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38 year old female recently admitted to ICU with sepsis.

Arterial blood gases provide a means to evaluate several critical aspects of a patient's condition. Specifically, arterial blood gases have long been considered the gold standard for initial evaluation of a patient's acid-base, ventilation and oxygenation status.

In addition, current blood gas machines also measure electrolytes, hemoglobin concentration and abnormal hemoglobin species, lactate, glucose, Blood Urea Nitrogen (BUN) or creatinine. Thus, Arterial Blood Gases in conjunction with these additional analytes provide a comprehensive view of the patient at a single point in time. Thus, the clinician can make a comprehensive assessment of the patient and often a differential diagnosis of acid-base and oxygenation disturbances.

Effective blood gas interpretation and application requires the clinician to have an organized and systematic approach. The method I suggest can be referred to as the ABC's of Blood Gas Application. These so-called ABC's are listed below. The clinician should think through each of these areas discretely and completely.

Acid-Base Assessment also includes Ventilation assessment as the PaC02 is the primary indicator of "respiratory" acid-base disturbances and ventilation. Blood Oxygenation looks at the effectiveness of the lungs in getting inspired oxygen into the pulmonary capillary blood. The simplest index of Blood Oxygenation is the P/F (PaO2/FIO2) ratio. Finally, Cellular oxygenation assessment looks beyond the PaO2 or SpO2 (Oxygen saturation as measured by pulse oximetry) in order to insure the tissues and cells are receiving adequate oxygenation.

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Case Application

Our patient in this case is a 39 year old female with suspected severe and septic shock. The patient is transferred to the ICU where arterial blood gases are drawn. A CVP is placed and is reading 6 mm Hg. The blood gas indicates a "Mixed Respiratory and Metabolic Acidosis with Mild Hypoxemia".
Arterial Blood Gases

FIO2           0.50
sO2            85%
pH             7.14
pCO2           55 mm Hg
pO2            75 mm Hg
cHCO3          18 mEq/L

Vital Signs

B/P            65/35 mm Hg
RR             33/min
HR             118/min
Temp           40C

Electrolytes

Na             143 mEq/L
CI             105 mEq/L
HCO3           14 mEq/L
K              5.3 mEq/L


Acid-base Balance

Patients with single acid-base disturbances are said to have "simple acid-base problems". Patients with two or more acid-base disturbances are said to have "mixed acid-base disturbances". This individual presents to the ICU with a mixed respiratory and metabolic acidosis. When both respiratory and metabolic conditions are moving pH in the same direction, sometimes the term "combined acidosis" is used.

The respiratory acidosis is likely due to a combination of excessive ventilation perfusion mismatch (i.e. pulmonary shunting) secondary to pulmonary fluid and exhaustion due to increased work of breathing. When either or both of these conditions are excessive, hypercapnia may ensue. Patients with severe sepsis frequently develop fluid accumulation in the lungs and severe pulmonary shunting.

The metabolic acidosis on the other hand is most likely secondary to tissue hypoxia and subsequent anaerobic metabolism and lactic acidosis. Metabolic adaptations to inflammatory mediators are also likely contributing to lactate formation. Lactate appears to be a strong indicator of morbidity and mortality in sepsis patients especially during early intensive care stay.

The anion gap ( Na--(HCO3 + CI) is normally less than 16 mEq/L . An elevated anion gap is highly suggestive of an accumulation of fixed acids. In this case, the anion gap 24 mEq/L and suggests the accumulation of fixed acid consistent with lactic acidosis. The significantly elevated lactate level confirms this cause of primary metabolic acidosis. Normal lactate should be less than (1.8-2.0) mMol/L.

Blood Oxygenation

Remember blood oxygenation looks at the efficiency of oxygen uptake in the lungs via the Oxygenation pO2/FIO2 (P/F) ratio. The very low P/F ratio of approximately .125 indicates severe pulmonary shunting. A P/F ratio in the absence of cardiogenic shock and a white out on the CXR is indicative non-cardiogenic edema and ARDS. The shunting appears to be true or absolute since it is poorly responsive to oxygen therapy. The diffuse white out on the CXR supports the likelihood of pulmonary edema. The white out could indicate the development of acute lung injury or acute respiratory distress syndrome, the decreased CVP along with the hypotension fever and other signs and absence of CHF suggests sepsis with related ARDS. Remember true alveolar shunting as seen in ARDS is poorly responsive to simple oxygen therapy.

Cellular Oxygenation

There appears to be little doubt that the patient's metabolic acidosis is a result of lactic acidosis secondary to sepsis and ARDS, Decreased mixed venous (or CVP) and presence of sepsis and ARDS. Although low venous oxygenation values usually support the conclusion of cardiovascular hypoxia; mixed venous oxygenation values may indicate arteriovenous shunting in these patients.

Sepsis can be defined as infection and related manifestations. Severe sepsis also includes sepsis induced organ dysfunction and/or tissue hypoxia. Septic shock is defined as hypotension unresponsive to fluid resuscitation. The "Surviving Sepsis Campaign 2008 evidence based protocol suggests maintaining CVP > 8 mm Hg and maintaining mixed venous saturation at 65-70%. Several of the key elements to patient management include: 1) During mechanical ventilation, target tidal volume at 6mL/Kg per ideal body weight or less, 2) maximize plateau pressure at 30 cm H20 or less to prevent volume induced lung injury, 3) allow for permissive hypercapnia to achieve the pressure and volume targets, 5) set PEEP to avoid alveolar collapse since alveolar de-recruitment leads to high alveolar opening pressures and lung damage, 6) minimize the use of pulmonary artery catheters due to no evidence based benefit, 6) use bicarbonate therapy if pH is < 7.15. The evidence for bicarbonate benefit in acidosis is scant.

Finally, there has been a close association with lactate measurements and Daily Sequential Organ Failure Assessment (SOFA). Especially in the early phases of resuscitation, serial lactate measurements may guide oxygen delivery and reduce mortality in early intensive care patients. Again, one should attempt to maintain lactate below 4mM/L and provide fluid resuscitation to keep SvO2 and mean arterial pressure.

Furthermore, maintaining lactate below 4 mom/L in occult emergency department hypotensive patients decreased mortality by approximately 50%. Indeed, if lactate can be maintained below 4 mM/L there is virtually 0% mortality versus nearly 50% if it cannot be controlled within 24 hours. Dr. Blow has added to the "golden hour of emergency management the concept of the silver hour based on serial lactate measurement. We should aggressively try to decrease lactate to levels below 2.5 mM/L. The longer it takes lactate to clear the the higher the incidence of multi- system organ failure and mortality. Forty three per cent of patients who could not clear lactate within twenty four hours leads to increased mortality. In addition to standard blood gases, lactate levels have become critical information for respiratory therapists to follow in trauma, sepsis, and ARDS patients. Serial lactate measurement is a critical component in the evidence based Surviving Sepsis Campaign.

William Malley MS, RRT, CPFT, is Administrative Director of Respiratory/Pulmonary Services at The Western Pennsylvania Hospital in Pittsburgh, PA. He can he contacted at bmailcy@icubed.com.

by William Malley, MS, RRT, CPFT, FAARC
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Title Annotation:BLOOD GAS CASE STUDIES; intensive care unit
Author:Malley, William
Publication:FOCUS: Journal for Respiratory Care & Sleep Medicine
Article Type:Report
Geographic Code:4EUUK
Date:Nov 1, 2010
Words:1189
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