Factors important in cardiopulmonary interactions include changes in intrathoracic pres- sure and lung volume during respiration, intravascular volume status, and baseline func- tion of the heart and lungs. In many situations the baseline condition of the patient, including the intravascular volume status and myocardial function, will determine the ulti- mate clinical effect, if any, the cardiac and respiratory systems may have on each other.
Anticipation of heart-lung interactions during support of the critically ill child will pro- mote early recognition of alterations in cardiopulmonary function and prevent deleterious effects of therapy (Fig. 3-17 ).
a
Alveoli
SVO2 = 70
SPVO2 = 100 SPVO2 = 70
SVO2 = 70
SLAO2 = 85%
Alveoli
SVO2 = 50
SPVO2 = 100 SPVO2 = 50
SVO2 = 50
SLAO2 = 75%
b FIGURE 3-16
( a ) The effect of a 50% pulmo- nary shunt on left atrial oxyhe- moglobin saturation ( S LA O 2 ) when cardiac output is normal. ( b ) With the same shunt, a decrease in venous saturation ( S V O 2 ) associ- ated with poor cardiac output will result in lower S LA O 2 . Increasing cardiac output ( a ) will result in improved arterial hemoglobin saturations in patients with pulmonary shunts. S PV O 2 pulmo- nary venous oxyhemoglobin saturation
Chest Wall
Pulmonary Vasculature
Lung
Lung Lung
RV LV
Systemic Vasculature 4
2 1
4 3 3
1 2
IVS Chest Wall
Pulmonary Vasculature
Lung
Lung Lung
RV LV
Systemic Vasculature
2 1
1
2
1 2 3
4
1
IVS
Negative Effects ¯LV compliance ( venous return and RV volumes causing a shift of the IVS into the LV)
LV aftrerload
Negative Effects
¯ Systemic vascular return.
PVR and RV afterload.
¯ LV dolumes causing (IVS shift into the LV from RV volumes; compression of) cardiac fossa).
¯ Contractility
(¯ coronary artery blood flow and/or myocardial depres- sant factors).
1
2
Positive effects
Pulmonary venous return (hypervolemic zone 3 conditions).
¯ LV afterload
1
Positive effect Systemic venous return
a
b
FIGURE 3-17
( a ) Summary of the primary effects of spontaneous respira- tion on cardiac function. ( b ) Summary of the primary effects of positive pressure ventilation on cardiac function. RV right ventricle, LV left ventricle, IVS intravascular septum
1. While evaluating an 11 month old, 10 kg infant with tachy- cardia (180 beats per minute) and poor perfusion; the critical care physician notes that the heart rate decreases momentarily to 170 beats per minute and the pulses became stronger after compressing the liver of the infant. Which of the following physiological changes most likely explains the response?
A. The compression on the liver produced a sudden increase in the systemic vascular resistance and the increased afterload resulted in the stronger pulses and decreased heart rate.
B. The compression on the liver produced a sudden vagal response clinically manifested by the decreased heart rate.
C. The compression on the liver produced a temporary increase in contractility with a resultant increase in cardiac output (stroke volume) and a slowing of the heart rate.
D. The compression on the liver reduced venous return to the heart thereby decreasing preload resulting in clinical deterioration manifested by the decreased heart rate.
E. The compression on the liver transiently increased preload with a resultant increase in cardiac output (stroke volume) and a slowing of the heart rate.
2. A 2 month old infant presents with renal failure secondary to an obstructive uropathy. The infant is tachypneic and noted to have a metabolic acidosis with a blood pH of 7.20. Sodium bicarbonate (1 mEq/kg) is administered intravenously in an attempt to im- prove the blood pH. Shortly thereafter, the perfusion of the infant is clinically noted to decline. Which of the following is the best potential explanation for the change in hemodynamics?
A. Despite bicarbonate therapy, the blood pH remained suffi ciently low to negatively affect the myocardium.
B. The increase in the blood pH further reduced the low serum ion- ized calcium levels, negatively effecting myocardial contractility.
C. The increase in the blood pH led to direct systemic vasodilata- tion resulting in hemodynamic compromise.
D. The rapid rise in sodium concentration resulted in a negative inotropic effect on the heart.
E. The respiratory rate increased further after the bicarbonate ther- apy resulting in further energy expenditure and cardiovascular compromise.
3. A previously healthy 10 year old child develops ARDS after sustaining abdominal and lower extremity trauma in a motor vehicle collision. She received crystalloid fl uid resuscitation and multiple blood product transfusions to achieve hemody- namic stability. She requires a positive end expiratory pressure (PEEP) of 15 cm H 2 O to maintain acceptable arterial oxygen- ation, but develops poor cardiac output with this ventilator strategy. Which of the following is the primary cause of her decreasing cardiac output?
A. A decrease in left ventricular contractility due to myocardial depressant factors.
B. A decrease in left ventricular fi lling due to intraventricular sep- tal shift into the left ventricle.
C. A decrease in systemic venous return secondary to increased mean airway pressure.
D. An increase in left ventricular afterload due to increased trans- mural wall pressure.
E. An increase in right ventricular afterload secondary to lung overdistension.
4. Which of the following is a contributing factor in the clinical presentation of pulsus paradoxus?
A. A decrease in left ventricular afterload B. A decrease in pulmonary vascular resistance C. A decrease in systemic venous return D. An increase in right ventricular volume E. A rightward shift of the intraventricular septum
5. A 3 year old male with a known cardiomyopathy and decreased left ventricular function is admitted to the pediatric intensive care unit with a presumed viral laryngotracheobronchitis. In addition to reducing the work of breathing, tracheal intubation and the use of positive pressure ventilation will benefi t cardiac function in this child in which of the following ways?
A. A decrease in left ventricular afterload B. A decrease in right ventricular afterload C. A decrease in systemic venous return D. An increase in cardiac contractility E. An increase in systemic venous return
6. A 12 year old male with a severe asthmatic exacerbation has worsening respiratory distress that is now accompanied by poor perfusion, worsening tachycardia (190 beats per minute), and hypotension (86/45 mm Hg). The physiologic explanations for the hypotension include relative hypovolemia secondary to increased insensible water losses and decreased intake, hypox- emia-induced myocardial dysfunction and:
A. decreased right ventricular afterload.
B. decreased systemic vascular resistance.
C. increased left ventricular afterload.
D. increased partial pressure of carbon dioxide.
E. untoward effect of steroid therapy.
7. A 12 year old male with a severe asthmatic exacerbation has now developed compromised perfusion with signifi cant tachy- cardia (185 beats per minute), and hypotension (82/42 mm Hg).
An appropriate initial hemodynamic intervention in this child would be to:
A. administer a crystalloid fl uid bolus (10 mL/kg) to augment preload.
B. initiate a low dose epinephrine infusion (0.05 mcg/kg/min) to augment contractility.
C. initiate an infusion of milrinone (0.5 mcg/kg/min) to augment contractility and foster ventricular relaxation.
D. initiate an infusion of sodium nitroprusside (1 mcg/kg/min) to decrease systemic afterload.
E. initiate inhaled nitric oxide (20 ppm) to decrease pulmonary vascular resistance.
REVIEW QUESTIONS
8. Which of the following statements regarding cardiovascular- pulmonary interactions is false?
A. An increase in systemic venous return or in right ventricular afterload during respiration will displace the intraventricular septum into the left ventricle and decrease left ventricular com- pliance and preload.
B. Extremes in lung volumes (both low and high) can result in elevations in pulmonary vascular resistance.
C. Adequate intravascular volume is important for both RV and LV output when initiating positive pressure ventilation.
D. Negative intrathoracic pressure generated during spontaneous breathing increases the pressure gradient from the systemic veins to the right atrium.
E. Negative intrathoracic pressure generated during spontaneous breathing has no effect on extra thoracic large veins.
1. E 2. B 3. C 4. D
5. A 6. C 7. A 8. E
ANSWERS
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65
Regional Circulations
LEARNING OBJECTIVES
Describe the relative proportions of blood fl ow and
■
oxygen consumption at the major tissue beds
Describe the mechanisms for changing regional blood
■
fl ow with stress and pathologic conditions
Describe the unique characteristics of the coronary
■
vasculature
Describe the importance of local regulation of blood
■
fl ow and mechanisms for achieving this control Review that the oxygen extraction ratio in the
■
myocardium is high at rest
Describe the unique characteristics of the pulmonary
■
vasculature
Describe hypoxic pulmonary vasoconstriction
■
Review causes of increased and decreased pulmonary
■
vascular tone
Review the importance of pulmonary vascular tone in
■
specifi c conditions
Describe the unique characteristics of the cerebral
■
vasculature
Discuss cerebral autoregulation and the effects of
■
carbon dioxide and oxygen on cerebral blood fl ow Review the causes of increased and decreased cerebral
■
vascular tone
Review the importance of control of cerebral vascular
■
tone in specifi c conditions
Describe the unique characteristics of the splanchnic and
■
renal vasculature
Understand the mechanisms and effects of control of
■
vascular tone
CHAPTER OUTLINE
Learning Objectives
Blood Flow and Oxygen Consumption at the Major Tissue Beds
Mechanisms of Regional Blood Flow Regulation During Stress and Pathologic Conditions
Coronary Circulation
Anatomy, Histology and Physiology Local Regulation of Coronary Blood Flow Specifi c Determinants of Coronary Blood Flow Adrenergic Control of Coronary Blood Flow Coronary Blood Flow During CPR
Effects of Acidosis, Hypocapnia, and Hypercapnia on Coronary Blood Flow
Cerebral Circulation Anatomy, Histology
Cerebral Circulation Autoregulation
Hypoxia and Carbon Dioxide Related Cerebral Autoregulation Flow Mediated Regulation
Pulmonary Circulation
Anatomy, Histology and Physiology Hypoxic Pulmonary Vasoconstriction
Pulmonary Vascular Tone and Clinical Implications Renal Circulation
Major Arteries
Renal Blood Flow and Autoregulation Medullary Blood Flow
Cortical Blood Flow Vasoactive Mediators Cyclooxygenase Inhibition Adenosine and Renal Circulation Splachnic Circulation
Vascular Anatomy and Distribution Baseline Vascular Tone Regulation Postprandial Blood Flow Regulation Pathologic States
Cutaneous Circulation
Neural Control of the Skin Blood Flow Review Questions
Answers
Suggested Readings