Authors: Jed Kinnick, MD and Destiny F. Chau MD – Arkansas Children’s Hospital/ University of Arkansas for Medical Sciences, Little Rock, AR.
A 5-year-old boy who recently immigrated to the country presents for dental abscess drainage and dental rehabilitation under general anesthesia. The caregivers report that the child will often squat during play and that his color improves after squatting. Physical examination reveals clubbing of fingers and toes and an SpO2 of 88% while breathing room air. A transthoracic echocardiogram confirms a diagnosis of Tetralogy of Fallot. What is the MOST LIKELY mechanism that causes an acute clinical improvement with a squatting posture in patients with Tetralogy of Fallot?
Correct!
Wrong!
Question of the Week 332
Tetralogy of Fallot (TOF) is the most common cyanotic congenital heart disease representing 10% of all congenital heart defects. It has been noted that a proportion of patients with TOF naturally resort to postural changes for symptomatic relief of acute desaturations from baseline. In ambulatory children and infants, postural changes include squatting and assuming a knee-to-chest position. TOF is characterized by a tetrad of heart abnormalities including the following: 1) ventricular septal defect (VSD), 2) right ventricular outflow tract obstruction (RVOT), 3) an overriding aorta, and 4) right ventricular hypertrophy.
Due to this constellation of defects, systemic deoxygenated blood can preferentially cross the VSD and into the systemic circulation rather than the pulmonary circulation. Depending on the severity of the RVOTO, the patient’s oxygen saturation may range from normal to extreme cyanosis. Acute hypercyanotic episodes or “tet spells” are characterized by severe oxygen desaturation, dyspnea, and even syncope. Common precipitating factors - such as physical exertion, crying, agitation or pain - result in increased sympathetic activity and spasm of the infundibular portion of the right ventricular outflow tract thereby leading to increased right to left intracardiac shunting. The infundibular spasm is caused by increased sympathetic tone and increased cardiac contractility. A decrease in ventricular afterload can also increase right to left intracardiac shunting, which may occur during a febrile illness, due to inhaled anesthetic agents, and with dehydration. Persistent hypoxemia may eventually lead to the development of metabolic acidosis, peripheral vasodilation, and further decreases in systemic vascular resistance. This may worsen the hypercyanotic episode and potentially result in syncope and cardiac arrest.
The modern therapeutic approach to TOF is surgical repair during infancy to avoid the pathophysiologic and anatomic changes related to worsening cyanosis and increasing RVOTO. Children with TOF in low- to middle-income countries may have limited access to health care and remain unrepaired for many years before gaining access to surgical care.
Textbooks commonly cite that postural changes of knee-to-chest positioning or squatting lead to increased venous return and increased systemic vascular resistance (SVR), which in turn increases pulmonary blood flow and oxygen saturation. Several reports have studied the physiologic and hemodynamic changes resulting from squatting. Many investigative studies that utilized unrepaired, older TOF patients are decades old. More contemporary reports of squatting have been conducted in healthy volunteers or acyanotic patients. The following is a general overview of relevant studies.
Lurie, in a 1953 study, performed several tilt tests on subjects with unrepaired TOF and demonstrated that oxygen saturation dropped with increasing tilt from the supine towards the standing position. Elastic bandages were placed to compress the abdomen and the lower extremities in two subjects; the decrease in oxygen saturation was lessened when going from supine to standing as compared to those in patients without compression bandages. The authors concluded that the improvement in oxygen saturation was related to the increased venous return to the heart secondary to the postural changes.
A 1957 study by Brotmacher et al. recruited nine subjects with unrepaired TOF who were routine squatters (experimental group), and ten subjects who had either acyanotic heart disease or non-cardiac conditions (control group). After oxygen desaturation from baseline was induced by exertion; subjects with TOF either squatted or remained standing. Those who squatted demonstrated an immediate rise in oxygen saturations with a faster return to baseline compared to the standing group. Those patients who remained standing after exertion demonstrated oxygen saturations that continued to drop after exertion was stopped and that took longer to increase and return to baseline.
A 1962 study by O’Donnell et al. recruited 14 subjects with unrepaired TOF and 14 acyanotic subjects. Overall, the results showed that going from standing to squatting, both groups demonstrated increased blood pressure and decreased heart rate (HR). The TOF group also showed increased saturations. Results from dye-dilution method on cardiac catheterization suggested that squatting led to a decrease in right to left intracardiac shunting.
More contemporary studies include those by Hanson (1995) and Murakami (2002). Hanson et al. analyzed the hemodynamic changes occurring in adult subjects when changing from a standing to a squatting position in nine normal and six heart transplant recipient subjects. Stroke volume was measured by thoracic impedance cardiography. The results showed that both groups had increases in stroke volume index and blood pressure. Heart rate decreased in the subjects with normal hearts but was unchanged in the heart transplant recipients. Interestingly, both groups had an initial decrease in systemic vascular resistance followed by a return to baseline after 20 seconds.
Murakami enrolled 12 healthy adults with the goal to study the changes in ventricular afterload by evaluating the aortic pressure waveform when going from standing to squatting. The results showed that squatting increased aortic pressures, specifically the systolic component. The aortic waveform showed an increased augmentation index [(peak systolic pressure- inflection point pressure)/ pulse pressure]. The author concluded that squatting increased the afterload to the left ventricle by enhancing the aortic wave reflection.
The above early research on postural behavior in patients with unrepaired TOF for acute symptomatic relief of oxygen desaturation provided important clinical insight. The management of acute oxygen desaturation in patients with unrepaired TOF focuses on decreasing the degree of right to left shunt across the VSD with the net effect of increasing the pulmonary blood flow. Decreasing sympathetic stimulation and/or removing/treating the stimulating trigger is a first step, ie administration of pain medication for painful experiences. Performing maneuvers to increase systemic vascular resistance and left ventricular afterload function to increase venous return, increase preload, maintain lower heart rate, and decrease right to left intracardiac shunting. Specific maneuvers to accomplish these goals include the assumption of a squatting posture or knee to chest position, administration of a fluid bolus, administration of phenylephrine and medications such as esmolol to reduce cardiac contractility and relieve infundibular muscle spasm.
References
1. Squatting in Fallot's tetralogy. Br Med J. 1968; 4(5629): 470.
2. Schmitz M, Ullah S, Dasgupta R. Anesthesia for Right-sided Obstructive Lesions. In: Andropoulos D, Stayer S, Mossad E, Miller-Hance W, eds. Anesthesia for Congenital Heart Disease. 3rd Edition. Hoboken, New Jersey: John Wiley & Sons, Inc.; 2015: 516-541.
3. Lurie PR. Postural effects in tetralogy of Fallot. Am J Med. 1953; 15(3): 297-306.
4. Brotmacher L. Haemodynamic effects of squatting during recovery from exertion. Br Heart J. 1957; 19(4): 567-573.
5. O'Donnell TV, McIlroy MB. The circulatory effects of squatting. Am Heart J. 1962; 64: 347-356.
6. Guntheroth W, Mortan B, Mullins G. Venous return with knee-chest position and squatting in tetralogy of Fallot. Am Heart J. 1968; 75(3): 313-318.
7. Hanson P, Slane PR, Rueckert PA, Clark SV. Squatting revisited: comparison of haemodynamic responses in normal individuals and heart transplantation recipients. Br Heart J. 1995; 74(2): 154-158.
8. Murakami T. Squatting: the hemodynamic change is induced by enhanced aortic wave reflection. Am J Hypertens. 2002; 15(11): 986-988.
9. Carano N, Tchana B. An equivalent posture to squatting is seen in an unoperated adult with tetralogy of Fallot. Cardiol Young. 2008; 18(6): 644.
Due to this constellation of defects, systemic deoxygenated blood can preferentially cross the VSD and into the systemic circulation rather than the pulmonary circulation. Depending on the severity of the RVOTO, the patient’s oxygen saturation may range from normal to extreme cyanosis. Acute hypercyanotic episodes or “tet spells” are characterized by severe oxygen desaturation, dyspnea, and even syncope. Common precipitating factors - such as physical exertion, crying, agitation or pain - result in increased sympathetic activity and spasm of the infundibular portion of the right ventricular outflow tract thereby leading to increased right to left intracardiac shunting. The infundibular spasm is caused by increased sympathetic tone and increased cardiac contractility. A decrease in ventricular afterload can also increase right to left intracardiac shunting, which may occur during a febrile illness, due to inhaled anesthetic agents, and with dehydration. Persistent hypoxemia may eventually lead to the development of metabolic acidosis, peripheral vasodilation, and further decreases in systemic vascular resistance. This may worsen the hypercyanotic episode and potentially result in syncope and cardiac arrest.
The modern therapeutic approach to TOF is surgical repair during infancy to avoid the pathophysiologic and anatomic changes related to worsening cyanosis and increasing RVOTO. Children with TOF in low- to middle-income countries may have limited access to health care and remain unrepaired for many years before gaining access to surgical care.
Textbooks commonly cite that postural changes of knee-to-chest positioning or squatting lead to increased venous return and increased systemic vascular resistance (SVR), which in turn increases pulmonary blood flow and oxygen saturation. Several reports have studied the physiologic and hemodynamic changes resulting from squatting. Many investigative studies that utilized unrepaired, older TOF patients are decades old. More contemporary reports of squatting have been conducted in healthy volunteers or acyanotic patients. The following is a general overview of relevant studies.
Lurie, in a 1953 study, performed several tilt tests on subjects with unrepaired TOF and demonstrated that oxygen saturation dropped with increasing tilt from the supine towards the standing position. Elastic bandages were placed to compress the abdomen and the lower extremities in two subjects; the decrease in oxygen saturation was lessened when going from supine to standing as compared to those in patients without compression bandages. The authors concluded that the improvement in oxygen saturation was related to the increased venous return to the heart secondary to the postural changes.
A 1957 study by Brotmacher et al. recruited nine subjects with unrepaired TOF who were routine squatters (experimental group), and ten subjects who had either acyanotic heart disease or non-cardiac conditions (control group). After oxygen desaturation from baseline was induced by exertion; subjects with TOF either squatted or remained standing. Those who squatted demonstrated an immediate rise in oxygen saturations with a faster return to baseline compared to the standing group. Those patients who remained standing after exertion demonstrated oxygen saturations that continued to drop after exertion was stopped and that took longer to increase and return to baseline.
A 1962 study by O’Donnell et al. recruited 14 subjects with unrepaired TOF and 14 acyanotic subjects. Overall, the results showed that going from standing to squatting, both groups demonstrated increased blood pressure and decreased heart rate (HR). The TOF group also showed increased saturations. Results from dye-dilution method on cardiac catheterization suggested that squatting led to a decrease in right to left intracardiac shunting.
More contemporary studies include those by Hanson (1995) and Murakami (2002). Hanson et al. analyzed the hemodynamic changes occurring in adult subjects when changing from a standing to a squatting position in nine normal and six heart transplant recipient subjects. Stroke volume was measured by thoracic impedance cardiography. The results showed that both groups had increases in stroke volume index and blood pressure. Heart rate decreased in the subjects with normal hearts but was unchanged in the heart transplant recipients. Interestingly, both groups had an initial decrease in systemic vascular resistance followed by a return to baseline after 20 seconds.
Murakami enrolled 12 healthy adults with the goal to study the changes in ventricular afterload by evaluating the aortic pressure waveform when going from standing to squatting. The results showed that squatting increased aortic pressures, specifically the systolic component. The aortic waveform showed an increased augmentation index [(peak systolic pressure- inflection point pressure)/ pulse pressure]. The author concluded that squatting increased the afterload to the left ventricle by enhancing the aortic wave reflection.
The above early research on postural behavior in patients with unrepaired TOF for acute symptomatic relief of oxygen desaturation provided important clinical insight. The management of acute oxygen desaturation in patients with unrepaired TOF focuses on decreasing the degree of right to left shunt across the VSD with the net effect of increasing the pulmonary blood flow. Decreasing sympathetic stimulation and/or removing/treating the stimulating trigger is a first step, ie administration of pain medication for painful experiences. Performing maneuvers to increase systemic vascular resistance and left ventricular afterload function to increase venous return, increase preload, maintain lower heart rate, and decrease right to left intracardiac shunting. Specific maneuvers to accomplish these goals include the assumption of a squatting posture or knee to chest position, administration of a fluid bolus, administration of phenylephrine and medications such as esmolol to reduce cardiac contractility and relieve infundibular muscle spasm.
References
1. Squatting in Fallot's tetralogy. Br Med J. 1968; 4(5629): 470.
2. Schmitz M, Ullah S, Dasgupta R. Anesthesia for Right-sided Obstructive Lesions. In: Andropoulos D, Stayer S, Mossad E, Miller-Hance W, eds. Anesthesia for Congenital Heart Disease. 3rd Edition. Hoboken, New Jersey: John Wiley & Sons, Inc.; 2015: 516-541.
3. Lurie PR. Postural effects in tetralogy of Fallot. Am J Med. 1953; 15(3): 297-306.
4. Brotmacher L. Haemodynamic effects of squatting during recovery from exertion. Br Heart J. 1957; 19(4): 567-573.
5. O'Donnell TV, McIlroy MB. The circulatory effects of squatting. Am Heart J. 1962; 64: 347-356.
6. Guntheroth W, Mortan B, Mullins G. Venous return with knee-chest position and squatting in tetralogy of Fallot. Am Heart J. 1968; 75(3): 313-318.
7. Hanson P, Slane PR, Rueckert PA, Clark SV. Squatting revisited: comparison of haemodynamic responses in normal individuals and heart transplantation recipients. Br Heart J. 1995; 74(2): 154-158.
8. Murakami T. Squatting: the hemodynamic change is induced by enhanced aortic wave reflection. Am J Hypertens. 2002; 15(11): 986-988.
9. Carano N, Tchana B. An equivalent posture to squatting is seen in an unoperated adult with tetralogy of Fallot. Cardiol Young. 2008; 18(6): 644.
