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Chapter 14Respiratory Monitoring in the

Intensive Care Unit

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Learning Objectives

After reading this chapter you will be able to: Identify the methods, normal values, and

significance of measuring the following lung volumes in the ICU: tidal volume, rapid-shallow breathing index, vital capacity, functional residual capacity

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Learning Objectives (cont’d)

Identify the methods, normal values, and significance of measuring the following airway pressures or related indices in the ICU: peak pressure, plateau pressure, compliance, airway resistance, mean airway pressure, maximum inspiratory pressure

List the definition, methods of detection, and methods of minimizing auto-PEEP

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Learning Objectives (cont’d)

Describe value of monitoring pressure, volume and flow waveforms, and pressure volume curves in mechanically ventilated patients

Describe methods and significance of measuring the fraction of inspired oxygen and exhaled carbon dioxide in the ICU

List the components of oxygen transport and their significance

List components of clinical evaluation of oxygenation and their significance

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Learning Objectives (cont’d)

Explain how these parameters can evaluate tissue oxygen delivery and use: Oxygen delivery and availability Oxygen consumption Mixed venous oxygen tension Venous saturation Arterial to mixed venous oxygen content

difference Oxygen extraction ratio Blood lactate

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Learning Objectives (cont’d)

Describe the value and limitations of pulse oximetry in monitoring oxygenation and oxygen delivery

Identify the techniques for monitoring tissue oxygenation and utilization

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Overview

Monitoring: repeated or continuous observations or measurements of the patient

Guide therapeutic interventions Assess interventions Alert clinicians to changes in patient’s

condition

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Ventilatory Assessment

PaCO2: standard for assessing ventilation Changes in metabolism, lung mechanics,

ventilatory efficiency, equipment function may precede changes in blood gas

Ventilatory parameters monitored Lung volumes and flows Airway pressures Fractional gas concentrations

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Lung Volumes and Flows

Why? They affect gas exchange They reflect changes in patient’s clinical status They indicate response to therapy They signal problems with patient/ventilator

interface Who?

Intubated and nonintubated patients

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Lung Volumes and Flows (cont’d)

What do we measure? VT (5-8 ml/kg IBW)

• VT <5 ml/kg may indicate respiratory problem• Pneumonia, COPD, CHF, ARDS, CNS depression• Large VT in metabolic acidosis, sepsis, neurological

injury VT = VA + VD

VD = 25% to 40% of the VT• VD >60% = need ventilatory support

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Lung Volumes and Flows (cont’d)

High VT ventilation with positive pressure: volutrauma

PEEP + smaller VT maintains FRC Discrepancies between set/measured VT

Compressible volume of the circuit Inspiratory and expiratory flow sensors Pneumothorax Leaks in the circuit or ETT

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Lung Volumes and Flows (cont’d)

Spontaneous breathing trial (SBT) failure: VT <300 ml or <4 ml/kg SpO2 <85% to 90% Blood pressure and heart rate change >20% Respiratory rate >35/min Change in mental status Accessory muscle use Diaphoresis

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Rapid shallow breathing index (RSBI) RSBI = f (breaths/min)/VT (liters) RSBI >105: prognostic of failure

VE = 5 to 6 L/min VE > 10 L/min: weaning not likely successful

Lung Volumes and Flows (cont’d)

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Vital capacity (VC) 65 to 75 ml/kg IBW FVC <20 ml/kg preoperative: risk of pulmonary

complications VC 10 to 15 ml/kg needed for deep breathing

and coughing VC >10 to 15 ml/kg for successful weaning and

extubation

Lung Volumes and Flows (cont’d)

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Functional residual capacity (FRC) FRC = 40 ml/kg IBW PEEP and CPAP increase FRC Beneficial in atelectasis and refractory

hypoxemia as occurs with ARDS

Lung Volumes and Flows (cont’d)

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Airway Pressures

Important to monitor airway pressures: Need for mechanical ventilation and readiness

for weaning Determine site and cause of impedance to

mechanical ventilation Evaluate elastic recoil, compliance of thorax Estimate amount of airway pressure

transmitted to heart and major vessels Assess respiratory muscle strength

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Airway Pressures (cont’d)

Peak pressure or PIP Pressure required to overcome opposition to

airflow in the lungs Increased resistance

• Bronchospasm, airway secretions, mucus plugging Decreased compliance (lung or chest wall) Patient-ventilator interface problem

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Airway Pressures (cont’d)

Plateau pressure Elastic recoil of lung and chest wall Static pressure during period of no gas flow Pressure required to maintain inflation

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Airway Pressures (cont’d)

Mean airway pressure (Paw) Affected by CPAP, PEEP, inspiratory time (TI),

VT, PIP, and rate Paw = = [½ (PIP – PEEP) × (inspiratory time/total

cycle time)] + PEEP Impacts oxygenation Caution when Paw >20 cm H2O

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_

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Airway Pressures (cont’d)

Maximum inspiratory pressure (PImax) Influenced by:

• Respiratory muscle strength• Patient effort/ventilatory drive• Lung volume• Phrenic nerve function• Nutritional status• Oxygenation/acid-base status

Normal PImax: –80 to –100 cm H2O PImax > –30 cm H2O may be useful to predict

successful weaning

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Airway Pressures (cont’d)

Auto-PEEP Total PEEP – set PEEP Reduction of auto-PEEP

• Bronchodilator therapy• Decreased TI (allows more time for exhalation)• Reduction of mechanical frequency• Reduction of VE is the most effective

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Airway Pressures (cont’d)

Compliance Dynamic compliance = {Corr VT – [(PIP – EEP)

× CF]}/PIP – EEP Static compliance = Corr VT - [(Pplat - EEP) ×

CF]/Pplat – EEP Normal static compliance in patients receiving

mechanical ventilation: 40 to 80 ml/cm H2O Compliance <20 – 25 ml/cm H2O associated

with failure to wean

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Airway Pressures (cont’d)

Airway resistance (Raw) (Ra-w = PIP (cm H2O) – plateau pressure (cm

H2O)/flow (L/sec) Normal = 1 to 3 cm H2O/L/sec

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Integrating Pressure, Flow, and Volume

Evaluating patient/ventilator interface At the patient: use of accessory muscles, color,

diaphoresis, heart rate, respiratory rate At the airway: type, size, integrity, stability At the ventilator circuit: leaks, temperature,

condensate At the ventilator settings and monitoring panel

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Integrating Pressure, Flow, and Volume (cont’d)

Monitoring pressure, flow, and volume Graphic display screen Scalar: a single parameter over time

• Pressure-time waveform• Volume-time waveform• Flow-time waveform

Loop: two parameters in a continuous tracing• Pressure-volume loop• Flow-volume loop

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Integrating Pressure, Flow, and Volume (cont’d)

Titrating PEEP and tidal volume with P/V Static pressure-volume curve Super syringe technique Time consuming and cumbersome Useful in acute lung injury Lower inflection point + 2 cm H2O: minimal

PEEP Upper inflection point: overdistention

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Fractional Gas Concentrations

FIO2

Exhaled CO2 Capnometry and capnography Affected by temperature changes, shivering,

seizures, trauma, high carbohydrate infusion Measure efficiency of ventilation PETCO2: accurate estimate of PaCO2

CPR effectiveness

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Dead Space-Tidal Volume Ratio

VD/VT Anatomic dead space: 1 ml/kg IBW Alveolar dead space VD/VT = PaCO2 – PE- CO2 /PaCO2

Normal: 25% to 40%

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Evaluation of Oxygenation

Evaluation of oxygen transport Oxygen consumption Oxygen delivery (DO2) Oxygen reserves Oxygen content

• CaO2 = (Hb × 1.34 × % saturation) + (PaO2 × 0.003) Cardiac output Oxyhemoglobin dissociation curve

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Evaluation of Oxygenation (cont’d)

Monitoring adequacy of arterial oxygenation Partial pressure of arterial oxygen

• Should be kept 60 to 80 mm Hg Alveolar-arterial oxygen tension difference

• P(A-a)O2

PaO2/FIO2 ratio• ALI: 200 to 300• ARDS: <200

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Monitoring adequacy of arterial oxygenation Oxygen index = Paw × FIO 2 × 100/PaO2

OI >40: mortality >80%

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Evaluation of Oxygenation (cont’d)

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Intrapulmonary shunt (QS/QT) Increased in atelectasis, pneumonia, ARDS,

pulmonary edema Pulse oximetry will reveal low SpO2 with

elevated FIO2

Co-oximeter useful to determine CaO2

. .

Evaluation of Oxygenation (cont’d)

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Monitoring Tissue Oxygenation

Oxygen delivery (DO2) Cardiac output × CaO2 × 10* Normal: 550 to 650 ml/min/m2

Oxygen consumption (VO2) Fick principle Normal: 100 to 140 ml/min/m2 *Change vol% to ml/L

.

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Monitoring Tissue Oxygenation (cont’d)

Mixed venous oxygen tension PvO2 Normal: 38 to 42 mm Hg Low: inadequate cardiac output, anemia,

hypoxia High: poor sampling technique, left-to-right

shunt, septic shock, increased cardiac output, cyanide poisoning

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Monitoring Tissue Oxygenation (cont’d)

Mixed venous oxygen saturation (SvO2) Fiberoptic reflectance oximetry Decreased: suctioning, shivering, extubation,

weaning, PPV

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Monitoring Tissue Oxygenation (cont’d)

Arterial-mixed venous oxygen content difference, C(a – v)O2 reflects: Normal: 4 to 6 vol% Increased: low cardiac output, increasing VO2 Decreased: septic shock, increased cardiac

output, anemia, left shift ODC

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.

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Monitoring Tissue Oxygenation (cont’d)

Oxygen extraction ratio [C(a – v)O2/CaO2] Normal: 25% to 30%

Increased: low cardiac output, increased VO2, decreased CaO2,

Decreased: high cardiac output, sepsis

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Monitoring Tissue Oxygenation (cont’d)

Blood lactate Anaerobic metabolism = lactic acid production Normal: <1.7 to 2.0 mM/L >3.83 mM/L = 67% mortality >8 mM/L = 90% mortality

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Summary

Common ventilatory measurements Lung volumes and flows Airway pressures Fractional gas concentrations

These measurements allow clinicians to determine the need for mechanical ventilation, monitor the patient, and determine the readiness for weaning