Assessment of the Cardiopulmonary Reserve

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Assessment of the Cardiopulmonary Reserve
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Assessment of the patients cardiopulmonary
reserve is the most important skill related to
mechanical ventilation.
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Cardiopulmonary Reserve

• Pulmonary Reserve
• Respiration
• Cardiovascular Reserve

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Pulmonary Reserve

• Clinical Observation
• Ventilatory Pattern
• Vital Capacity
• FEV 1
• Shunt
• Deadspace

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Respiration (ABGs)

• Alveolar Ventilation
• Acid-Base
• Oxygenation

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1st step in ABG interpretation is to classify the
PaCO2
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PaCO2 determines adequacy of alveolar ventilation

• lt 30 torr alveolar hyperventilation
• 30 - 50 torr acceptable alveolar ventilation
• gt 50 torr alveolar hypoventilation (ventilatory
  failure)

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Next, assess pH in relation to PaCO2
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Alveolar hyperventilation with

• pH gt 7.50 acute
• pH 7.40 - 7.50 chronic
• pH 7.30 - 7.40 compensated metabolic acidosis
• pH lt 7.30 partly compensated metabolic acidosis

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Alveolar hypoventilation with

• pH lt 7.30 acute ventilatory failure
• pH 7.30 - 7.40 chronic ventilatory failure
• pH gt 7.50 partly compensated metabolic alkalosis

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Acceptable alveolar ventilation and

• pH gt 7.50 uncompensated metabolic alkalosis
• pH lt 7.30 uncompensated metabolic acidosis

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Patient 1

• 61 yo woman who is comatose is brought into the
  ER with the following vital signs
• Respiratory rate 8 breaths/min
• Vt 320 mL
• Body temperature 33o C
• 5 ft tall and weighs 135 lb.
• ABGs pH 7.42 PaCO2 38 torr HCO3 23
  mEq/L

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Bradypnea but not hypoventilation

• Vt and RR are decreased, resulting in a decreased
  VA, which would normally produce an increased
  PaCO2
• However, the decreased metabolic rate from the
  decreased body temperature and activity reduce
  the VCO2 allowing for a normal PaCO2 at a lower VA

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Patient 2

• 28 yo male, 54 tall and 155 lb with suspected
  multiple pulmonary emboli and the following vital
  signs is seen in the ER
• Respiratory rate 28 BPM
• Vt 550 mL
• Blood pressure and pulse are normal
• ABG pH 7.38 PaCO2 39 torr HCO3 25
  mEq/L

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Tachypnea but not hyperventilation

• A respiratory rate of 28 BPM in a patient this
  size would normally reduce PaCO2 to 20 torr.
• However, the increased deadspace requires higher
  minute ventilation just to keep PaCO2 normal

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Patient 3

• 65 yo woman on the surgical ward 12 hrs after
  exploratory abdominal surgery. Shes alert with a
  pulse of 94 bpm, other vital signs normal.
  Nasogastric suction is in place and functioning
• ABG pH 7.46 PaCO2 46 torr HCO3 32
  mEq/L

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Metabolic alkalosis

• Gastric suctioning caused loss of stomach acids
  with resultant increase in HCO3
• Lungs may have attempted to compensate by
  hypoventilation, but the resulting hypoxemia has
  kept the PaCO2from rising much above normal.

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Patient 4

• A 14 yo girl is brought to the ER with dx of
  suspected drug overdose
• Vt 175 mL and respiratory rate 8 BPM
• ABG pH 7.23 PaCO2 68 torr HCO3 28
  mEq/L

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Classify the blood gas

• Acute respiratory acidosis

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Patient 5

• 78 yo man with no history cardiopulmonary disease
  falls down a flight of stairs. He is seen in the
  ER and CXR reveals 2 broken ribs.
• ABGs pH 7.27 PaCO2 56 torr HCO3 25
  mEq/L

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Classify the blood gas

• Acute respiratory acidosis. The patient is
  treated and sent home with pain medication.
• Three days later, patient is seen in the ER with
  SOB and pain on inspiration
• ABG pH 7.36 PaCO2 56 torr
  HCO3 31 mEq/L
• Compensated respiratory acidosis

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2nd Step in ABG interpretation - assess hypoxemic
state

• Give oxygen - does PaO2 increase?
• a/A Ratio .25 - .50 moderate hypoxemia
• a/A Ratio lt .25 severe hypoxemia

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3rd Step - Assess Tissue Oxygenation

• Assess cardiac status
• Assess peripheral perfusion status
• Blood oxygen content mechanism

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Assess cardiac output and peripheral perfusion by

• Blood pressure
• Heart rate
• ECG
• Coolness of extremities
• Sensorium
• Electrolyte balance
• Urine Output

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If cardiac output and perfusion are adequate,
only blood oxygen content can be interfering with
tissue oxygenation.
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Blood oxygen content(CaO2 )
SaO2 (Hgb x 1.34) PaO2 x .003

• PaO2
• Hemoglobin
• Oxygen saturation
• Hemoglobin-oxygen
• affinity

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Oxygen content is decreased with

• Decreased Saturation
• Hypercarbia, acidemia, hyperthermia -
  oxyhemoglobin curve shift to the right.
• Hypoxemia and acidemia - assume tissue hypoxia
• Anemia, carboxyhemoglobin, methemoglobin

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Hemoglobin-oxygen affinity

• Alkalemia and hypothermia increase Hb-O2 affinity
  - shift to the left - the gt affinity the lt
  oxygen released to the tissues

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Evaluation of tissue oxygenation is not a simple
task - it is a clinical evaluation enhanced with
proper interpretation of blood gas measurements.
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Last Patient

• 52 yo male admitted to the ICU following sudden
  onset of SOB and severe chest pain. After 30 min
  he coded. Was successfully resuscitated after 10
  min.
• After resuscitation he was hypotensive, RR 40 BPM
  and HR 120/min. Comatose, cyanotic, cool
  extremities, crackles, and a weak pulse.

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Initial blood gas measurements after
resuscitation were

• pH 7.16 PaCO2 40 torr PaO2 60
  torr on 100
• SaO2 77 CaO2 11 vol
• BE(D) (-14) HCO3 14 mEq/L
• Classify the blood gas

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Metabolic acidosis

• pH well below normal
• HCO3 is reduced
• PaCO2 is normal

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How do you expect the PaCO2 to compensate in
acute metabolic acidosis?

• Winters formula
• Expected PaCO2 (1.5 x HCO3) 8 /-2
• Expected PaCO2 (1.5 x 14) 8 29 torr

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What else is occurring since the actual PaCO2 is
40 torr?

• Increased deadspace from a lack of pulmonary
  perfusion

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How do you evaluate oxygenation?

• PaO2 of 60 on 100 indicates significant shunt.
• The SaO2 (77) and CaO2 (11 vol) are
  significantly below normal.
• Since CaO2 is reduced proportionately more than
  the SaO2, the patient must be anemic.
• How do you know CaO2 is more reduced that SaO2?

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• .77 ( 1.34 x 15 ) 15.4 not 11 vol
• OR another way - what is Hgb?
• .77 (1.34 X) 11 vol
• X Hgb 10.6 g
• anemia

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How do you evaluate tissue oxygenation?

• There are clinical signs of tissue hypoxia.
• The lack of adequate oxygenation and circulation
  is resulting in anaerobic metabolism and lactic
  acidosis.

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• Whenever hypoxemia and the clinical signs of
  inadequate perfusion occur together, metabolic
  acidosis from the production of lactic acid is a
  definite possibility.

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What are the signs of inadequate perfusion?

• 52 yo male admitted to the ICU following sudden
  onset of SOB and severe chest pain. After 30 min
  he coded. Was successfully resuscitated
• After resuscitation he was hypotensive, RR 40 BPM
  and HR 120/min. Comatose, cyanotic, cool
  extremities, crackles, and a weak pulse.

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Getting back to the initial blood gas
measurements after resuscitation

• pH 7.16 PaCO2 40 torr PaO2 60
  torr on 100
• SaO2 77 CaO2 11 vol
• BE(D) (-14) HCO3 14 mEq/L
• Classify the blood gas

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What is meant by base excess/deficit?

• Normally /- 2 mEq/L
• It is the standard deviation of the standard HCO3
• Reflects the non-respiratory portion of acid/base
  imbalances.
• Base excess or deficit is not simply the
  difference between actual and normal HCO3
• It includes the buffers used to normalize pH so
  BE/D will always be greater than the normal -
  actual HCO3 difference.

Davenport, H. The ABC of acid-base chemistry.
1971 p 58-60.
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• Calculation of BE is made from pH, PaCO2, and
  Hct.
• Hematocrit considered because RBC contain
  significant blood buffers.
• Can use the base deficit in treating metabolic
  acidosis. Generally want to treat the source
  rather than give bicarbonate

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Tx Metabolic acidosis

• Give half the base or HCO3 deficit over 5 - 10
  min
• Give the other half if needed over the next 12 hr.

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Calculating HCO3 Deficit

• HCO3 Deficit Total body water x (desired -
  actual HCO3)
• Total body water (L) 60 of the total body wt.
  (kg)
• Ex. 70 kg x .6 x (24 - 14) 420 mEq/L of
  bicarbonate. HCO3 Deficit is in mEq/L of HCO3
  Give half (210)
• Another formula
• Deficient mEq/L HCO3 BD x wt(kg)
• 
  4
• Ex. 14 x 70 kg 245 mEq/L No its not
  the same
• 4

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What is standard HCO3?

• What the HCO3 would be if the PaCO2 were 40 torr.

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Different from the T40 HCO3

• T40 HCO3 predicts the influence of hypercapnia on
  plasma HCO3
• Assumes that the plasma HCO3 increases 1 mEq for
  every 10 - 15 torr increase in PaCO2.

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What is the anion gap?

• AG (NA K) - (CL HCO3)
• Normally 8 - 16 mEq/L
• Ex. (135 4.0) (105 24) 10

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Use AG to

• distinguish between metabolic acidosis caused by
  loss of HCO3- from other types of metabolic
  acidosis caused by the addition of H.
• Loss of HCO3 - causes hyperchloremic metabolic
  acidosis

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Corrected AG P(veno-art )CO2 Index
1. Correct for BUN
uncor AG BUN Factor corr AG
10 15 0 10
11 22 -1 10
12 29 -2 10
13 36 -3 10
14 43 -4 10
15 50 -5 10
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2. Correct for Albumin
uncorr AG Albumin Factor Corr AG
10 4 0 10
10 3 2.5 12.5
10 2 5 15
10 1 7.5 17
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Find PCO2 (veno-arterial)

• Venous PCO2 must be mixed venous or from internal
  jugular
• If Index 15 100 survival
• 24 50
• 32 0

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With Hyperchloremia metabolic acidosis - AG is
normal. HCO3 lost due to

• Diarrhea
• Pancreatic fistula
• Drugs which cause renal wasting of HCO3

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Other types Metabolic Acidosis - AG is Elevated

• Lactic Acidosis
• Diabetic Ketoacidosis
• Uremia
• Toxic Ingestion
• Salicylate Intoxication (aspirin)

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There is no AG in metabolic alkalosis

• Urine Chloride levels more useful
• If lt 10 mEq/L - Give NaCl
• If gt 20 mEq/L - treatment varies with causative
  agent

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Mixed Acid-Base Disorders

• May have elevated AG without acidosis in
  dehydration or mixed acid/base disorders.

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Examples of Mixed Acid/Base Disorders

• Primary metabolic alkalosis in pts with COPD and
  chronic respiratory acidosis who are treated with
  diuretics - may have K depletion
• May have respiratory alkalosis and metabolic
  acidosis from salicylate intoxication.

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Summary

• Assess alveolar ventilation via PaCO2 and relate
  to the pH
• Degree of compensation
• Oxygenation status