f, and E • Low VT and high f often indicate distress • VD/VT • Normal 0.20 to 0.40 • Higher ratio indicates more wasted ventilation • ICU common to be >0.70 • >0.60, patient is unlikely to sustain spontaneous ventilation . . V V . .

12. Compliance • Compliance is ΔV/ΔP or effective VT/(Pplat  PEEP) . • Normally, this is 60 to 100 ml/cm H2O . • In severe ARDS, it may be <25 ml/cm H2O . • Many pulmonary diseases alter compliance. • See Box 46-7.

13. Resistance • Resistance (Raw) = (PIP Pplat)/flow • Normally 1 to 2 cm H2O/L/sec • Intubated, probably 5 to 10 cm H2O/L/sec or more • See Box 46-7 for diseases that alter Raw.

14. Auto-PEEP • If exhalation is incomplete, auto-PEEP occurs. • Causes ⇑FRC and mean alveolar pressure • Often causes patientventilator asynchrony • Ways to decrease auto-PEEP • Decrease VE • Increase ET • Decrease IT .

15. Auto-PEEP (cont.) • Adding extrinsic PEEP may overcome the trigger sensitivity issue and facilitate lung emptying. • Slowly increase in 1-2 cm H2O increments till either: • Patient can trigger the ventilator • Auto-PEEP increases

16. Measuring Auto-PEEP • Presence noted by expiratory flow at the end of expiration • Measured by • End-expiratory hold: most common method • Allows alveolar pressure to equalize with the ventilator pressure. • Esophageal balloon • Increase PEEP until end-expiratory flow is zero • PEEP applied estimates auto-PEEP

17. Monitoring Breathing Effort and Pattern • P0.1 assesses ventilator drive • Occlusion pressure 100 ms after initiation of inspiration • <6 cm H2O is indicative of patient’s ability to wean from MV. • RSBI (f/VT) • Respiratory muscle fatigue tends toward rapid shallow breathing. • RSBI < 100 indicates patient likely to wean from MV • The lower the RSBI the better

18. Monitoring Breathing Effort and Pattern (cont.) • Vital capacity (VC) • Effort dependent • VC less than 10 to 15 ml/kg, need for MV • Maximal inspiratory pressure (MIP) • Not effort dependent, as prolonged occlusion of airway stimulates maximal effort • More negative is better • 20 to 30 cm H2O acceptable

19. Monitoring During Lung Protective Ventilation • Commonly used for ALI/ARDS patients to avoid ventilator-induced lung injury (VILI) • Three principles confirmed • Limit Pplat to <30 cm H2O • Reduce VT to 6–8 ml/kg • Use adequate PEEP to avoid opening/closing injury • Permissive hypercapnia is often used as a lung protective strategy avoid VILI.