Ammonia and ventilation: site and mechanism of action

Fish gill chemosensing: knowledge gaps and inconsistencies

In this review, we explore the inconsistencies in the data and gaps in our knowledge that exist in what is currently known regarding gill chemosensors which drive the cardiorespiratory reflexes in fish. Although putative serotonergic neuroepithelial cells (NEC) dominate the literature, it is clear that other neurotransmitters are involved (adrenaline, noradrenaline, acetylcholine, purines, and dopamine). And although we assume that these agents act on neurons synapsing with the NECs or in the afferent or efferent limbs of the paths between chemosensors and central integration sites, this process remains elusive and may explain current discrepancies or species differences in the literature. To date it has been impossible to link the distribution of NECs to species sensitivity to different stimuli or fish lifestyles and while the gills have been shown to be the primary sensing site for respiratory gases, the location (gills, oro-branchial cavity or elsewhere) and orientation (external/water or internal/blood sensing) of the NECs are highly variable between species of water and air breathing fish. Much of what has been described so far comes from studies of hypoxic responses in fish, however, changes in CO2, ammonia and lactate have all been shown to elicit cardio-respiratory responses and all have been suggested to arise from stimulation of gill NECs. Our view of the role of NECs is broadening as we begin to understand the polymodal nature of these cells. We begin by presenting the fundamental picture of gill chemosensing that has developed, followed by some key unanswered questions about gill chemosensing in general.

Brain and gills as internal and external ammonia sensing organs for ventilatory control in rainbow trout, Oncorhynchus mykiss

Ammonia is both a respiratory gas and a toxicant in teleost fish. Hyperventilation is a well-known response to elevations of both external and internal ammonia levels. Branchial neuroepithelial cells (NECs) are thought to serve as internal sensors of plasma ammonia (peripheral chemoreceptors), but little is known about other possible ammonia-sensors. Here, we investigated whether trout possess external sensors and/or internal central chemoreceptors for ammonia. For external sensors, we analyzed the time course of ventilatory changes at the start of exposure to high environmental ammonia (HEA, 1 mM). Hyperventilation developed gradually over 20 min, suggesting that it was a response to internal ammonia elevation. We also directly perfused ammonia solutions (0.01-1 mM) to the external surfaces of the first gill arches. Immediate hypoventilation occurred. For central chemoreceptors, we injected ammonia solutions (0.5-1.0 mM) directly onto the surface of the hindbrain of anesthetized trout. Immediate hyperventilation occurred. This is the first evidence of central chemoreception in teleost fish. We conclude that trout possess both external ammonia sensors, and dual internal ammonia sensors (perhaps for redundancy), but their roles differ. External sensors cause short term hypoventilation, which would help limit toxic waterborne ammonia uptake. When fish cannot avoid HEA, the diffusion of waterborne ammonia into the blood will stimulate both peripheral (NECs) and central (brain) chemoreceptors, resulting in hyperventilation. This hyperventilation will be beneficial in increasing ammonia excretion via the Rh metabolon system in the gills not only after HEA exposure, but also after endogenous ammonia loading from feeding or exercise.

Neonatal Presentations of Metabolic Disorders

Metabolic disorders in a neonate can present with involvement of any organ system and can be challenging to diagnose. A newborn can present with an acute metabolic crisis such as hyperammonemia or seizures needing immediate management, with a more chronic clinical picture such as cholestatic liver disease, or with structural abnormalities such as skeletal manifestations. Early detection of treatable metabolic conditions is important to improve outcomes. Newborn screening has facilitated early detection and initiation of therapy for many metabolic disorders. However, normal testing does not rule out a metabolic disorder and a high index of suspicion should remain when caring for any critically ill neonate without a diagnosis. Whole exome sequencing (WES) or whole genome sequencing (WGS) can be powerful tools in rapid diagnosis of a potentially treatable metabolic condition in a critically ill neonate. This review presents classic clinical presentations of neonatal metabolic disorders and also highlights some uncommon neonatal manifestations of metabolic disorders to improve the recognition and diagnosis of these conditions.

Ventilatory sensitivity to ammonia in the Pacific hagfish (Eptatretus stoutii), a representative of the oldest extant connection to the ancestral vertebrates

Ventilatory sensitivity to ammonia occurs in teleosts, elasmobranchs and mammals. Here, we investigated whether the response is also present in hagfish. Ventilatory parameters (nostril flow, pressure amplitude, velar frequency and ventilatory index, the last representing the product of pressure amplitude and frequency), together with blood and water chemistry, were measured in hagfish exposed to either high environmental ammonia (HEA) in the external sea water or internal ammonia loading by intra-vascular injection. HEA exposure (10 mmol l^-1 NH₄HCO₃ or 10 mmol l^-1 NH₄Cl) caused a persistent hyperventilation by 3 h, but further detailed analysis of the NH₄HCO₃ response showed that initially (within 5 min) there was a marked decrease in ventilation (80% reduction in ventilatory index and nostril flow), followed by a later 3-fold increase, by which time plasma total ammonia concentration had increased 11-fold. Thus, hyperventilation in HEA appeared to be an indirect response to internal ammonia elevation, rather than a direct response to external ammonia. HEA-mediated increases in oxygen consumption also occurred. Responses to NH₄HCO₃ were greater than those to NH₄Cl, reflecting greater increases over time in water pH and P _NH3  in the former. Hagfish also exhibited hyperventilation in response to direct injection of isotonic NH₄HCO₃ or NH₄Cl solutions into the caudal sinus. In all cases where hyperventilation occurred, plasma total ammonia and P _NH3  levels increased significantly, while blood acid-base status remained unchanged, indicating specific responses to internal ammonia elevation. The sensitivity of breathing to ammonia arose very early in vertebrate evolution.

Does ammonia trigger hyperventilation in the elasmobranch, Squalus acanthias suckleyi?

We examined the ventilatory response of the spiny dogfish, to elevated internal or environmental ammonia. Sharks were injected via arterial catheters with ammonia solutions or their Na salt equivalents sufficient to increase plasma total ammonia concentration [TAmm]a by 3-5 fold from 145±21μM to 447±150μM using NH4HCO3 and a maximum of 766±100μM using (NH4)2SO4. (NH4)2SO4 caused a small increase in ventilation frequency (+14%) and a large increase in amplitude (+69%), while Na2SO4 did not. However, CO2 partial pressure (PaCO2) also increased and arterial pHa and plasma bicarbonate concentration ([HCO3(-)]a) decreased. NH4HCO3 caused a smaller increase in plasma ammonia resulting in a smaller but significant, short lived increases in ventilation frequency (+6%) and amplitude (36%), together with a rise in PaCO2 and [HCO3(-)]a. Injection with NaHCO3 which increased pHa and [HCO3(-)]a did not change ventilation. Plasma ammonia concentration correlated significantly with ventilation amplitude, while ventilation frequency showed a (negative) correlation with pHa. Exposure to high environmental ammonia (1500μM NH4HCO3) did not induce changes in ventilation until plasma [TAmm]a increased and ventilation amplitude (but not frequency) increased in parallel. We conclude that internal ammonia stimulates ventilation in spiny dogfish, especially amplitude or stroke volume, while environmental ammonia only stimulates ventilation after ammonia diffuses into the bloodstream.

Rh protein expression in branchial neuroepithelial cells, and the role of ammonia in ventilatory control in fish

Bill Milsom has made seminal contributions to our understanding of ventilatory control in a wide range of vertebrates. Teleosts are particularly interesting, because they produce a 3rd, potentially toxic respiratory gas (ammonia) in large amounts. Fish are well known to hyperventilate under high environmental ammonia (HEA), but only recently has the potential role of ammonia in normal ventilatory control been investigated. It is now clear that ammonia can act directly as a ventilatory stimulant in trout, independent of its effects on acid-base balance. Even in ureotelic dogfish sharks, acute elevations in ammonia cause increases in ventilation. Peripherally, the detection of elevated ammonia resides in gill arches I and II in trout, and in vitro, neuroepithelial cells (NECs) from these arches are sensitive to ammonia, responding with elevations in intracellular Ca(2+) ([Ca(2+)]i). Centrally, hyperventilatory responses to ammonia correlate more closely with concentrations of ammonia in the brain than in plasma or CSF. After chronic HEA exposure, ventilatory responsiveness to ammonia is lost, associated with both an attenuation of the [Ca(2+)]i response in NECs, and the absence of elevation in brain ammonia concentration. Chronic exposure to HEA also causes increases in the mRNA expression of several Rh proteins (ammonia-conductive channels) in both brain and gills. "Single cell" PCR techniques have been used to isolate the individual responses of NECs versus other gill cell types. We suggest several circumstances (post-feeding, post-exercise) where the role of ammonia as a ventilatory stimulant may have adaptive benefits for O2 uptake in fish.

Sensitivity of ventilation and brain metabolism to ammonia exposure in rainbow trout, Oncorhynchus mykiss

Ammonia has been documented as a respiratory gas that stimulates ventilation, and is sensed by peripheral neuroepithelial cells (NECs) in the gills in ammoniotelic rainbow trout. However, the hyperventilatory response is abolished in trout chronically exposed (1+ months) to high environmental ammonia [HEA; 250 μmol l(-1) (NH4)2SO4]. This study investigates whether the brain is involved in the acute sensitivity of ventilation to ammonia, and whether changes in brain metabolism are related to the loss of hyperventilatory responses in trout chronically exposed to HEA ('HEA trout'). Hyperventilation (via increased ventilatory amplitude rather than rate) and increased total ammonia concentration ([TAmm]) in brain tissue were induced in parallel by acute HEA exposure in control trout in a concentration-series experiment [500, 750 and 1000 μmol l(-1) (NH4)2SO4], but these inductions were abolished in HEA trout. Ventilation was correlated more closely to [TAmm] in brain rather than to [TAmm] in plasma or cerebrospinal fluid. The close correlation of hyperventilation and increased brain [TAmm] also occurred in control trout acutely exposed to HEA in a time-series analysis [500 μmol l(-1) (NH4)2SO4; 15, 30, 45 and 60 min], as well as in a methionine sulfoxamine (MSOX) pre-injection experiment [to inhibit glutamine synthetase (GSase)]. These correlations consistently suggest that brain [TAmm] is involved in the hyperventilatory responses to ammonia in trout. The MSOX treatments, together with measurements of GSase activity, TAmm, glutamine and glutamate concentrations in brain tissue, were conducted in both the control and HEA trout. These experiments revealed that GSase plays an important role in transferring ammonia to glutamate to make glutamine in trout brain, thereby attenuating the elevation of brain [TAmm] following HEA exposure, and that glutamate concentration is reduced in HEA trout. The mRNAs for the ammonia channel proteins Rhbg, Rhcg1 and Rhcg2 were expressed in trout brain, and the expression of Rhbg and Rhcg2 increased in HEA trout, potentially as a mechanism to facilitate the efflux of ammonia. In summary, the brain appears to be involved in the sensitivity of ventilation to ammonia, and brain ammonia levels are regulated metabolically in trout.

Seven things fish know about ammonia and we don't

In this review we pose the following seven questions related to ammonia and fish that represent gaps in our knowledge. 1. How is ammonia excretion linked to sodium uptake in freshwater fish? 2. How much does branchial ammonia excretion in seawater teleosts depend on Rhesus (Rh) glycoprotein-mediated NH(3) diffusion? 3. How do fish maintain ammonia excretion rates if branchial surface area is reduced or compromised? 4. Why does high environmental ammonia change the transepithelial potential across the gills? 5. Does high environmental ammonia increase gill surface area in ammonia tolerant fish but decrease gill surface area in ammonia intolerant fish? 6. How does ammonia contribute to ventilatory control? 7. What do Rh proteins do when they are not transporting ammonia? Mini reviews on each topic, which are able to present only partial answers to each question at present, are followed by further questions and/or suggestions for research approaches targeted to uncover answers.

Ammonia as a stimulant to ventilation in rainbow trout Oncorhynchus mykiss

Ammonia is the third most important respiratory gas in ammoniotelic fish after oxygen and carbon dioxide. We here investigated the effects of elevated plasma ammonia on ventilation in freshwater rainbow trout. Intact trout fitted with indwelling dorsal aortic catheters were given injections (over 5 min) of Cortland saline, isotonic high ammonia solutions (NH(4)HCO(3), (NH(4))(2)SO(4), NH(4)OH at pH 8.0, and NH(4)OH at pH 9.0), and other solutions as controls for acid-base effects, while ventilatory rate (VR) and buccal pressure amplitude (DeltaP(buccal)) were recorded. All high ammonia solutions resulted in immediate elevations of plasma Tamm(a), Pa(NH3), and [NH(4)(+)](a), and increases in ventilatory DeltaP(buccal) and VR to different degrees. However, while Pa(O2) remained constant, in every case there was a confounding change in one or more components of acid-base status (decreases in pH(a) or increases in [HCO(3)(-)](a) or Pa(CO2) in different treatments), although the ventilatory responses to ammonia injections were generally larger than could be explained by changes in acid-base status. Therefore a series was performed in which normal blood perfusion of the gills was replaced by ventral aortic perfusion with either Cortland saline or Cortland saline plus high ammonia in which pH, [HCO(3)(-)], P(CO2), and P(O2) remained unchanged. Although ventilation was depressed in these anaesthetized, spontaneously ventilating preparations, perfusion with high ammonia saline increased DeltaP(buccal). In a final series, trout were infused for 24h with Cortland saline, isotonic NH(4)HCO(3), or isotonic (NH(4))(2)SO(4) solutions. The two ammonia solutions both caused persistent elevations in VR and DeltaP(buccal), together with similar large increases in plasma Tamm(a), Pa(NH3), and [NH(4)(+)](a). As there was no changes in Pa(O2), pH(a), Pa(CO2), or [HCO(3)(-)](a) in the (NH(4))(2)SO(4) infusion series, this, together with the ventral aortic perfusion experiment, provides the most convincing evidence that ammonia stimulates ventilation. We suggest several circumstances (post-feeding, post-exercise) where the role of ammonia as a ventilatory stimulant may have adaptive benefits for O(2) uptake, and propose that ammonia-induced hyperventilation may also facilitate ammonia excretion in rainbow trout.

A rare case of dilatation of pulmonary veins and pulmonary emphysema in both lower lobes

A 34-year-old man was admitted to hospital because of persistent shortness of breath and diffuse vascular dilation at both lower fields of his CXR. Bronchiectasis had been suspected during childhood because of abnormal chest shadows. However, a chest CT scan obtained on admission failed to show bronchiectasis, but rather there was a dilation of blood vessels and low attenuation areas in both lower lobes. A pulmonary angiogram showed normal pulmonary arteries in the arterial phase and diffuse dilated veins in the venous phase. Although the patient also had liver cirrhosis type B with portal hypertension, no association could be found between his liver cirrhosis and the lung lesions. This is a rare case of possible congenital or idiopathic diffuse dilatation of the pulmonary veins.

Blood ammonia and ventilation at maximal exercise

This study, intended to evaluate the role of ammonia (NH3) as a ventilatory stimulus, was conducted in three groups of subjects: 14 sedentary individuals, 12 triathletes, 5 patients with a glycolytic deficiency (Mc Ardle disease). All subjects performed maximal exercise tests on a cycle ergometer. Ventilation measured at maximal oxygen consumption (VE 100%) was correlated with lactatemia (lactate 100%) and ammonemia (NH3 100%) in the sedentary group, but only with ammonemia in triathletes, although NH3 100% and lactate 100% were correlated in both groups, which suggests that correlation between VE 100% and NH3 100% is not a false correlation. In patients with Mc Ardle disease, unable to produce lactate during exercise, VE 100% was correlated with NH3 100%. NH3 may act indirectly by increasing the production of lactate in cereberal tissue. Another hypothesis rests on the fact that the catabolism of ammonia leads to an increase in intracerebral glutamate which may act as a ventilatory stimulus.

Effects of changes in plasma pH, CO2 and ammonia on ventilation in trout

We investigated ventillatory responses to a plasma alkaloids and hypocapnia,a nd the basis for the ventilatory response to sodium bicarbonate (NaHCO3) infusion in rainbow trout. Plasma alkalosis and hypocapnia created by infusion of sodium hydroxide (NaOH) did not cause hypoventilation, whereas infusion of hydrochloric acid (HCl) caused vigorous hyperventilation, associated with an acidosis, a reduction in blood O2 content (CaO 2) and a release of circulating catecholamines. Infusion of NaHCO3 stimulated ventilation and caused an increase in plasma pH, total carbon dioxide content (CaCO 2) and catecholamine levels, and a reduction in oxygen tension (PaO 2). Infusion of ammonium bicarbonate (NH 4HCO3) caused hyperventilation and was associated with an increase in CaCO 2 and plasma total ammonia (Camm) and ammonia gas (NH3) concentration. Infusion of sodium chloride (NaClI) and Cortland's saline had no effect on ventilation. The results indicate that trout do not exhibit the ventilatory sensitivity to pH seen in terrestrial vertebrates. Ventilatory responses to NaHCO3 appear to have been a result of reductions in PaO 2, a release of catecholamines and an increase in CaCO 2 whereas responses to NH4HCO3 appear to have been a result of increases in CaCO 2 and Camm.

Ammonium ion and the anaerobic threshold in man

To determine if ammonium ion plays a role in the lactate and ventilatory thresholds of incremental exercise, we investigated the effects on blood lactate and ventilation of NH(4+)-buffering by monosodium glutamate. Six normal volunteers underwent intravenous loading with MSG, 9 g, in a randomized, double-blind, saline placebo controlled crossover study. Four of the six subjects had a greater than 10 percent fall in peak (NH4+) following MSG (37 +/- 2.0 vs 25 +/- 4.3 micrograms/dl p = 0.003, PLB vs MSG). When MSG blunted the rise in venous (NH4+) during exercise, uncoupling of the LT and VT was observed. Specifically, with suppression of peak exercise (NH4+) by MSG, the LT was delayed (r = -0.84, p = 0.03), the VT was earlier (r = 0.86, p = 0.02), and the VO2 difference between the LT and VT widened (r = -0.90, p = 0.02). We conclude that NH4+ plays a role in determining the LT and VT of incremental exercise and that the VT may not be exclusively dependent on blood lactate.

On octanoic acid-induced hyperventilation--implications for hepatic encephalopathy and Reye's syndrome

Medium chain fatty acid sodium octanoate was infused into rabbits as a 0.2 M solution over 4 h resulting in blood and brain octanoate levels of 200-800 mumol/l. The infused animals developed marked hyperventilation leading to a mild respiratory alkalosis. Additionally, octanoate infusion brought about hyperammonemia and hyperlactate acidemia. Another group of rabbits also infused with octanoate but pretreated with indomethacin (10 mg/kg b.wt.) developed neither hyperventilation nor hyperammonemia. Therefore, the conclusion made was that octanoate causes the above mentioned disorders through stimulation of prostaglandin synthesis and especially the PGE2 synthesis. Patients with hepatic encephalopathy and Reye's syndrome have elevated levels of plasma octanoate. The present study suggests that octanoate might be the cause for both the hyperventilation and hyperammonemia observed in patients with hepatic encephalopathy and Reye's syndrome.

The pathogenesis of vascular headaches in patients with hypertension; the role of the ammonia-potassium axis

Headaches may occur in as many as 25% of hypertensive patients and generally bears little relationship to level of diastolic blood pressure. Previous observations, in normotensive patients, suggested that abnormalities in both potassium and ammonia metabolism might be related to the pathogenesis of these headaches. The present study was undertaken to see whether these factors also occurred in hypertensive patients with headaches. The present observations were made in thirteen hypertensive patients with vascular headaches. The major findings include potassium levels of 3.45 +/- 0.25 mEq/L; CO2, 29.85 +/- 1.21 mEq/L; blood ammonia, 41 +/- 8.40 U mol/L and an alkaline pH of the urine. The blood ammonia levels, when factored by the BUN, yielded elevated ammonia to BUN ratios (3.81 +/- 1.82). These findings are similar to those previously observed in normotensive patients with vascular headaches. The profile of hypokalemia and/or alkalosis, increased blood ammonia to BUN ratios and a relatively alkaline urine appears to be a commonly observed pattern in patients with vascular headaches. These data suggest that a biochemical basis exists for the genesis of vascular headaches in patients with hypertension.

Disorders of the serum electrolytes, acid-base balance, and renal function in alcoholism

This chapter reviews the disturbances of the serum sodium and potassium concentrations, acid-base imbalances, and acute renal dysfunction that are seen frequently in alcoholic patients. The hyponatremia common in decompensated cirrhotics is caused by an impairment of renal free water clearance and concomitant water ingestion. Excessive proximal renal tubular sodium reabsorption and nonosmotic vasopressin release underlie the defect in renal water excretion in cirrhosis. Restriction of water intake is the principal therapeutic measure for hyponatremia. Hypokalemia is common in alcoholics but when observed does not always represent true potassium depletion. Although most cirrhotics have a diminished total body potassium content, intracellular potassium concentration is usually normal. In some patients gastrointestinal and renal potassium losses and nutritional potassium deficiency may cause true potassium depletion. Respiratory and metabolic alkalosis are the acid-base disturbances seen most frequently in alcoholics. Acidosis is relatively uncommon and is usually due to renal insufficiency, lactic acid or keto-acid accumulation. Toxin ingestion (methanol, ethylene glycol, or isopropanol) may also cause severe acidosis. Rhabdomyolysis, common in severe alcoholism, may produce various electrolyte disturbances and acute renal failure. The prognosis for recovery is good although temporary dialysis may be necessary.

Ascites, renal abnormalities, and electrolyte and acid-base disorders associated with liver disease

Ascites and renal dysfunction are often associated with decreased liver function and reflect the complex abnormalities of water, protein, electrolyte, and acid-base metabolism that may complicate severe liver disease. This article discusses the pathophysiology and management of ascites, polydipsia and polyuria, decreased renal function, and acid-base and electrolyte alterations that can complicate liver disease.

Acid-base and ventilatory adaptation in conscious dogs during chronic hypercapnia

Ventilation and cisternal cerebrospinal fluid (CSF) and arterial acid-base balance were measured in awake dogs during air control and from 1 h to 26 days of breathing 5% CO2 in air. Ventilation increased 4-fold during acute hypercapnia and then declined to a minimum at 5-10 days. Between 1-3 days and 16-26 days of hypercapnia ventilation was relatively stable at 2.5 times control. [HCO3-]CSF increased rapidly by 12 h of hypercapnia and in the steady-state [HCO3-]CSF was correlated with PCSFCO2. Between 1 h and 1.5 days of hypercapnia, increase in [HCO3-]CSF was also correlated with increase in [NH3]CSF. Despite increase in [HCO3-]CSF, there was no compensation of [H+]CSF throughout 26 days of hypercapnia. Hydrogen ion may have contributed to the control of ventilation during chronic hypercapnia since ventilation was correlated with [HCO3-]a and [HCO3-]CSF. However, a relationship between ventilation and [H+] of arterial blood and CSF during chronic hypercapnia was relatively poor or absent. Ventilatory adaptation to chronic hypercapnia could not be related to metabolism or to [NH3]CSF. The mechanism(s) by which the increase in PCO2 during chronic respiratory acidosis results in sustained elevation of ventilation remains to be resolved.

The hypokalemic, bowel, bladder, headache relationship; a new syndrome. The role of the potassium ammonia axis

A conceptual approach that relates vascular headaches, bowel and bladder dysfunction to abnormalities of the "ammonia potassium axis" is presented. Hypokalemia alters smooth muscle function of both the bowel and bladder and results in the elaboration of an alkaline urine. The occurrence of an alkaline urine, along with bladder dysfunction and urinary stasis, predisposes to recurrent urinary tract infections. Hypokalemia and/or alkalosis increases the renal return of ammonia, exposes the brain to chronically higher concentration of ammonia and facilitates its passage into the central nervous system. Increased levels of blood ammonia predispose to hyperventilation which results in a superimposed respiratory alkalosis on a pre-existing hypokalemia and/or alkalosis therefore causing intense cerebral vasoconstriction. Varying degrees of cerebral ischemia and hypoxia occur and give rise to higher brain concentrations of ammonia. Vasodilatation occurs during the headache phase and may be a consequence of the sudden increase of brain ammonia and/or due to the release of other vasoactive mediators. As a consequence of increased blood ammonia, a reduction of protein intake may result in the alterations of amino acid precursors for brain uptake and therefore further interferes with the modulation of cerebral blood flow and brain function.

Induction of fetal breathing by metabolic acidemia and its effect on blood flow to the respiratory muscles

Sustained and vigorous fetal breathing activity was produced in a chronic fetal lamb preparation by infusion into the fetus of either NH4Cl or HCl. Over a 2 to 3 hour period, 20 to 25 mEq/kg were infused. All of the fetuses tolerated blood pH values of 6.7 to 6.8 and survived. The breathing activity began after the completion of the infusion, and consisted of regular 30 to 50 torr inspirations at a rate of 60 to 120 breaths/min. This activity was continuous for as much as 8 hours, and persisted with pauses and decreased amplitude for 24 to 36 hours. During fetal breathing, blood flow to the diaphragm and intercostal muscles increased approximately 12- and sixfold, respectively.